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Body reconstruction and size estimation of plesiosaurs
Ruizhe Jackevan Zhao İD
JackevanChaos@outlook.com
Abstract
Body size is the key to understanding many biological properties. Sizes of extinct animals
are usually estimated from body reconstructions since their masses can not be weighed
directly. Plesiosaurs were Mesozoic marine reptiles that were diverse in both body plan and
size. Attempts to estimate body masses of plesiosaurs were rare in the past two centuries,
possibly due to lack of knowledge about their postcranial anatomy and body shapes in
life. The burst of plesiosaur studies in the past two decades has greatly expanded our
cognition of their physiology, taxonomy, potential behavior and even soft body outlines.
Here I present a comprehensive review of relevant knowledge, and propose a uniform set of
methodology for rigorous body reconstruction of plesiosaurs. Twenty-two plesiosaur models
were constructed under these criteria, and they were subsequently used as samples to find
proxies for body mass. It is revealed that multiple skeletal elements are good indicators
of plesiosaur size. This study offers scaling equations for size estimation, enabling quick
acquisition of body mass information from fragmented fossils. A summary of body size
evolution of different plesiosaur clades is also provided.
Contents
1 Introduction 3
1.1 Institutional abbreviations ........................... 4
1.2 Unit abbreviations ............................... 4
2 Preliminaries to plesiosaur reconstruction 4
2.1 Postcranial anatomy of plesiosaurs ...................... 5
2.1.1 Vertebral column ............................ 5
2.1.2 Rib cage ................................. 10
2.1.3 Limbs .................................. 12
2.2 Soft tissue reconstruction ............................ 13
2.2.1 Plesiosaur fossils with soft tissues ................... 13
2.2.2 Head and neck ............................. 14
2.2.3 Rib cage ................................. 15
2.2.4 Tail .................................... 18
2.2.5 Flipper profile .............................. 18
2.3 Inconsistent definitions and measuring criteria ................ 18
2.3.1 Skull ................................... 18
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2.3.2 Spinal segmentation .......................... 20
2.3.3 Ribs ................................... 20
2.4 Ontogeny .................................... 21
2.5 Phylogeny and Taxonomy ........................... 21
2.6 A review of plesiosaur body mass estimation ................. 22
3 Reconstructing missing puzzles in plesiosaur fossils 23
3.1 Skull size .................................... 24
3.2 Rib length .................................... 25
3.3 Tail length .................................... 25
3.4 Trunk length .................................. 26
3.5 Limb length ................................... 26
4 Criteria for plesiosaur reconstruction 26
4.1 Body length calculation ............................ 26
4.1.1 Length of vertebral column ...................... 27
4.1.2 Skull length ............................... 27
4.2 Rib cage reconstruction ............................. 28
4.2.1 Glenoid cross-section .......................... 28
4.2.2 Acetabulum cross-section ........................ 28
4.2.3 Maximum cross-section ......................... 28
4.3 Flipper elements ................................ 29
4.4 Soft tissue reconstruction ............................ 29
4.4.1 Skull and neck ............................. 29
4.4.2 Rib cage ................................. 29
4.4.3 Tail and flippers ............................. 30
4.5 Body density .................................. 30
5 Methods 31
5.1 Model construction ............................... 31
5.2 Body volume and surface area calculation .................. 31
5.3 Morphometric analysis ............................. 32
5.4 Regressions ................................... 32
6 Results 33
7 Discussion 34
7.1 On some large Jurassic pliosaurs ........................ 34
7.2 A brief summary of plesiosaur body size evolution .............. 38
7.2.1 Rhomaleosauridae ............................ 38
7.2.2 Pliosauridae ............................... 39
7.2.3 Microcleididae and basal Plesiosauroidea ............... 40
7.2.4 Cryptoclididae ............................. 41
7.2.5 Elasmosauridae ............................. 42
7.2.6 Polycotylidae .............................. 44
7.2.7 Leptocleididae and freshwater plesiosaurs from China ........ 44
7.3 Enlightment for future studies ......................... 45
8 Conclusions 46
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9 Acknowledgements 46
1 Introduction
Body size is one of the most important biological properties, which is the key to under-
standing various aspects of an animal, including physiology [
1
,
2
,
3
,
4
] and ecology [
5
,
6
,
7
].
Among all proxies for body size, mass has the advantage over linear measurements that it
enables comparison among different taxa without being affected by distinct body propor-
tions. While the masses of extant animals can be weighed directly, those of extinct species
are unavailable in most cases. Numerous methods have been developed to estimate the
body masses of extinct animals for this reason. One category of methods requires rigorous
body reconstructions and transforms them into masses using physical laws, mathematical
calculation or computer techniques [
8
,
9
,
10
,
11
,
12
]. Another category, namely scaling
methods, advocates digging the relationships between skeletal elements and body mass
using regressions [
13
,
14
,
15
]. Each category of mass-acquring method has its strengths
and shortcomings [
16
]. Mass estimates based on reconstructions were included as sample
data for scaling in some previous studies (e.g., [
17
,
18
]). These methods were termed as
“hybrid approaches” in [16].
Plesiosaurs were a group of extinct aquatic reptiles which possessed a cosmopolitan
distribution and a wide stratigraphic range [
19
,
20
]. They first appeared in the latest
Triassic [
21
], radiated in Jurassic and Cretaceous [
22
,
23
], and finally went to extinction
at the end of Mesozoic together with non-avian dinosaurs [
24
]. Since the discovery of the
first plesiosaur two centuries ago, these animals have been known for their diverse body
plans, which were traditionally divided into two morphotypes: long-necked, small-headed
“plesiosauromorph” and short-necked, large-headed “pliosauromorph” [
25
]. They were
highly adapted to aquatic life, with four hydrofoil-like flippers, rigid trunks and possible
tail fins [
26
,
27
,
28
]. Although they belong to diapsid Sauropterygia [
26
], plesiosaurs
possessed many physiological features analogous to mammals (e.g., viviparity [
29
,
30
],
endotherm [31], high metabolic rates [32]).
Besides their great diversity in body plans, plesiosaurs also varied greatly in body size,
from small taxon like Thalassiodracon hawkinsi that measures less than 2 meters [
33
],
to gigantic thalassophoneans and elasmosaurs (
>
10 m [
34
,
35
]). Numerous studies at-
tempted to estimate the body sizes of different plesiosaur taxa, most of which focused on
body length (e.g., [
36
,
34
,
37
,
38
]). However, these estimations were carried out under
different criteria and the results are often conflicting. In addition, linear measurements
like body length may not be good proxies for plesiosaur size due to the high plasticity
and diversity of their body proportions. On the other hand, attempts to estimate body
masses of plesiosaurs are very rare, possibly precluded by lack of knowledge about the
three-dimensional arrangement of their postcranial skeletons. Therefore, the body massses
of many plesiosaur taxa remain mysterious to date.
Thanks to the burst of plesiosaur researches in the past two decades, our knowledge
on their anatomy, physiology and evolutionary history has been greatly expanded. The
discovery of some fossils with soft tissue imprints also sheds light on their body outlines.
Now it is the suitable time to review previous studies and propose a set of criteria for
plesiosaur body reconstruction.
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The goal of this study is to: (1) perform a comprehensive review of relevant knowl-
edge, criteria and discrepancies about plesiosaur reconstructions in previous studies; (2)
propose a uniform set of methodology for body reconstruction and size estimation that
can be applied to the whole Plesiosauria; (3) create multiple rigorous plesiosaur models
and calculate their masses and surface areas; (4) use these models to generate scaling
equations that enable quick body mass prediction from incomplete fossils. Results of this
study would illuminate the great size disparity among plesiosaurs, offer tools for future
studies and finally lead to better understanding of their evolutionary history.
1.1 Institutional abbreviations
NHMUK (formerly BMNH), Natural History Museum, London, U.K.; SMU SMP,
Shuler Museum of Paleontology, Southern Methodist University, Dallas, U.S.A; SMNS,
Staatliches Museum für Naturkunde, Stuttgart, Germany; MB, Naturkundemuseum
Berlin, Berlin, Germany; CAMSM, Sedgwick Museum of Geology, Cambridge, U.K.;
MCZ, Museum of Comparative Zoology, Harvard University, USA; FHSM, Fort Hays
State University, Sternberg Museum of Natural History, U.S.A.; UANL, Facultad de
Ciencias de la Tierra, Universidad Autonóma de Nuevo León, Mexico. MJACM, Museo
El Fósil, Vereda Monquirá, Colombia; DMNH, Denver Museum of Nature and Science,
Denver, U.S.A. UCMP, Museum of Paleontology, University of California at Berkeley,
Berkeley, California; QM, Queensland Museum, Queensland, Australia; GPIT, Geologisch-
Paläontologisches Institut Tübingen, Tübingen, Germany; PMO, Palaeontology Museum,
Natural History Museum, Oslo, Norway; DORCM, Dorset County Museum, Dorchester,
U.K.; USNM, NationalMuseum of Natural History,Washington, D.C., U.S.A.; MOZ,
Museo Profesor J. Olsacher, Zapala, Argentina; OUMNH, Oxford University Museum of
Natural History, Oxford, U.K.; ABGCH, Abingdon County Hall Museum, Abingdon, U.K.;
YORYM, Yorkshire Museum, York, U.K.; MNA, Museum of Northern Arizona, Flagstaff,
U.S.A.; MNHNL, Muséum national d’histoire naturelle du Luxembourg, Luxembourg-
ville, Luxembourg; MChEIO, Museum of Chuvash Natural Historical Society, Chuvashia,
Russia; TMP, Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada; KUVP,
Natural History Museum, University of Kansas, Lawrence, U.S.A.; INAH, Instituto
Nacional de Antropología e Historia, Saltillo, Mexico; YPM, Yale Peabody Museum, New
Haven, U.S.A.; SGO.PV, Museo Nacional de Historia Natural, Santiago, Chile;
1.2 Unit abbreviations
m, meter; mm, millimeter; t, tonne;
2 Preliminaries to plesiosaur reconstruction
Attempts to reconstruct plesiosaurs are often hampered by incomplete or fragmented
nature of fossils, which might be crushed or distorted during taphonomic processes. Our
knowledge on three-dimensional arrangement of plesiosaur postcranial skeletons is limited.
Even in well-preserved, articulated fossils, skeletal elements necessary for reconstructions
are sometimes obscured and thus unavailable (e.g., Sachicasaurus vitae [
39
]; Meyerasaurus
victor [
40
]). Here I present a review of preliminary information relevant to plesiosaur
reconstruction. If previous studies have already reviewed some certain areas related to
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A
Cervicals
Pectorals
Dorsals
Sacrals
Caudals
Trunk
Tail
B C
Figure 1: A, skeletal reconstructions of Sachicasaurus vitae and Aristonectes quiriquinensis
(limbs removed), with different regions marked by colored lines. B, pectorals and the
first dorsal of Muraenosaurus leedsi, vectored from Andrews ([
41
]: Fig. 52) using Vector
Magic.C, photo of a cast of Rhomaleosaurus cramptoni (NHMUK PV R34) showing the
intervertebral cartilages in the neck region, provided by Frederick Dakota.
this theme, I do not repeat all the details but attribute to them and add extra information
if necessary.
2.1 Postcranial anatomy of plesiosaurs
Paul [
42
] emphasized that precise life reconstructions of vertebrates should be based
on rigorously created skeletons. Therefore, a comprehensive understanding of skeletal
elements and their patterns of three-dimensional arrangement is essential for plesiosaur
body reconstructions.
2.1.1 Vertebral column
Traditionally, five types of vertebrae were recognized in plesiosaurs: cervicals, pectorals,
dorsals, sacrals and caudals ([
41
,
43
,
38
,
34
]; Fig. 1A). Pectoral vertebra is a special
concept which is rarely used in other groups of vertebrates [
35
]. It was introduced by
Seeley [
44
] and refers to the type of vertebrae with ribs partly attached to the centra and
partly attached to the neural arches (see “pectoral shifts” in [
45
]; Fig. 1B). Sometimes
it is tricky to identify the actual vertebral formulas in osteologically mature plesiosaurs
because of fusion of neural arches to the centra [
38
]. Although widely used, the concept of
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pectoral vertebrae is, however, not accepted by all researchers (e.g., [
46
,
47
]). Some recent
studies opposed this traditional concept by combining pectorals into cervicals or dorsals
([
48
,
49
,
50
]; [
51
,
52
,
53
]). Opinions on whether this terminology should be retained have
not reached consensus [
54
], and inconsistent definitions have led to conflicting arguments
in previous studies (e.g., 76 vs 75 cervicals in Albertonectes vanderveldei [
46
,
55
]; 60 vs
62 cervicals in Hydrotherosaurus alexandre [
36
,
48
]). Despite the discrepancy in previous
studies, I retain the concept of pectoral vertebrae in this paper for unambiguous references.
In vertebrates, there exist cartilages separating adjacent vertebrae, and their forms vary
across different clades [
45
]. Intervertebral cartilages are also present in plesiosaur fossils
(Fig. 1C), but their type has rarely been discussed until recently. Synovial intervertebral
articulation is typical in extant reptiles like crocodiles and lepidosaurs [
56
]. Wintrich et
al [
57
] argued that conventionally hypothesized synovial joints would greatly reduce the
mobility of necks in plesiosaurs because they possessed platycoelous centra rather than
procoelous ones seen in crocodiles, so they proposed that plesiosaurs had intervertebral
discs analogous to those of mammals. In a later study, Wintrich et al [
58
] once again
suggested the presence of intervertebral discs in eosauropterygians (an extinct clade in-
cluding Plesiosauria [59]) and non-avian dinosaurs. On the other hand, some researchers
argued that central scars on articulated faces of plesiosaur vertebral corpora indicate
synovial joints (Eberhard Frey, pers. comm, 2022). Since this problem is still in debate, I
avoid designating specific type of vertebral joints but use the terminologies “intervertebral
cartilage” or “intervertebral distance” in this paper.
Intervertebral cartilages are important components of body length, and neglecting them
would lead to a significant underestimation of body size [
35
]. Some previous studies
measured intervertebral distances from articulated fossils, most of which focused on in-
tercervical cartilages. The sizes of intercervical cartilages vary greatly depending on
position, taxonomic status and how the neck was arranged during taphonomic processes
[
54
]. Andrews [
60
] described the intervertebral cartilages in the cervical and thoracic
regions of Leptocleidus superstes NHMUK PV R4828, which is about 5 mm each [
61
].
In Libonectes morgani SMU SMP 69120, the intercervical cartilage is 7 mm by average
[
62
]. The intercervical cartilages in Nichollssaura borealis range from 7 to 16 mm [
63
].
Some studies designated intercervical distances when handling disarticulated cervical series
(e.g., 1-3 mm in [
64
,
57
], 2 mm in [
65
,
66
]). A collection of intercervical distances in
different plesiosaur taxa is shown in Table 1. It is notable that intervertebral distances
increase with size of the animal (e.g., 11.54 mm by average in
∼
5 m Brachauchenius
lucasi [
35
] vs 18.25 mm in
∼
10 m Sachicasaurus vitae, [
39
]: Fig. 5). Hence Troelsen
[
54
] argued that it is better to use ratios rather than absolute lengths. In addition, ple-
siosaurs with fewer cervicals tend to possess proportionally larger intercervical distances,
which probably worked as compensations for reduction in cervical number to increase
neck mobility [
57
]. In the pectoral, dorsal and sacral regions, the ratios of interverte-
bral cartilages to adjacent vertebrae seem to be constant (
∼
10 %) in a broad range of
plesiosaur taxa (e.g., Brachachenius lucasi [
35
]; Rhomaleosaurus thorntoni, [
67
]: Plate
13-20; Elasmosaurus platyurus and Seeleyosaurus guilelmiimperatoris, [
55
]: Fig. 3). This is
possibly because the rib cages in plesiosaurs were quite stiff in life so that intervertebral car-
tilages did not undertake the task to adjust vertebral mobility as in the cervical region [
26
].
Many previous studies stated that tails of plesiosaurs are too short to be used for propul-
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Table 1: Collection of intercervical distances (either measured or assumed) from previous
studies.
Taxon Clade Description Ref
Leptocleidus superstes Leptocleididae
intercervical distance about 5
mm each;
[61]
Nichollssaura borealis Leptocleididae
intercervical distances ranging
from 7∼16 mm;
[54]
Libonectes morgani Elasmosauridae
neck length 5618 mm, including
421 mm intercervical cartilages;
[62]
Hydrotherosaurus alexandrae Elasmosauridae
about 1 cm between anterior
cervical spines, and posterior
cervicals almost touch;
[36]
Elasmosaurus platyurus Elasmosauridae
intervertebral distances in the
middle cervical region larger
than 1 cm;
[34]
Kawanectes lafquenianum Elasmosauridae
(assumed) intervertebral dis-
tance 2 mm each;
[66]
Aristonectine indet Elasmosauridae
(assumed) intervertebral dis-
tance 2 mm each;
[65]
Multiple elasmosaurids Elasmosauridae
(assumed) intercervical dis-
tance 1∼3 mm each;
[64]
Cryptoclidus eurymerus Cryproclididae
15%
∼
26% of anterior cervical
lengths;
[68]
Cryptoclidus eurymerus Cryproclididae
(assumed) intercervical dis-
tance 1∼3 mm each;
[57]
Muraenosaurus leedsi Cryproclididae
4%
∼
19% of anterior cervical
lengths;
[68]
Ophthalmothule cryostea Cryproclididae
intercervical distances 2.7
∼
5.8
mm (5.5%
∼
15.5% of cervical
lengths, 9.3% by average);
[54]
Multiple plesiosauroids Plesiosauroidea
(assumed) intercervical dis-
tance around 1 cm each;
[69]
Trinacromerum bentonianum Polycotylidae
intervertebral distances about
3∼6 mm;
[70]
Thalassiodracon hawkinsi Pliosauridae
intercervical distances 0.8
∼
3.4
mm (4%
∼
19% of cervical
lengths, 10.1% by average);
[54]
Brachauchenius lucasi Pliosauridae
intercervical distances 9
∼
18
mm (24.8% of cervical lengths
by average);
[35]
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sion and can only function as tools for direction control, stabilization or streamlining
[
71
,
1
,
72
,
73
,
47
]. However, this cognition may be biased due to the poor preservation
of tails in many plesiosaur fossils [
74
,
75
,
50
,
22
]. In some early Jurassic plesiosaurs,
the tails were almost equal in length with the trunks (e.g., Hauffiosaurus tomistomimus
[
53
], Atychodracon megacephalus [
67
], Thalassiodracon hawkinsi [
76
], Plesiosauroidea in-
det SMNS 51945 [
77
]; see the next section for definitions of trunk and tail). Such a
condition continued into late Jurassic and Cretaceous: thalassophoneans, cryptoclidids
and elasmosaurids also possessed long tails comparative in length with their trunks (e.g.,
Peloneustes philarchus [
43
], Cryptoclidus eurymerus [
38
], Albertonectes vanderveldei [
46
]).
But in polycotylids, the tails were much shortened, exemplified by Dolichorhynchops
osborni and Mauriciosaurus fernandezi [78,47].
Owen [
79
] studied the tails of some plesiosaur species from Liassic. He noticed that
the intercaudal distances in Plesiosaurs dolichodeirus are large, similar to the condition
in the spine of fishes. In Microcleidus homalospondylus, the tall caudal centra and cor-
responding spines are indicative to a large amount of muscles attached to the tail and
greater mobility than the neck [
79
]. Evidence of tail flexibility is also present in the fossils
of rhomaleosaurids, cryptoclidids and elasmosaurids, all of which possess large intercaudal
distances [
80
,
81
,
27
,
82
,
83
,
84
]. Due to the flexibility of tails in plesiosaurs, Sennikov [
85
]
proposed that they played important roles in propulsion, diving and predation. On the
other hand, the intercaudal cartilages are extremely narrow in Mauriciosaurus [
47
], which
indicate minor flexibility and may represent a synapomorphy of polycotylids (Eberhard
Frey, pers. comm, 2022).
Many plesiosaur fossils are dorsalventrally crushed, hence the curvatures of their ver-
tebral columns in life are unknown. Dating back to the 19th and 20th centuries, plesiosaurs
were often reconstructed as animals patrolling around the water surface with swan-like
or snake-like necks (Fig. 2AB). However, recent studies have demonstrated that such a
bending of the neck is impossible for plesiosaurs [
64
,
86
]. This posture never appears in
articulated fossils either. In some early reconstructions, plesiosaurs have arched dorsal
regions (“hump” in [
87
]), exemplified by the classic reconstructions of plesiosaurs from Ox-
ford Clay Formation by Andrews ([
41
,
43
]; Fig. 2D, Fig. 3A). The pliosaur reconstruction
by Newman and Tarlo also possesses a humped back ([
88
]; Fig. 3F). Arched backs can still
be seen in some modern reconstructions (e.g., Fig. 2F). On the other hand, the vertebral
columns in other reconstructions are straight or gently arched (e.g., Fig. 2CG, Fig. 3E).
To date whether plesiosaurs had humped backs or not is still in debate (Richard Forrest,
pers. comm, 2021; Leslie Noè, pers. comm, 2022). Robinson [
89
] argued that an arched
vertebral column, analogous to a bow, could function as a transmitter of propulsive forces
through the trunk. But this is not agreed by Smiths and Benson, who reviewed previous
reconstructions and listed some well-preserved fossils as counterexamples against a humped
back [
87
]. Richards [
90
] used wedging angles and face angles of plesiosaur dorsal vertebrae
to reconstruct the columns. In Richards’ reconstructions, the spines of Tatenectes and
Cryptoclidus are gently curved as proposed by Smiths and Benson [
87
], but Muraenosaurus
has a humped back, similar to the reconstruction produced by Andrews [
41
]. Therefore, it
is possible that spinal curvature may vary across plesiosaur taxa and there doesn’t exist a
uniform rule that the back must be humped or not.
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A
C
D
B
EF
G
H
I
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Figure 2 (previous page): Body reconstructions of long-necked plesiosaurs from different
times. A, 1840 painting The Sea-Dragons as They Lived, from [
91
]. B, 1959 painting of
an elasmosaur fighing against a mosasaur, from [
92
]. C,Plesiosaurus dolichodeirus (1824),
from [
93
]. D,Cryptoclidus eurymerus (1910), from [
41
]. E,Cryptoclidus eurymerus (1981),
from [
38
]. F,Brancasaurus brancai (2016), from [
94
]. G,Tatenectes laramiensis (2010),
from [
81
]. H,Microcleidus homalospondylus (2022), from [
95
]. I,Styxosaurus, from this
study. Figure C-H were vectored using Vector Magic.
2.1.2 Rib cage
The rib cages of plesiosaurs are immobile structures, which are stiffened by enlarged girdles
articulated by rows of gastralia ([
26
]; Fig. 4A). The rib cage consists of the dorsal basket
(vertebral column and ribs) and ventral basket (girdle elements and gastralia) [
89
]. The
small size of ilium and the dorsal blade of scapula suggests that the ventral basket was
loosely attached to the dorsal basket in life and could be removed as a whole structure
[89,99,100].
All dorsal ribs in plesiosaurs are single-headed, attached to the transverse processes
of the neural arches [
41
,
101
,
38
]. Therefore, the orientation of each rib can be determined
from the shape of corresponding transverse process [
102
,
90
]. Richards [
90
] proposed two
slant angles which can be measured directly from transverse processes. They are adopted
in this study (Fig. 4B). The angle between rib and vertical plane from side view is defined
as “angle
α
” here, and the angle between rib and vertical plane from dorsal view is defined
as “angle
β
”. The sizes of slant angles vary greatly across plesiosaur species. In Cryptoclidus
eurymerus, the maximum angle
α
is about 50
◦
[
90
] while in Dolichorhynchops herschelensis
it is only about 10
◦
([
103
]: Fig. 7). Judging from well-preserved plesiosaur fossils, the
sizes and slant angles of transverse processes shift gradually. The transverse processes
often increase in size till middle of the dorsal region and become shortened afterwards
[
41
,
38
]. Angle
α
also increases gradually from 0
◦
in anterior dorsals to maximum in
middle of the dorsal region, and keeps almost constant in the posterior half of dorsal series
but decreases to 0
◦
again rapidly when approaching sacral region (Fig. 4C). On the other
hand, angle βincreases monotonically from 0 ◦to maximum, and remains constant with
no recovery. Such an arrangement of rib orientation, which seems to be widespread among
plesiosaurs, was described by previous researchers or can be observed from well-preserved
fossils (e.g., Cryptoclidus eurymerus [
38
]; Muraenosaurus leedsi [
41
]; Tatenectes laramien-
sis [
102
]; Rhomaleosaurus thorntoni [
87
]; Attenborosaurus conybeari [
50
]; Fluvionectes
sloanae [
104
]; Colymbosaurus svalbardensis [
105
]; Avalonnectes arturi, pers. comm with
Frederick Dakota, 2023). The ribs also change in shape and size throughout the dorsal
region. They typically increase in length gradually in anterior half of the dorsal region,
then become shorter and straighter in posterior half (e.g., Cryptoclidus eurymerus [
38
];
Monquirasaurus boyacensis [
74
]; Attenborosaurus conybeari [
50
]; Dolichorhynchops osborni
[78]; Brancasaurus brancai [94]).
The ventral basket consists of pectoral girdle, pelvic girdle and gatralia connecting them.
The pectoral girdle contains interclavicle, clavicles, scapulae and coracoids, and the
pelvic girdle contains pubes, ischia and illia [
41
,
43
]. Fusion of interclavicle and clavi-
cles can be observed in many osteologically mature plesiosauroids and rhomaleosaurids
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AB
CD
E
FG
HI
J
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Figure 3 (previous page): Body reconstructions of short-necked plesiosaurs from different
times. A,Peloneustes philarchus (1913), from [
43
]. B, historical mount of Peloneustes
philarchus (GPIT/RE/3182, photographed in 1920), from [
96
]. C, mount of Liopleurodon
ferox, photo provided by Anna Krahl. D, restoration of Kronosaurus/Eiectus (1924), from
[
97
]. E, mount of Kronosaurus/Eiectus (1956), from [
37
]. F, “a typical giant pliosaur”
(1967), from [
88
]. G,Monquirasaurus boyacensis (1992), from [
98
]. H,Liopleurodon ferox
(2022), from [
95
]. I,Luskhan itilensis (2023), from [
75
]. J,Kronosaurus/Eiectus, from
this study. Figure A, F-H were vectored using Vector Magic.
[
38
,
106
,
40
,
107
,
104
], but they are not present in some elasmosaurids (e.g., “Mauisaurus
haasti” [
108
]; Wapuskanectes betsynichollsae [
52
]; Aristonectes quiriquinensis [
109
]). In
addition, interclavicles are generally abesent in the fossils of thalassophonean pliosaurs
[
110
]. An asymmetrical, triangular structure is present in Peloneustes philarchus and was
identified as an interclavicle [
43
,
111
]. However, it has been reinterpreted as a clavicle by
Ketchum and Benson [
112
], but the re-evaluation is uncertain in the current stage [
113
].
The coracoids, pubes and ischia of plesiosaurs are plates of bones. Two symmetric elements
of these two bones meet with each other in the sagittal plane, forming median symphyses
and intersection angles. Each of them forms a V-shape with corresponding symmetric
element [
43
]. The sizes of intersection angles vary across species. In the reconstruction of
Hydrotherosaurus alexandre by Welles [
36
], the intersection angles between coracoids and
pubes are around 150
◦
and 130
◦
respectively. Andrews [
43
] stated the angle between the
two coracoids of Peloneustes philarchus is about 90
◦
. Williston [
78
] reported a 125
◦
angle
between the pubes of Dolichorhynchops osborni. From side views, the positions where
girdle elements meet and articulate are V-shaped embayments for limb insertion (glenoid
in pectoral girdle and acetabulum in pelvic girdle) [41,43].
Each row of gastralia in plesiosaurs often contains three types of elements: a median
structure uniting bilaterally symmetric ossicles [
38
,
104
], while evidence for the existence
of medial elements is absent in some species (e.g., Aristonectes quiriquinensis [
109
]). The
number of rows of gastralia varies across species, ranging from 6 to 12 [
76
]. They were
united with each other and with the ribs by ligaments and cartilages in life [71,89].
2.1.3 Limbs
Plesiosaurs are secondarily aquatic tetrapods, possessing four hydrofoil-like flippers [
71
,
28
,
114
]. The flippers are significant components of body mass and surface area due
to their large sizes, thus precise limb reconstructions are essential for plesiosaur size
estimation [
4
]. Skeletal arrangement of plesiosaur limbs is homologous to those of other
tetrapods, each of which contains a propodial, epipodials, mesopodials, metapodials, and
phalanges [
115
,
116
,
117
]. They also display hyperphalangy, indicative of smooth bending
and additional twisting, which were hydrodynamically advantageous during swimming
[118,28].
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A
vertebral column rib
scapula coracoid gastralia pubis ischium
ilium
dorsal basket
ventral basket
B
abc
C
Figure 4: A, rib cage of Peloneustes philarchus.B, different views of plesiosaur dorsal
vertebra, showing two slant angles: a, right side view; b, front view; c, dorsal view. C,
rib cage of Cryptoclidus eurymerus, with blue arrows representing rib orientations in side
view, reproduced from [90].
2.2 Soft tissue reconstruction
2.2.1 Plesiosaur fossils with soft tissues
Some plesiosaur fossils are preserved with imprints of soft tissues. The earliest reports
of soft tissue outlines in plesiosaur fossils date back to the 19th century. Sollas [
50
]
noticed possible soft tissue preservation in the holotype of Attenborosaurus conybeari,
which was unfortunately destroyed in World War II [
77
]. Seeleyosaurus guilelmiimperatoris
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MB.R.1992 has soft tissue outlines around its tail and right forelimb ([
119
]; Fig. 5A). This
fossil is, however, currently covered by paint, which obscures the soft tissue profiles [
80
].
There were 2 plesiosaur fossils with soft tissue imprints described in the 20th century:
the holotype of Microleidus brachypterygius and a single flipper described by Watson
[
120
,
121
], but their authenticity was questioned by previous researchers [
115
,
117
]. In
general, all plesiosaur fossils with soft tissue preservation described in the 19th and 20th
centuries are currently unavailable or of doubtful reliability.
Vincent et al [
77
] described SMNS 51945, a long-necked plesiosaur fossil with soft tis-
sue preservation around its vertebral column and hindlimbs. The imprints around the
hindlimbs in SMNS 51945 suggest that soft tissues expand the flippers to a proportionally
greater extent than in those of Microleidus brachypterygius [
77
]. Although it partially
illuminates the body outlines of plesiosaurs in life, SMNS 51945 offers very limited reference
for quantitative soft tissue reconstruction of plesiosaur trunks. It should be noted that the
profile around the body ([
77
]: Fig. 1) is an artifact created during the preparation of this
fossil rather naturally preserved outline (Peggy Vincent, pers. comm, 2022).
Frey et al [
47
] described Mauriciosaurus fernandezi, a polycotylid plesiosaur with ex-
tensive soft tissues around its body (Fig. 5B). Although the thickness of soft tissues might
be slightly exaggerated during taphonomic processes, this specimen is the best reference
for plesiosaur soft tissue reconstruction to date. The soft tissue imprints indicate that the
body outline of M. fernandezi in life was streamlined, similar to that of a leatherback
turtle (Dermochelys coriacea) [
47
]. It is notable that the rib cage of M. fernandezi is
collapsed, with ribs detached from the vertebral column ([
47
]: Fig. 8). Therefore, a precise
reconstruction of the rib cage is required before quantifying the amount of soft tissues.
There are also soft tissues in the limb regions of this fossil, but outlines anterior to the
flippers are not preserved.
2.2.2 Head and neck
No study to my knowledge quantitatively discussed the amount of soft tissues in the
head regions of plesiosaurs. Several researchers described boss-like structures on ventral
symphyses of some elasmosaurid mandibles [
125
,
106
,
126
]. This kind of structure has
been interpreted as attaching site of geniohyoid muscle [
109
]. The morphology of ventral
boss in Aristonectes quiriquinensis indicates a loose mouth floor [
127
], but the soft tissue
outline of its skull is unknown. Due to the lack of direct evidence for cranial soft tissue
outline, modern aquatic tetrapods are needed as references. Aquatic mammals possess
thick fat tissue around their skulls. Odontocetes (toothed whales) possess melons, which
function as echolocation organs consisting of fat and connective tissue [
128
]. Mysticetes
(baleen whales) also possess thick soft tissues around their skulls and fatty structures for
sound transmition [
129
]. All pinnipeds possess well-developed cranialfacial soft tissues
and suction feeders have larger masseter muscles [
130
]. In modern aquatic or semiaquatic
reptiles like sea turtles and crocodiles, however, there is little fat tissue around their
skulls [
131
,
132
]. It has been proved that the limited amount of soft tissues around the
skull of leatherback turtle (Dermochelys coriacea) are sufficient to minimize heat loss [
133
].
The fossil of Mauriciosaurus fernandezi reveals that the necks of plesiosaurs were thicker
in life than traditionally reconstructed [
54
]. Many modern aquatic tetrapods possess thick
necks. The neck outlines of cetaceans and pinnipeds are smooth curves connecting their
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AB
CDE
Figure 5: Plesiosaur fossils with soft tissue preservation and thoracic CT scans of modern
marine mammals. A, illustration of the holotype of Seeleyosaurus guilelmiimperatoris,
from [
119
]. B, the holotype of Mauriciosaurus fernandezi, with yellow curves representing
preserved body outline, reproduced from [
47
]. C, the thoracic cross-section of a harp seal
(Pagophilus groenlandicus), from [
122
]. D, the thoracic cross-section of a harbor porpoise
(Phocoena phocoena), from [
123
]. E, the thoracic cross-section of a bottlenose dolphin
(Tursiops truncatus), from [124].
heads and rib cages. In leatherback turtle (Dermochelys coriacea), the cervical vertebrae
are also covered by thick fat [
133
]. It is likely that plesiosaurs possessed thick necks as well
to prevent heat loss in high latitude regions or deepwater environments [
134
,
135
]. Having
thick necks could also give plesiosaurs hydrodynamic advantages, which might compensate
for energy consumption in growing and nurturing them [
136
]. There exist large amounts
of muscles in the necks and temporal regions of crocodiles, enabling them to produce great
force during predation [
137
]. Some thalassophonean pliosaurs were also macrophagous,
thus they might possess well-developed muscles in their necks like crocodiles [
138
,
35
,
139
].
2.2.3 Rib cage
Previous studies seldom discussed the amount of soft tissues around the rib cages of
plesiosaurs, possibly due to the late discovery of Mauriciosaurus fernandezi. In most
published reconstructions, the body outlines are not provided or drawn close to the skele-
tons (e.g., Fig. 2C-H, Fig. 3FH). To my knowledge, only the recent study carried out by
Gutarra et al in 2022 [
4
] quantitatively restored the amount of soft tissues in the trunk
region according to M. fernandezi. In spite of the marvelous preservation of M. fernandezi,
many aspects of soft tissue arrangement in plesiosaurs remain unclear (e.g., the amount of
soft tissues in dorsalventral sides of the trunk). This leaves room for interference, which
can be based on plesiosaur physiology, muslce reconstructions and comparison with modern
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aquatic tetrapods.
Plesiosaurs were physiologically similar to modern aquatic mammals, and have been
termed as “long-necked dolphins” in recent studies (e.g., [
136
]). They were endothermic
animals, having metabolic rates within the range of avians [
32
]. Judging from oxygen
isotopes, the body temperatures of them ranged from 33
◦
C to 37
◦
C [
31
]. Evidence for
decompression syndrome is present in many plesiosaur fossils, hence some species might
adopt deep-diving habits [
140
]. Two plesiosaur fossils were found as evidence for viviparity:
Polycotylus latipinnis [
29
] and Abyssosaurus nataliae [
30
]. Given the presence of viviparity
in other eosauropterygians like nothosaurs and pachypleurosaurs [
141
], and the distinct
relationship between P. latipinnis and A. nataliae, it is possible that all plesiosaurs gave
birth to infants. Compared with maternal adults, the fetuses were proportionally large,
indicative of a k-selected lifestyle [
29
]. The fossil of M. fernandezi also demonstrates
that at least polycotylids, but possibly all plesiosaurs, possessed extensive soft tissues
around their rib cages [
47
]. Due to their similarity in physiology, the soft tissue anatomy
of modern endothermic aquatic tetrapods may shed light on plesiosaur reconstructions.
Modern marine mammals like cetaceans and pinnipeds possess thick blubber beneath the
skin, which play the roles of thermal barrier, energy storage, hydrodynamic streamlining
and buoyancy regulation [
142
,
143
,
144
,
145
]. The function of blubber as thermal barrier
is essential for them to sustain body temperature because of the high thermal conductivity
of water [
146
]. Plesiosaurs, on the other hand, seemed to prefer high latitude and cold
water environments. Many species are known to inhabit in or seasonally migrate to polar
regions [
134
,
147
,
148
,
149
,
150
]. Without fur or hair, plesiosaurs were also likely to
possess thick fat tissue to prevent heat loss. This is supported by the abundant soft
tissues in Mauriciosaurus fernandezi, which was, however, a species dwelling in warm
water environment [
151
,
47
]. Cetaceans from high latitude regions generally tend to
possess thicker blubber than those from warmer seas [
152
,
153
]. It is possible that ple-
siosaurs also follow this rule, but a quantitative study is not applicable in the current stage.
Besides a geographic view, musculoskeletal and kinematic properties of plesiosaurs should
also be taken into consideration. The rib cages of modern marine tetrapods are surrounded
by muscles and ligaments (known as “core” in relevant studies, see [
154
] for example), out-
side of which are fat (blubber) and skin. The thoracic cross-sections in aquatic mammals are
usually similar in shape to corresponding core profiles formed by muscles ([
122
,
124
,
123
];
Fig. 5CDE), thus muscle arrangement of plesiosaurs is also needed to reconstruct their
body shapes. The attempts for muscular reconstruction of plesiosaurs have lasted for a
century, most of which focused on the myological mechanism of their limbs and girdles.
The earliest study on this theme dates back to 1924, when Watson [
155
] reconstructed the
muscles attached to pectoral girdles and humeri of some long-necked plesiosaurs. Tarlo
studied the forelimb muscles of Pliosaurus cf. kevani CAMSM J. 35990, of which the
“scapula” turned out to be an ilium later [
156
,
157
]. Robinson [
71
] reconstructed the
muscular system of Cryptoclidus eurymerus and proposed that the locomotory strategy of
plesiosaurs was underwater flight, analogous to the method applied by modern penguins
and sea turtles. Carpenter et al [
158
] reconstructed the girdle musculature of Dolichorhyn-
chops, and Araújo and Correia [
159
] applied a phylogenetic bracket method to infer pectoral
musculature in plesiosaurs. The latest study is the new muscular reconstruction of C.
eurymerus girdles by Krahl and Witzel [
160
], in which a review and comparison of muscle
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A B
Figure 6: Illustration showing the main elevator (m. latissimus dorsi ) and main depressor
(m. pectoralis) of plesiosaur (Cryptpclidus eurymerus) forelimbs, reconstructed based on
conclusions from [
158
,
159
,
160
] and marked in red. A, front view of rib cage, with fat
tissue and skin colored in blue. B, side view of rib cage.
restoration criteria can be found. Figure 6shows the main elevator and depressor musles
of plesiosaur forlimbs, based on recent myological studies. Although the muscle restora-
tions from different studies are somewhat inconsistent, a conclusion can be drawn that
well-developed locomotory muscles were attached to the ventral sides of plesiosaur girdles
in life. Thus thick soft tissues beneath the rib cage should be added during reconstruction.
Currently there is a consensus that plesiosaurs applied a lift-based appendicular swim-
ming mode, namely underwater flight [
73
,
161
,
116
,
162
]. In spite of their similarity in
locomotory style, types of muscle arrangement in underwater fliers are distinct from each
other. Sea turtles (except leatherback turtle Dermochelys coriacea [
163
]) are ectothermic
animals, having low metabolic rates and slow cruising speeds [
164
,
165
]. Their major
forelimb muscles for locomotion, which gather at their pectoral girdles and plastrons
without attachment to the vertebrae, are distinct in arrangement to those of plesiosaurs
[
132
]. The presence of carapace and their preference for a mixed diet including vegetation
and benthic animals also suggest that they are not suitable references for soft tissue
reconstructions of plesiosaur rib cages [
166
,
167
]. Penguins have high metabolic rates and
are active predators [
168
,
169
]. The main elevator and depressor muscles of their wings
are, however, on the ventral side of the pectoral girdle, similar to those of flying avians
[
170
]. Such muscle arrangement is quite distinct from that in plesiosaurs, of which the
main elevators of limbs originate from dorsal side of their thoraxes ([
160
]; Fig. 6). Some
pinnipeds like otariids also adopt underwater flight [
171
,
172
]. The extension range of
their glenohumeral joints is limited, leading to asymmetric strokes during swimming: the
downstroke is propulsive, and the upstroke is a passive recovery [
173
,
174
]. This requires
the limbs to possess large range of motion anteroposteriorly, which can not be achieved by
plesiosaurs judging from their girdle anatomy [
158
]. Therefore the stroke style applied by
plesiosaurs, which predominantly consisted of dorsalventral motions, was different from
that applied by pinnipeds [115].
In cetaceans, epaxial swimming muscle m. longissimus dorsi runs along the postcra-
nial skeleton and surrounds the neural arches [
175
]. The main elevator of plesiosaur
forelimb, m. latissimus dorsi, originated from the vertebral column and ranged from the
1st to 12th dorsal vertebrae [
160
]. The tall neural arches and long transverse processes of
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plesiosaurs resemble those of cetaceans, suggesting that they also possessed well-developed
epaxial muscles for flipper elevation. Therefore, thick soft tissues should also be added to
dorsal side of the trunk when reconstructing a plesiosaur.
2.2.4 Tail
The tail of Mauriciosaurus fernandezi in life contained contour fat, which extended the
trunk outline to the tail without constriction, making a streamlined body [
47
]. It is
possible that counter fat also existed in the tails of other plesiosaurs for hydrodynamic
advantage, and caudal vertebral anatomy suggests larger amount of muscles and greater
tail mobility in clades other than polycotylids [
79
]. The last several caudals in many
plesiosaur fossils are fused to form pygostyle-like structures. It is phylogeneticly widespread
among plesiosaurs, and a summary of plesiosaur taxa with fused terminal caudals has been
carried out by Clark et al [
176
]. This structure has been intepreted to indicate a tail fin
[
85
]. Many researchers mentioned the possible presence of tail fins in plesiosaurs based on
caudal morphologies or soft tissue imprints (e.g., [
119
,
88
,
81
,
80
,
85
]). But whether their
tail fins were vertically or horizontally oriented is still in debate ([
27
,
80
]; [
127
,
85
]), and
their shapes are not known to date. In addition, it remains unclear whether the tail fins (if
existed) of different plesiosaur clades were homologous structures or evolved independently
for multiple times.
2.2.5 Flipper profile
Various methods and criteria have been used to reconstruct the flipper outlines of plesiosaurs.
Bakker [
177
] reconstructed flippers with outlines close to the bones, but this is not realistic
due to the presence of soft tissue imprints in fossils mentioned above. Despite the potential
unreliability of Seeleyosaurus guilelmiimperatoris and Microleidus brachypterygius, some
researchers still used them as references for flipper restoration in recent studies (e.g.,
[
178
,
158
,
73
]). DeBlois [
179
] reconstructed plesiosaur flippers based on hydrodynamic
properties. But the absence of soft tissues on leading edges of flippers is not realistic, as
argued by Muscutt [
115
], who quantitatively reconstructed plesiosaur flippers according to
the wing of a penguin. No fossil discovered so far sheds light on the amount of soft tissues
anterior to the leading edge of a plesiosaur flipper, thus the limbs of modern underwater
fliers may be demanded as references.
2.3 Inconsistent definitions and measuring criteria
There isn’t a uniform set of measuring standards in plesiosaur studies, thus measurements
of the same fossil may be distinct from each other in different studies. A uniform set
of definitions and measuring protocols is vital to precise estimation of body size. This
demand was proposed in other areas of paleobiology (e.g., sphenacodontids [
180
]) but was
rarely emphasized in plesiosaur studies. Therefore, a review of different measuring criteria
relevant to body reconstruction is provided below.
2.3.1 Skull
Multiple terminologies and standards have been introduced as proxies for skull size of
plesiosaurs. To stipulate the measuring criteria, I adopt the terminologies used by McHenry
[
35
], who first introduced measuring methods from biomechanics to plesiosaurs. In many
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DCL
BSL
SKL
ML
arc length
chord length
A
B
Figure 7: Illustrations showing the measuring criteria for skull and rib. A, different
measuring methods for the skull (Sachicasaurus vitae). B, arc length and chord length of
a dorsal rib. See text for detailed definitions.
cases, researchers ambiguously used the concepts “skull length” or “cranial length” without
explaining their measuring standards (e.g., [
181
,
182
,
40
,
183
,
53
,
126
,
184
]). Even when
clearly stated, “cranial length” or “skull length” in plesiosaurs might have various definitions
in different studies. Figure 7A shows the different measuring methods for plesiosaur skull
used in this paper.
Skull Length (SKL): SKL refers to anteroposterior length from tip of the snout to
midline between the quadrates. It was also termed as “cranial length” in some previous
studies (e.g., [185,74]). SKL is a good proxy for skull size since other measuring criteria
can be affected by taphonomy or anatomy (see below).
Basal Skull Length (BSL): BSL refers to length from tip of the snout to the basioccip-
ital condyle [
186
,
187
]. This criterion has a long history, dating back to the 19th century
[
188
]. In vertebrates, the occipital condyle connects with atlas, the first cervical [
45
].
Therefore BSL is a good proxy for skull size since the sum of it and length of vertebral
column is the total body length. BSL also has the advantage that it is less likely to be
affected by taphonomic processes because the condyle lies at the same horizontal plane
with the snout. However, it is not a good measuring method when comparing skull sizes of
different clades of plesiosaurs. The quadrates extend far behind the basioccipital condyle
in some taxa (e.g., Rhomaleosaurus cramptoni [
189
]; Aristonectes quiriquinensis [
127
]),
thus BSL would have unsatisfactory performances in representing their overall skull size.
Dorsal Cranial Length (DCL): DCL refers to length along the dorsal midline of the
skull. High sagittal crests are present in skulls of some plesiosaur species (e.g., [
190
,
191
]),
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thus DCL can be heavily affected by taphonomy (i.e., whether the skull is dorsalventrally
crushed).
Mandibular Length (ML): There are two methods to measure the ML. The first
principle is to measure from tip of the mandibular symphysis to midpoint of posterior
ends of retroarticular processes [
79
,
113
,
75
]. The other principle is to measure along the
ramus [
192
,
193
,
194
]. Here I avoid selecting a preferred principle, but advocate clarifying
the measuring criterion in future studies.
2.3.2 Spinal segmentation
Neck: As stated above, the concept of neck is an ambiguous definition in plesiosaur
studies due to inconsistent opinions on vertebral segmentation. The definition of neck
length varies across studies. It may refer to length of the cervical series [
38
,
36
,
40
], length
of cervical and pectoral series [
50
,
195
] or length of cervical series and some (but not all)
pectorals [
35
,
46
]. In this paper, neck length is defined as the sum of cervical lengths and
intervertebral distances anterior to them (i.e., the distance between basioccipital condyle
and atlas is included, but the distance between the last cervical and the first pectoral is
excluded).
Trunk: Some concepts were used as proxies for rib cage lengths of plesiosaurs, including
trunk, glenoid-acetabulum length and torso. Trunk often refers to the combined length
of pectorals, dorsals and sacrals in a horizontal line or along the column [
34
,
22
]. But in
some studies it means the distance between glenoid and acetabulum (e.g., [
4
]), identical to
the definition of glenoid-acetabulum length [
182
]. McHenry [
35
] observed the positions
of glenoids and acetabulums in thalassophonean pliosaurs and proposed the concept of
torso. It refers to combined length of the last pectoral, all dorsals and the first two sacrals.
However, the position of glenoid is not fixed in plesiosaurs due to the difference in relative
scapula size. In some articulated plesiosaur fossils, anterior margin of the scapula is in the
level of the first pectoral vertebra (e.g., Albertonectes vanderveldei [
46
]; Monquirasaurus
boyacensis [
74
]), but this is not a uniform rule for all plesiosaurs (e.g., Avalonnectes arturi
[
22
]). In this paper, the terminology trunk is used and is defined as the distance between
the anterior margin of scapula to acetabulum in a horizontal line (Fig. 1A).
Tail: Tail is also a concept with inconsistent definitions in previous studies. In some
cases it refers to the caudal series only (e.g., [
196
]), while in others it also includes sacrals
posterior to the acetabulum level (e.g., [
35
]). The second definition is adopted in this
paper (Fig. 1A).
2.3.3 Ribs
To my knowledge, the criteria for measuring plesiosaur ribs have rarely been discussed. Most
previous studies provided rib lengths directly without clear definition (e.g., [
37
,
197
,
150
]).
There are generally two types of measuring methods for rib length in anatomy: chord
length and arc length ([
198
,
199
]; Fig. 7B). Chord length refers to the distance between
two ends of the rib, and arc length is the measurement along the inner curve or outer
curve of the rib. In this paper, the arc length, defined as inner curve of the rib, is preferred
because it is less likely to be affected by taphonomic processes than chord length.
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2.4 Ontogeny
Plesiosaurs underwent ontogenetic shifts in anatomic features during growth. The earliest
ontogenetic study on plesiosaurs was carried out on Cryptoclidus eurymerus [
200
]. Some
osteological characters were selected as indicators of maturity in plesiosaurs. The most
widely used criterion is the fusion of neural arches to the centra, proposed by Brown
[
38
]. However, whether this criterion is suitable for all plesiosaurs has been questioned
and challenged in recent years, because lack of vertebral fusion has been observed in
some relatively large individuals from Pliosauridae, Leptoclididae and Rhomaleosauridae
[193,187,201,202].
Most ontogenetic studies on plesiosaurs focused on Cretaceous clades. For elasmosaurids,
the concentration is the ontogenetic changes in cervical morphology and ratio. O’Keefe and
Hiller [
203
] discovered complex ontogenetic allometry in elasmosaurids. Brum et al [
204
]
enhanced this conclusion by revealing a morphological shift in cervicals from disc-like to
can-shaped in all elasmosaur groups. For polycotylids, O’Keefe et al [
205
] and Byrd [
206
]
reported allometric growth in propodials and girdle elements respectively. In pliosaurids
(exemplified by Stenorhynchosaurus munozi), evidence of allometric growth is present in
their skulls, but little is known about the ontogenetic changes of their postcranial skeletons
[207].
Araújo et al [
208
] reported paedomorphism (i.e, histologically mature but osteologically im-
mature) in some aristonectines, and they suggested using the terminologies “osteologically
mature/immature” if an osteohistological analysis is absent. In a more recent study, Araújo
and Smiths [
202
] discovered that paedomorphism was widespread among plesiosaur groups,
especially in Rhomaleosauridae, Elasmosauridae, Pliosauridae and Polycotylidae. Given
the existence of paedomorphism, results from some previous studies might be misleading.
For example, UANL-FCT-R2 (“the monster of Aramberri”) was estimated to be a “15 m
juvenile pliosaur” [
209
]. Although not stated clearly, this result made an impression that
there existed adult pliosaurs much larger than the 15 m juvenile in Jurassic oceans. The
juvenile status of UANL-FCT-R2 was identified according to lack of fusion between neural
arches and centra [
209
]. However, Benson et al [
193
] argued that sutural fusion in cervicals
and dorsals might be delayed or absent in thalassophonean pliosaurs. In Monquirasaurus
boyacensis MJACM 1, the neural arches are fused to the centra in most vertebrae except
cervicals [
74
]. If thalassophoneans also followed a front-to-back fusion order like other
plesiosaurs [
83
], neural arches might never become fused with cervical centra in these large
pliosaurs. Another possibility is that they followed a different fusion pattern. Given these
potential conditions, UANL-FCT-R2 can not be confidently identified as a juvenile, but
more likely to be an adult due to its large size (see discussion for a re-evaluation of its
body size).
2.5 Phylogeny and Taxonomy
Phylogeny and taxonomy also play important roles in plesiosaur body reconstructions.
References to close relatives are often needed when restoring incomplete fossils, and this
process requires knowledge on interspecific relationships.
The phylogeny of plesiosaurs has been revealed to be unstable [
19
]. In early studies
Plesiosauria was divided into Pliosauroidea and Plesiosauroidea [
36
]. Pliosauroidea used
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to contain Pliosauridae, Rhomaleosauridae and Polycotylidae, and Plesiosauroidea used to
contain Plesiosauridae, Elasmosauridae and Cryptoclididae [
210
,
38
,
211
]. This taxonomic
system was affected by body proportions, which have been revealed to be volatile among
different clades (e.g., shortening of the neck evolved independently for multiple times
[
212
,
213
]). Benson and Drunkenmiller [
20
] constructed a large character matrix containing
270 morphological characters and 80 operational taxonomic units. Most contemporary
phylogenies are based on revised version of this matrix, and a consensus on relationships
among major plesiosaur clades has been established in recent years [
23
,
107
]. Pliosauridae
has been revealed to form a monophyletic clade with Plesiosauroidea, known as Neople-
siosauria [
22
]. Polycotylidae is a group within Plesiosauroidea [
213
]. On the other hand,
there exists discrepancy in topologies inside each clade among the phylogenies from recent
years, due to the incompleteness of fossil materials and the usage of incompatible matrices.
2.6 A review of plesiosaur body mass estimation
Body length and body mass are the most commonly used proxies for body size. McHenry
[
35
] argued that body length has the advantage that it is easier to be acquired and less
likely to fluctuate for an animal. Numerous studies tried to estimate body lengths of
different plesiosaur taxa. A splendid example was the body length estimates for multiple
elasmosaurid species provided by Welles, who offered very detailed measurements in his
publications [36,34,214].
The body plans of plesiosaurs vary greatly across different taxa. Thus body mass may be
a better proxy than length because it enables the comparison of species which are distinct
in body morphotypes [
35
]. However, body masses of plesiosaurs remain poorly studied to
date, with only a few relevant publications.
Henderson [
215
] reproduced three plesiosaur models according to museum mounts or
published reconstructions and calculated their body volumes using mathematical slicing [
9
].
The three models were then used to test the stability and floating properties of plesiosaurs.
Although the main theme of Henderson’s study was not plesiosaur size estimation, it is
the first thesis offering precisely calculated plesiosaur masses to my knowledge. However,
it can not be guaranteed that old reconstructions from the 20th century are completely
reliable. For example, the Liopleurodon ferox by Henderson was reproduced from the
pliosaur model created by Newman and Tarlo [
88
], which possesses barely any soft tissues
around its rib cage (Fig. 3F).
McHenry’s [
35
] methods of reconstruction and establishing comparative vertebral datasets
have important enlightenment and referential significance. McHenry calculated the body
lengths of different pliosaurid species precisely, then estimated their body masses using a
commercial model (BMNH model). However, it is unknown which fossil this model was
based upon, and interspecific variations were neglected during the body mass estimations.
Observations from Jurassic and Cretaceous pliosaurids suggest that body proportions were
diverse within Thalassophonea [
216
,
217
], thus the mass results provided by McHenry may
not be reliable.
Paul [
95
] created side view images of multiple plesiosaur skeletons and provided body
mass estimates for many species. The highlight of this study is that each plesiosaur taxon
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possesses an independent reconstruction instead of sharing a model with proportionally
similar species. However, there also exist some shortcomings. For instance, barely any soft
tissues were added to the rib cages, and all dorsal ribs in each model were oriented to the
same direction without gradual changes. Paul [
95
] stated that “approximations are often
inevitable” in determining body height, but the detailed criteria were not described.
Gutarra et al [
4
] reproduced several plesiosaurs from orthogonal photos of museum mounts
or articulated fossils, and subsequently tested their hydrodynamic performances. This is
the only study to date that quantitatively restored soft tissues according to Mauriciosaurus
fernandezi. However, museum mounts, especially those constructed more than 100 years
ago, can not be fully trusted. Some old mounts have been criticized by previous researchers.
For example, the Kronosaurus/Eiectus mount MCZ 1285 probably contains too many ver-
tebrae, as argued by McHenry [
35
]; a mount of Dolichorhynchops osborni (FHSM VP404)
was constructed with humeri and femurs reversed [
218
]. In addition, acquiring body widths
or heights from photos of articulated fossils may be problematic as well. The rib cages in
some plesiosaur fossils are collapsed (e.g., Monquirasaurus boyacensis [
74
]), thus measur-
ing body width from photos directly would probably lead to an overestimation of body size.
In general, all previous studies estimating body masses of plesiosaurs have their short-
comings, and the results may be unreliable. The key problem is that rigorously created
skeletons are lacking, hence a set of protocols for plesiosaur reconstruction is demaned to
solve this issue.
3 Reconstructing missing puzzles in plesiosaur fossils
Plesiosaur fossils are often incomplete or fragmented, lacking key structures for body
reconstruction. Therefore some methods and criteria are required to restore the missing
parts. There are two types of methods to restore the missing structures of plesiosaur
skeletons: comparison and regression. The principles and applying scopes of these two
methods are different.
The principle of comparison is to restore the missing parts using body ratios of close
relatives. The body plans of plesiosaurs vary greatly across clades, but congeneric species
normally have similar proportions and vertebral numbers (e.g., Rhomaleosaurus spp. [
76
];
Hauffiosaurus spp. [
53
]; Styxosaurus spp. [
219
,
34
]; but see the significant difference in cer-
vical number between Microcleidus melusinae and other Microcleidus species [
79
,
183
,
220
]).
Thus during the process of comparison, phylogeny and taxonomy should be taken into
consideration. Comparison has been frequently used in plesiosaur size estimations (e.g.,
[
221
,
222
,
149
,
223
]). However, subjectivity can never be ruled out from this method
because one can freely change the referred individual, and normally this leads to different
results.
Regression is to dig the correlations between different skeletal elements in plesiosaurs
using well preserved fossil materials. Previous researchers have developed some regression
equations for reconstructing skeletal elements of plesiosaurs. These equations are reviewed
and commented in this paper (see below). Some new relationships are also discovered and
presented (Fig. 8). A common question when using regression methods is that whether
the studied species follow these relationships [
16
]. For instance, a regression based solely
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lg(Cervical Number)
−lg(SKL/(SKL+Neck Length)
lg(Dorsal Vertebral Dimension)
lg(Max Rib Arc Length)
lg(Trunk Length)
lg(Tail Length)
lg(Propodial Measurements)
lg(Trunk Length)
Proxies
Femur Chord
Femur Length
Humerus Chord
Humerus Length
A
B C
D
Figure 8: Regressions for reconstructing missing body parts of plesiosaurs. All measure-
ments are in mm. A, Neck-SKL regression. B, Dorsal Vertebrae-Rib regression. C,
Trunk-Tail regression. D, Propodial-Trunk regressions.
on large pliosaurs may not have good performances in predicting the values of elasmosaurs
due to their great difference in body proportions. To ensure that the equations can be
applied to the whole Plesiosauria, all regressions in this study were summarized from
datasets which are phylogeneticly widespread, unless available samples are extremely rare.
3.1 Skull size
Knutsen et al [
187
] established two regression equations to estimate the BSL of pliosauroids
based on cervical width and condylar width respectively. These are the only regression
equations applied for skull size in plesiosaurs to date. However, there exist some prob-
lems in the datasets from current perspective. In condylar width-BSL regression, half
of the specimens are from Rhomaleosauridae, a clade revealed to be relatively distinct
from Pliosauridae in recent plesiosaur phylogenies (e.g., [
19
,
23
]). In cervical width-BSL
regression, three out of eight samples included in the dataset are juveniles. The growth
pattern of pliosaur vertebrae has not been studied, but it is possible that cervical ratios
of juveniles are different from those of adults, as in elasmosaurids [
203
,
204
]. In cervical
width-BSL regression, the juveniles gather at lower ends of the axes due to their small
sizes, and this may lead to biased estimates.
The width of basioccipital condyle serves as a proxy for anterior cervical width, so the two
regression equations both try to reveal the relationship between skull size and cervical
width (Espen Knutsen, pers. comm, 2022). However, it is uncertain whether cervical
width is a good proxy for skull size in plesiosaurs. There exist some conspecific plesiosaurs
in which the ratio of skull size to cervical width varies greatly (e.g., Muraenosaurus leedsi,
see [41]).
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There is an empirical consensus that long-necked plesiosaurs possess small skulls and short-
necked forms possess large skulls [
25
]. Inspired by this empirical conclusion, I established
a new regression equation based on SKL, neck length and cervical number. SKL was
selected because it is a better proxy than BSL when comparing skull size, as stated above.
To enlarge the dataset, individuals with a complete but disarticulated cervical series were
included, with their intercervical distances restored according to close relatives. The two
variables in this regression are:
x= log10 (Cervical Number)y=−log10 SKL
SKL +Neck Length
I used a log-logistic function to perform a nonlinear regression [
224
], which corroborates
the conjecture that there is a negative correlation between skull size and neck length in
plesiosaurs (Fig. 8A). The math equation behind this correlation is:
y=−1.1729
1 + x
1.573911.9871 + 1.3651 (1)
The dataset is phylogeneticly widespread and contains 40 species, ranging from Mon-
quirasaurus boyacensis (11 cervicals [
74
]) to Albertonectes vanderveldei (75 cervicals [
46
]).
Therefore this equation can be applied to the whole Plesiosauria.
3.2 Rib length
Ribs are key structures for rib cage reconstructions. However, rib length information is
often lacking because many plesiosaur fossils contain incomplete or distorted ribs. In
addition, the mounted status of some plesiosaur skeletons also precludes measurements.
For this reason, I developed a regression method to estimate maximum rib length using
measurements of dorsal vertebrae. The two variables in this regression are:
x= log10 (Average Dorsal Length ×Average Dorsal Width ×Average Dorsal Height)
y= log10 (Maximum Rib Arc Length)
and the equation behind their relationship is
y= 0.3520x+ 0.7177 (r2= 0.8853, p < 0.001) (2)
Only plesiosaur individuals with relatively complete dorsal rib series were included as
samples to ensure that all data were measured from the longest rib of each individual.
The requirement for completeness of the fossil restricted the dataset to 15 species, with
rhomaleosaurids and leptoclidids absent due to the lack of their vertebral measurements.
The regression shows that there is a positive correlation between combined dorsal vertebral
dimensions and maximum rib length (Fig. 8B).
3.3 Tail length
No regression equation has been established to estimate tail length to my knowledge. I
gathered samples from different plesiosaur clades and established a regression to estimate
tail length using trunk length (Fig. 8C):
y= 0.7912x+ 0.6513 (r2= 0.8217, p < 0.001) (3)
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where
x= log10 (Trunk Length)y= log10 (Tail Length)
The dataset contains only 15 samples due to the generally poor preservation of plesiosaur
tails. In addition, polycotylids are not included in this regression since regression diagnostics
revealed that they are outliers and would cause significant impact. Although it is consistent
with the conjecture above that short tail is a synapomorphy of polycotylids, this result was
summarized from limited amount of samples (Dolichorhynchops and Mauriciosaurus) and
might be biased by poor fossil sampling. It is possible that other polycotylids possessed
longer tails, but more complete fossils are required to illuminate this issue. Before that, I
recommend not to predict tail lengths of polycotylids using this equation.
3.4 Trunk length
O’Gorman et al [
149
] revealed the correlation between femur length and dorsal region length
in elasmosaurids based on eight species. This regression was restricted to elasmosaurids.
To develop equations that can be applied to the whole Plesiosauria, I collected trunk
length and propodial measurements from literature. Trunk length, instead of dorsal region
length, was selected because it enables the inclusion of fossils which show their ventral
sides. It is also a better proxy for body size than dorsal region length due to the variation
of pectoral and sacral numbers in plesiosaurs. Four variables (humerus length, humerus
chord, femur length and femur chord) were tested, and the results are:
log10 (Trunk) = 0.9954 log (Humerus Length) + 0.5776 r2= 0.8985, p < 0.001(4)
log10 (Trunk) = 1.0877 log (Humerus Chord) + 0.6964 r2= 0.8757, p < 0.001(5)
log10 (Trunk) = 0.9676 log (Femur Length) + 0.6424 r2= 0.8782, p < 0.001(6)
log10 (Trunk) = 1.0647 log (Femur Chord) + 0.7515 r2= 0.9629, p < 0.001(7)
Only articulated fossils with measurable trunks were included, and the dataset contains 20
samples which are phylogeneticly widespread. Regression results suggest that propodial
measurements are good indicators of trunk length (Fig. 8D), and the best proxy is femur
chord with r2over 0.96.
3.5 Limb length
Sanders [
117
] established several regression equations to estimate limb lengths of plesiosaurs
from chord propodial, chord epipodial and chord digits respectively. The dataset behind
these equations contains 45 plesiosaur species from a wide phylogenetic range, thus they
can be applied to the whole Plesiosauria. Since the prododial chords can be obtained
easily, the equation based on it is adopted in this paper:
ln (Chord Propodial)=0.9952 ln (Flipper Length)−1.6985 (r2= 0.8937) (8)
4 Criteria for plesiosaur reconstruction
4.1 Body length calculation
The first step to reconstruct a plesiosaur is to calculate its body length. Body length along
the vertebral column consists of basal skull length (BSL), sum of vertebral lengths, sum of
intervertebral distances and thickness of soft tissues.
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4.1.1 Length of vertebral column
The criteria for calculating spinal length are described first since estimating skull size
requires neck length and cervical number. If the plesiosaur fossil is articulated or broken
into several blocks, its spinal length can be acquired by summing the measurements along
the column. But the fragmented and disarticulated nature of many plesiosaur fossils
suggests that a set of methodology is needed for estimating their spinal lengths. When
dealing with relatively complete vertebral columns, of which only a few vertebrae are
missing or unmeasurable, lengths of those vertebrae can be restored by taking the average
values of adjacent ones. If many vertebrae are missing or badly preserved, a close relative
with a complete vertebral column is selected for reference. McHenry [
35
] emphasized
the importance of comparative datasets of vertebral lengths in plesiosaur reconstructions.
Order of vertebrae in the studied individual should correspond to that of the referential
individual. The assumption behind this criterion is that closely related species have similar
vertebral length distributions. For example, if the longest dorsal vertebrae are positioned
in middle of the trunk in one species, it is possible that its close relatives also follow this
pattern.
The intervertebral distances can be measured directly if the fossil is articulated. But
these cartilages are often not present in disarticulated columns. Instead of assuming
absolute values, I advocate estimating intervertebral distances with ratios from closely
related species which have articulated columns, following Troelsen [
54
]. As stated in the
preliminaries, the ratio of intervertebral distances to vertebral lengths in the pectoral,
dorsal and sacral regions seems to be uniformly around 10% in different plesiosaur clades,
hence only the estimation of intercervical distances requires comparison with relatives.
Before estimating the tail lengths of incomplete individuals, their trunk lengths need
to be acquired. Trunk length can be measured directly if the fossil contains gridles in
situ. If this condition can not be satisfied, a comparison method should be applied: the
ratio of length along the vertebral column to length in a horizontal line of the trunk
region in a referential model can be used to estimate the trunk length of the studied
species. Afterwards the tail length can be predicted using Trunk-Tail regression (Eq. 3) or
comparison method.
4.1.2 Skull length
Basal skull length (BSL) in plesiosaurs is not always available. In some plesiosaur fossils
exposed in dorsal view, their basioccipital condyles may be obscured by the overlying skull
rooves, thus the BSLs are unmeasurable. Comparison with relative species is required in
such cases to convert their SKLs to BSLs. The skulls of some fossils are fragmented or even
missing. The Neck-SKL regression (Eq. 1) is used to predict the SKLs of these individuals
if their neck lengths can be measured or estimated. Comparison with congeneric species
which possess neck and skull measurements is also applied to provide other possible results.
Then average value of the results is taken as the estimated SKL, which is finally converted
to BSL by comparison.
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4.2 Rib cage reconstruction
Trunk length of the studied plesiosaur is measured or estimated in body length calculation
stage, then curvature of the spine can be reconstructed by setting the scapulae at the level
of the first pectoral and acetabulum between the first two sacrals. To enable the calculation
of body volume and surface area, three thoracic cross-sections are reconstructed for each
individual: the glenoid cross-section (the vertical plane containing the glenoid; Fig. 9B),
the acetabulum cross-section (the vertical plane containing acetabulum; Fig. 9D) and the
maximum cross-section (which is set to be at the middle of the other two cross-sections;
Fig. 9C). Each cross-section is split into the dorsal part and ventral part. All the dorsal
parts of the three cross-sections include vertebrae and ribs. In the glenoid and acetabulum
cross-sections, the ventral part consists of the coracoids and pubes respectively, and the
ventral part of maximum cross-section is gastralia. The size and shape of each cross-section
are determined in this stage.
4.2.1 Glenoid cross-section
The two coracoids formed a V-shape in life [
41
], but the angle between them can not always
be confidently inferred from fossils. A mathematical method is applied here to determine
the width and height of the glenoid cross-section. In some previous reconstructions or
mounted plesiosaurs (e.g., Thalassomedon haningtoni DMNH 1588 and Hydrotherosaurus
alexandrae UCMP 33912 [
36
]), the dorsal part and ventral part at the glenoid level are
of the same width. This assumption is inherited in this paper to reconstruct the glenoid
cross-section. The first step is to obtain lengths of the ribs in the glenoid plane. Arc
lengths of these ribs can be calculated using the rib length distribution (i.e., the ratio
of arc length in glenoid cross-section to that of the longest rib) of closely related species
which have complete rib series. Lengths of transverse processes in glenoid cross-section
can be acquired in a similar manner if they are not preserved. The ribs are firstly placed
with angle
α
and
β
both set to 0 degree (Fig. 9B), for which the reason is clarified in the
preliminaries. This determines width of the glenoid cross-section and height of the dorsal
part. With width of coracoid at hinder angle of glenoid, height of the ventral part can be
calculated by the Pythagorean theorem.
4.2.2 Acetabulum cross-section
The pubes and ischia of plesiosaurs also formed V-shapes in life [
41
]. However, width
and height of the acetabulum cross-section can not be calculated using the same method
applied in the glenoid region. This is because the dorsal ribs of plesiosaurs shrink in size
and tend to become stragit in posterior half of the dorsal region (see preliminary section).
In this paper, it is assumed that width of the acetabulum cross-section is the same with
that of the glenoid cross-section. This assumption is corroborated by the in situ fossil of
Maurciosaurus fernandezi [
47
]. The lower end of the ventral part is set to be in the same
horizontal line with the glenoid. Using these criteria, width and height of the acetabulum
cross-section can be calculated (Fig. 9D).
4.2.3 Maximum cross-section
Dorsal part of the maximum cross-section can not be reconstructed with vertebrae and a
single pair of ribs because of the presence of slant angles. The method applied by Welles
to reconstruct Hydrotherosaurus alexandrae [
36
], which requires multiple pairs of dorsal
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ribs to obtain the cross-sectional profile, is adopted here. Arc length of the longest rib is
first estimated using regression (Eq. 2) if it is unavailable, and it is assumed that several
ribs in middle of the trunk are similar in length. Dorsal ribs are initially placed without
inclination, then heights and widths of them are multiplied by
cos α
and
cos β
repestively.
The maximum cross-section can be regared as part of a vertical plane that truncates
the rib cage, and the positions of dorsal ribs intersecting with the plane are determined.
Finally a smooth curve is used to link all the intersection points (Fig. 9C). Height of the
ventral part can be acquired with the following approach: average height of the ventral
parts of glenoid cross-section and acetabulum cross-section is taken, then the result is
scaled by multiplying the inverse of rib length distributional proportion which is used in
estimating rib lengths in glenoid level.
4.3 Flipper elements
Plesiosaur flippers can be completely preserved only in rare cases (e.g., [
225
,
219
]), thus
restoration is frequently required. If propodial of the studied flipper is preserved, total
length of the whole limb is first estimated using regression (Eq. 8) or comparison. Then
missing elements are restored according to the shapes and ratios of corresponding ones in
close relatives. If a pair of forelimbs or hindlimbs are entirely missing, flipper lengths are
restored using comparison with congeneric or kin species.
4.4 Soft tissue reconstruction
4.4.1 Skull and neck
The soft tissues added to the skull region are very thin, as suggested by the anatomy of
leatherback turtle (Dermochelys coriacea) [
133
], but enough soft tissues beneath the skull
are reconstructed to create a smooth curve connecting skull and neck so that there doesn’t
exist an abruptly humped chest. No soft tissues are reconstructed anterior to the head to
increase body length. This is unrealistic due to the presence of skin, but its thickness is
negligible comparing to the whole body and would not cause significant impact on overall
body size.
4.4.2 Rib cage
Soft tissues around the rib cage are reconstructed according to Maurciosaurus fernandezi,
and the detailed criteria are described below. The rib cage of M. fernandezi is collapsed
([
47
]: Fig. 8), thus the soft tissues were proportionally thicker in life than the fossilized
condition. For this reason, the rib cage of M. fernandezi was first reconstructed according
to the criteria described above. Muscles, fat and skin can not be differentiated from the
imprints around the fossil, hence soft tissues are only restored numerically. The fossil of M.
fernandezi demonstrates that plesiosaurs possessed thicker musles for locomotion in the
girdle regions than in middle of their trunks. The soft tissue outlines of the glenoid and
acetabulum cross-sections are 45% thicker than the rib cage, and this proportion decreases
to 30% in middle of the trunk. The fossil of M. fernandezi provides no evidence for the
amount of dorsalventral soft tissues, but in modern marine mammals the cross-sectional
outlines share the same shape with corresponding cores (Fig. 5CDE). Hence a smooth
curve is first drawn along the rib cage, leaving enough space for musles around neural
arches and giving the studied plesiosaur a rounded cross-section. Outline of the acetabulum
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A
BC D
Figure 9: Reconstruction of Pliosaurus cf. kevani (CAMSM J. 35990) under the criteria
proposed in this study. A, main body of the model, with limbs removed. B, glenoid
cross-section. C, maximum cross-section. D, acetabulum cross-section.
cross-section is constructed by scaling and stretching that of the glenoid cross-section.
Then the soft body profiles are obtained by magnifying the cores. Figure 9shows the
Pliosaurus model reconstructed under the criteria proposed above (see supplementary
materials for detailed methods).
4.4.3 Tail and flippers
The fossil of M. fernandezi shows a compressed tail, and trunk outline continues to it [
47
].
It is assumed here that there wasn’t abrupt undulation of the body outline between trunks
and tails in other plesiosaurs either. Lower and upper ends of the acetabulum cross-section
are linked to tip of the tail (5% extra length representing soft tissues added according to
Seeleyosaurus guilelmiimperatoris [
119
]) with smooth curves. Tail fins of plesiosaurs are
provisionally not reconstructed since their shapes and orientations are not certain in the
current stage, and neglecting the tail fin would not make a significant impact on total
mass and area. No fossil evidence so far sheds light on complete outlines of plesiosaur
flippers in life, hence I follow the method proposed by Muscutt [
115
] to reconstruct flipper
profiles according to penguins (see [115] for details).
4.5 Body density
The last step is assigning density to transform volume into mass. Animals are not solid
objects but possess cavities including lungs and digestive tracts, which should be taken into
consideration during mass estimation [
226
,
11
,
16
]. However, this task is tricky in studying
extinct animals since direct evidence is often lacking. Anatomy of modern animals may
shed some light on this issue, but it is not always certain that two different clades share the
same pattern. Henderson [
215
] assigned lungs occupying 10% the total body volume for
plesiosaurs, which is the upper end of relative lung size range in extant reptiles. Richards
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[
90
] proposed lungs occupying 9.8% of total body volume for the same reason. However,
it is not certain that whether plesiosaurs followed the typical reptile pattern. Another
tricky problem is that lung size varies across species within a clade. Deep-diving whales
are known to possess smaller lungs than their shallow-diving relatives [
227
]. It is likely
that lung size also varies among different plesiosaurs. In addition, lung sizes do not remain
the same in modern marine mammals since they collapse during diving [
228
,
229
,
230
].
Sea turtles also possess collapsible lungs [
231
,
232
,
233
]. Currently the diving mechanism
of plesiosaurs are unclear, so I avoid designating lung sizes but assume that they were
neurally buoyant (overall body density set to 1025 kg/m3).
5 Methods
5.1 Model construction
I made 22 plesiosaur models under the criteria proposed above (reconstruction details can
be found in supplementary materials). All models are two dimensional and are in lateral
view, with three cross-sections (glenoid, maximum and acetabulum) and limbs. Each
model is representative of a genus (except Pliosaurus cf. kevani and P. funkei, see below
for reason) since intrageneric differences in body plans can not be quatatively studied in
the current stage. The models are phylogeneticly widespread, consisting of genera from all
major plesiosaur clades. All models were made in AutoCAD 2020, which has high precision
and has been applied in reconstructing extinct animals in previous studies [
9
,
215
]. The
accuracy of each model was set to four digits after the decimal point during construction.
Skeletal measurements were cited from literature or acquired from high-resolution photos
provided by colleagues.
5.2 Body volume and surface area calculation
After the models were constructed, I used the cross-sectional method (CSM) to calculate
their volumes and surface areas [
234
]. CSM processes cross-sectional profiles directly
instead of assuming an elliptical or superelliptical approximation. It integrates areas
(or circumferences) of the cross-sections to mass (or surface area). I assume that body
cross-sections of plesiosaurs shifted gradually in life, which is also the principle behind
CSM. Each model was partitioned into 4 parts (“slabs” in cross-sectional method [
234
])
by the three cross-sections. Identidy segments, which are maximum heights of the three
cross-sections in this case, were drawn and measured. Then area and circumference of each
cross-section were obtained using measuregeom command in AutoCAD. Parameters
φ
and
ψ
required in cross-sectional method were calculated afterwards. The slabs at two ends of
the sagital axis were treated as cones with constant
φ
and
ψ
values. The other two in the
trunk region were treated as frustums with gradually changing cross-sections. Each slab
was sliced into 1000 subslabs using arrayrect and trim in AutoCAD, and dataextraction
was applied to export the identity segments into Excel, where the final calculation took
place. Volumes and lateral areas of the four slabs were added together to acquire the total
volume and surface area of the main body respectively. All limbs were treated as cones
with costant
φ
and
ψ
values. Assumed cross-section of limbs was reproduced from Muscutt
[115]. Their volumes and areas were calculated in the same way with the main body.
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hf
cpi
1
23
4
5
6
7
1
23
4
5
6
7
1
2
3
4
5
6
12
3
4
5
6
123
4
5
Figure 10: Landmarking schemes of the selected bones. Red points represent landmarks.
Semilandmark curves are marked in blue. Abbreviations: h, humerus; f, femur; c, coracoid;
p, pubis; i, ischium.
5.3 Morphometric analysis
To clarify whether there exist correlations between sizes of skeletal elements and body
volume, centroid sizes of some bones were obtained and tested. I collected images of
humeri, femurs, coracoids, pubes and ischia of the 22 model individuals from literature.
Scapulae were not included due to their complex three-dimensional structures, and they
are not preserved in some model individuals. If a skeletal element is not preserved or
incomplete, it was not included in the dataset. All images were scaled to correct size in
tps.dig2 using centimeters before landmarking. I used landmarks and semilandmarks to
quantify the geometry of the skeletal elements. Placement schemes of the constellations
are shown in figure 10, and the detailed explanation behind each landmark can be found in
supplementary materials. After the landmark files were constructed, they were imported
into R 4.1.3 [
235
] using geomorph package [
236
,
237
]. A Procrustes analysis was performed,
then centroid sizes were extracted for analyses.
Besides centroid sizes, some linear measurements were also collected, including lengths
and widths of the skeletal elements mentioned above. Measuring criteria of each element
can be found in supplementary materials. Trunk lengths and dorsal dimensions (defined
as average dorsal vertebral length
×
average dorsal vertebral width
×
dorsal vertebral
height) data were also gathered.
5.4 Regressions
I used linear regressions to generate equations for predicting the sizes of missing body parts
in plesiosaurs, except for Neck-SKL equation. Regression diagnostics were performed to
winnow and eliminate outliers. I avoided using confidence intervals or prediction intervals
since plesiosaur reconstructions allow only point estimations to enable volume and area
calculation. For Neck-SKL equation, a nonlinear least sqaure method was used, and I
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Table 2: Volumes and areas of the models
Model Catalogue Number Body Length Volume Surface area
Abyssosaurus MChEIO PM/1 6767 mm 1.3862 m312.7017 m2
Albertonectes TMP 2007.011.0001 12134 mm 4.3496 m326.0802 m2
Aristonectes SGO.PV.957 10000 mm 5.7628 m332.5928 m2
Cryptoclidus NHMUK PV R2860 3739 mm 0.4280 m35.4443 m2
Dolichorhynchops FHSM VP404 2958 mm 0.5250 m35.3066 m2
Hydrotherosaurus UCMP 33912 8835 mm 2.2297 m318.6510 m2
Kronosaurus/Eiectus MCZ 1285 10390 mm 13.0233 m346.2238 m2
Liopleurodon GPIT-RE-3184 5854 mm 2.9214 m317.3344 m2
Macroplata NHMUK PV R5488 4627 mm 0.8940 m38.0467 m2
Martinectes KUVP 40002 4585 mm 1.4144 m311.7607 m2
Mauriciosaurus INAH CPC RFG 2544 P.F.1. 2007 mm 0.0829 m31.6625 m2
Monquirasaurus MJACM 1 9108 mm 13.6487 m343.6918 m2
Peloneustes GPIT-RE-3182 4337 mm 1.4363 m310.7135 m2
Pliosaurus cf. kevani CAMSM J. 35990 9776 mm 12.5749 m342.5029 m2
Pliosaurus funkei PMO 214.135 9776 mm 12.7247 m346.7229 m2
Polycotylus YPM 1125 4676 mm 1.1222 m39.5767 m2
Rhomaleosaurus NHMUK PV R4853 6820 mm 3.4062 m321.6122 m2
Sachicasaurus MP111209-1 10233 mm 16.7108 m353.2342 m2
Stenorhynchosaurus VL17052004-1 5979 mm 2.7952 m316.9390 m2
Styxosaurus SDSM 451 11283 mm 4.0494 m326.9084 m2
Thalassomedon DMNH 1588 11978 mm 7.2257 m334.8474 m2
Vegasaurus MLP 93-I-5-1 6186 mm 1.2219 m311.0328 m2
selected a four-parameter log-logistic function to perform fitting [
224
], which is in the form
of
y=A1−A2
1 + x
θλ+A2
All data were collected from published literature, and the datasets were uploaded as
supplementary materials.
Linear regression was also applied to test which skeletal element is a good indicator
of body size and to generate scaling equations for quick body size estimation. I used
body volume rather than mass in order to leave room for altering body density if future
discoveries shed light on this issue. I used r
2
and p-values to evaluate the regression models.
All data were imported into R 4.1.3 [
235
] and
log10
transformed before analyses. Linear
regressions were performed using lm( ) function. Non-linear regression was performed
using drm( ) from drc package [
238
]. Function type was set to LL.4( ), which activates a
self-starting four-parameter log-logistic curve fitting. Results of regressions were plotted
using ggplot2 [239], ggpubr [240] and cowplot packcage [241].
6 Results
Body volumes and surface areas of the 22 plesiosaur models are listed in Table 2. Figure
11 shows the results of linear regressions tesing correlations between skeletal elements
and body volume. Coefficients, r
2
values and p-values of them are shown in Table 3.
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lg(Element Size)
lg(Volume)
Element
SKL*CN
Skull Width*CN
lg(Trunk Length)
lg(Volume)
Trunk
Trunk Length
lg(Dorsal Vertebral Dimensions)
lg(Volume)
Dorsal Vertebrae
Dimensions
lg(Humerus Size)
lg(Volume)
Humerus
Centroid Size
Chord
Length
lg(Coracoid Size)
lg(Volume)
Coracoid
Centroid Size
Length
Width
lg(Pubis Size)
lg(Volume)
Pubis
Centroid Size
Length
Width
lg(Ischium Size)
lg(Volume)
Ischium
Centroid Size
Length
Width
lg(Femur Size)
lg(Volume)
Femur
Centroid Size
Chord
Length
Figure 11: Linear regressions showing correlations between skeletal sizes and body volume.
CN is short for cervical number. Skull width represents maximum width of skull across
quadrates.
p-value smaller than 0.001 is considered significant in this study. Regression results
suggest that skull width multiplied by cervical number is not significantly related to body
size, other selected elements all show positive correlations with volume. Therefore, sizes
of these skeletal elements can be regarded as indicators for plesiosaur body size. The
best two proxies are trunk length and dorsal vertebral dimensions (defined as avergae
length
×
average width
×
average width), with r
2
values larger than 0.95. These scaling
equations offer tools for quick body size estimation and can generate predictions from very
fragemented materials. Centroid sizes of some skeletal elements were tested because they
evaluate the overall size better than linear measurements, but it turns out that they don’t
always have better performances in predicting body size.
7 Discussion
7.1 On some large Jurassic pliosaurs
Since the erection of genus Pliosaurus [
242
], much attention, from either paleontologists or
the public [
243
], has been paid to the body sizes of thalassophonean pliosaurs. They were
one of the first groups of giants discovered, and accompained the development of modern
paleontology [
35
]. With the burst of plesiosaur research during the past two decades, the
body sizes of large Cretaceous pliosaurs gradually come to light: from Monquirasaurus
boyacensis (14 t [
74
]), to Kronosaurus/Eiectus (QM F2454 weighed 15.5 t [
35
]), to Sachi-
casaurus vitae (17 t [
39
]). However, body sizes of some large Jurassic pliosaurs remain
mysterious due to the fragmented nature of their fossils. A review of these materials was
carried out by McHenry [
244
,
35
], but here I present a comprehensive review again since
the methods and criteria used by McHenry were quite different from those proposed in
this paper.
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Table 3: Coefficients of volume scaling equations. Trunk lengths before
log10
transformation
are measured in m, other linear measurements are in mm. Unit for body volume is m
3
. n
represents sample number and CN is short for cervical number.
Element n Slope Intercept r2p-value
Skull skull length×CN 15 2.1643 -8.8515 0.8847 <0.001
skull width×CN 11 0.3218 -0.7794 0.0419 0.5461
Trunk trunk length 22 2.7938 -0.4095 0.9701 <0.001
Dorsal Vertebrae dimensions 13 0.9959 -5.4570 0.9561 <0.001
Humerus centroid size 17 2.6135 -5.4451 0.7817 <0.001
length 21 2.6053 -6.5481 0.7569 <0.001
chord 21 3.4785 -7.9156 0.7091 <0.001
Coracoid centroid size 13 2.8724 -5.5306 0.7995 <0.001
length 14 2.5376 -6.7231 0.8065 <0.001
width 20 2.9160 -6.9805 0.8724 <0.001
Pubis centroid size 12 2.7708 -5.8606 0.8397 <0.001
length 15 2.3471 -5.8830 0.7760 <0.001
width 17 3.4380 -8.6042 0.9062 <0.001
Ischium centroid size 14 2.5175 -5.1651 0.7069 <0.001
length 17 2.2407 -5.5707 0.7669 <0.001
width 17 2.9425 -6.9815 0.8311 <0.001
Femur centroid size 15 2.4302 -5.0194 0.7089 <0.001
length 19 2.3287 -5.8636 0.7248 <0.001
chord 19 3.1413 -7.1047 0.8430 <0.001
It should be noted that estimating body size from fragmented fossils may be compli-
cated, and the results can be problematic. Some previous studies compared incomplete
fossils with different referential species and provided intervals to bracket the true sizes (e.g.,
[
35
,
223
]). However, some results produced this way may not be reliable given the flexible
body plans of plesiosaurs during their evolutionary history [
25
], thus restricting referential
models to congeneric species or coeval relatives may be a better option. For example, the re-
constructions of Jurassic pliosaurs are sometimes affected by species from Brachaucheninae,
a monophyletic Cretaceous clade consisting of pliosaurs with extremly short necks [
20
,
245
]
(except for Stenorhynchosaurus munozi [
246
]; Lorrainosaurus keileni from Bajocian was
also revealed to be a brachauchenine recently, but this result is questionable due to the
incompleteness of its holotype [
247
]). There is, however, no evidence that extremely short
necks have been evolved in Jurassic thalassophoneans, and observations from Liopleurodon,
Peloneustes and Pliosaurus fossils suggest that their necks are comparative to or just
slightly shorter than SKLs [111,43,248].
Before the review, the referential models used for comparison merit an introduction.
Fragmented materials were compared with either Liopleurodon model or Pliosaurus model
established in this study to estimate their body sizes, depending on their stratigraphic
horizons. The Liopleurodon model was based on GPIT/RE/3184, an old mount that was
described by Linder [
111
]. The vertebral count in the current mount exceed the number
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2 m
A
B
Figure 12: Body reconstructions of Liopleurodon ferox and Pliosaurus funkei.A,L. ferox
GPIT/RE/3184. B,P. funkei PMO 214.135. Limbs are set in a vertical plane for display.
The skull of P. funkei is reproduced from that of P. kevani since it is not preserved. Its
morphology may not be reliable, but this would not cause significant influence on body
size estimation.
recorded by Linder (pers. obs; Fig. 3C) and approaches the vertebral formula of NHMUK
PV R3536 [
43
]. The caudal series is incompletely preserved (only 13 caudals were reported
by Linder), and tail of the mount was restored, being too short if Liopleurodon had a
trunk-tail proportion similar to that of Peloneustes [
43
]. Hence the tail length in the
model was restored using Trunk-Tail regression (Eq. 3), and the spinal curvature was
rearranged (Fig. 12A).
The Pliosaurus model is a mixed model, based on P. cf. kevani CAMSM J. 35990
and P. funkei PMO 214.135 [
110
,
187
]. As stated above, cervical width may not be a good
proxy for skull size, and vertebral lengths in corresponding regions indicate that these two
individuals were comparative in body size. This is further supported by their similarity in
coracoid size ([
110
]: Fig. 4; [
187
]: Fig. 9). From this perspective, the other individual of
P. funkei, PMO 214.136, was also similar in size to the model. In addition, estimated SKL
of the model is almost identical to that of P. kevani DORCM G. 13675 [
193
]. Therefore,
all these four individuals were around 9.8 m and weighed over 12 t (Fig. 12B), behind
which the core assumption is that different Pliosaurus species possessed very similar body
plans.
Large pliosaurs that might reach or exceed 10 m in length first appeared in Callovian.
Gilbert [
249
] mentioned a giant individual from Oxford Clay Formation that posesses “a
right-hand paddle 7 feet 6 inches in length”. Newman and Tarlo [
88
] also reported briefly
that the “distance across the paddles is 21 feet” and estimated a 36 feet (nearly 11 m) body
length for this individual. Comparison of its hindlimb with that of Liopleurodon ferox
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suggests a 9.4 m body length. But I would agree with McHenry [
35
] that it might fall
within the range of 9
∼
11 m if taking limbspan into consideration. This individual, known
as “the Stewartby pliosaur”, is currently housed in NHMUK, with catalogue number R8322.
A more detailed body size estimation is not applicable in the current stage, pending the
reassessment of this fossil. Its existence demonstrates that giant pliosaurs already occurred
in Callovian oceans. Another fossil, which is a large mandible fragment known as the
“NHM symphysis” in [
35
], further corroborates this conclusion. McHenry [
35
] mentioned
an isolated vertebra from Peterborough (PETMG R272). Size of its owner was once
thought as 15
∼
18 m, and later decreased to 11.6
∼
14.2 m. Although McHenry mentioned
the possibility that it is in fact from a sauropod, its dinosaur identity was not agreed by
everyone at that time. It is worth mentioning that this fossil has been restudied recently
and revealed to be a sauropod indeed [250].
Megalneusaurus rex from Oxfordian of Wyoming was regarded as a very large pliosaur
[35]. The incomplete propodial was first identified as a 1.2 m femur [251] but turned out
to be a 0.99 m humerus later [
252
]. McHenry [
35
] proposed 11
∼
12 m body length for this
individual. However, if the propodial is indeed a humerus, its restored length is actually
shorter than that of Pliosaurus funkei. The cervical and dorsal vertebrae, which might
be lost in the last century [
253
], also indicate a similar size with the Pliosaurus model [
251
].
Fragmented fossils suggest that 9
∼
11 m pliosaurs were common in late Jurassic oceans of
Europe. DORCM G. 123, an almost complete hindlimb, measures about 2 m in length
and is comparative to the hindlimb of CAMSM J. 35990 [
254
]. Philips [
255
] listed some
large anterior cervicals and referred them to Pliosaurus macromerus ([
255
]: p. 354, d,
e, f; note that a, b, c have been provisionally recalassified to P. brachydeirus by Tarlo
[
110
]). These three cervicals are by average 8% larger than corresponding vertebrae of
the Pliosaurus model, suggesting a 10.5 m individual. The posterior cervical figured by
Philips ([
255
]: Fig. 149) is comparative in length with the largest cervical of CAMSM
J. 35990, thus its owner might be 9.8
∼
10.3 m in life. Sauvage [
256
] reported a complete
pliosaur mandible that slight exceeds 2 m in length, which may come from an individual
similar to or slightly smaller than the Pliosaurus model. Some large pliosaur materials
discovered in the 18th and early 19th centuries were confused with the fossils of sauropods
or other animals. For example, a posterior cervical (85 mm in length), which is slightly
larger than corresponding vertebrae in the Pliosaurus model, was once misidentified as
a sauropod and named as “Cetiosaurus rigauxi ” [
257
]. Later Sauvage reclassified it to
Pliosaurus [
258
]. Some French pliosaur materials bore the name “Tapinosaurus” in early
literature [
259
,
260
], and they were referred to Pliosaurus in a recent study [
198
]. Most of
these large individuals were within the range of 9
∼
11 m ([
259
]: Plate 9, Fig. 2-3, Fig. 11).
Large pliosaurs also inhabited in the Jurassic oceans of South America. As stated in the
preliminaries, body size of “the monster of Aramberri” was exaggerated. Its vertebral dimen-
sions indicate that it was similar in size to the Pliosaurus model. Another pliosaurid from
Caja formation, UANL-FCT-R7, comprises four and a half posterior cervicals, and their
average length is approximately 90 mm [
261
]. Their maximum height and width are 145
mm and 140 mm respectively, indicating that they are suprisingly thin for thalassophonean
cervicals (widths are typically twice the lengths of cervicals in late Jurassic pliosaurs [
101
]).
To my knowledge, the only other example of such kind is the last two cervicals of the
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holotype of Brachauchenius lucasi USNM 4989 [
35
]. In addition, the high positions of
rib facets on the centra, together with the contact between centra and coracoid, make
the “cervical” identification questionable ([
261
]: Fig. 9). If some of the four vertebrae are
actually pectorals, then this individual might not be significantly larger than the Pliosaurus
model due to the 91 mm pectoral in CAMSM J. 35990 [
110
]. Two large Tithonian pliosaurs
(MOZ 6144P and MOZ 6141P) from Neuquén Basin, Argentina were briefly mentioned
and classified to Liopleurodon sp. in a 1999 thesis [
262
]. MOZ 6144P had a 2.1 m skull,
with a 4.6 m complex of cervical and partial dorsal series. The incomplete mandible of
MOZ 6141P was 1.7 m along the ramus. Unfortunately, these two fossils were broken
and lost before being studied (Zulma Gasparini, pers. comm, 2021). A precise estimation
of body size is not applicable since the measuring criteria are unknown and no photos
were provided in the original paper, but they were also likely to fall in the range of 9
∼
11 m.
There exist fossils indicating that some Kimmeridgian pliosaurs from Europe might
far exceed the Pliosaurus model. OUMNH PAL-J.010454, a famous mandible that was
classified to Stretosaurus,Liopleurodon and Pliosaurus before, was restored as 2875 mm
in length [
110
]. Length of the imperfect mandible before restoration was 7 feet (about
2134 mm), as briefly mentioned by Prestwich [
263
]. There exists a breakage behind the
dentary on each ramus of the mandible, and length of mandible anterior to breakage
matches the value provided by Prestwich. Tarlo [
110
] argued that “...the posterior part of
the left ramus has come to light... the total length would have been more than 3000 mm”.
There are indeed two associated lower jaw fragments from a single individual (OUMNH
PAL-J.050376 and OUMNH PAL-J.050377) discovered in the same pit with OUMNH
PAL-J.010454 and they match in size [
264
]. It is not certain which specimen Tarlo [
110
]
referred to, but if it was OUMNH PAL-J.050376, total length of the mandible should be
around 2.6 m (Shinya Noguchi, pers. comm, 2023). This suggests an individual which
was 11.8 m,
>
20 t in size based on the Pliosaurus model. Prestwich [
263
] mentioned
another large pliosaurid skull in Dorset Museum measuring 7.5 feet (about 2286 mm)
long. This fossil, however, can no longer be traced (Shinya Noguchi, pers. comm, 2023).
OUMNH PAL-J.010454 is not the only example indicating that Jurassic pliosaurs might
reach or exceed 20 t. Martill et al [
223
] described four large posterior cervicals (ABGCH
1980.191.1038
∼
1041) from Abingdon, Britain. Comparison with the Pliosaurus model
suggests this individual was 10.7
∼
11.8 m in body length. Another individual from Ely
(YOYRM: 2006.19), described in the same paper, was 11.7
∼
13 m if the same method is
applied. An isolated dorsal rib from France, housed in Museum of Le Havre, is 122 cm
in chord length [
198
], suggesting a similar body size to the three individuals mentioned
above.
7.2 A brief summary of plesiosaur body size evolution
A comprehensive analysis of plesiosaur body size evolution trends through deep time is
beyond the scope of this study. Here I just present a brief description of body size evolution
of different clades. Body sizes of all species other than the 22 models were estimated using
Trunk-Volume or Dorsal Vertebrae-Volume regressions unless otherwise specified.
7.2.1 Rhomaleosauridae
In the early stage of their evolution, some rhomaleosaurids, represented by Macroplata
tenuiceps (4.6 m, 0.92 t [
195
]) and Atychodracon megacephalus (5 m, 1 t [
67
]), were larger
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than other Hettangian plesiosaurs. Smaller species like Lindwurmia thiuda (190 kg [
265
])
and Avalonnectes arturi (170 kg [
22
]) were also present. Sinemurian rhomaleosaurids like
Thaumatodracon wiedenrothi [
194
] and Archaeonectrus rostratus [
79
] were generally smaller
than their largest Hettangian relatives. The largest genus, Rhomaleosaurus, appeared
in Toarcian [
76
]. Both R. cramptoni and R. thorntoni were around 6.8 m and weighed
over 3 t [
87
]. There also exist fragmented materials suggesting that some rhomaleosaurids
might reach 8 m [
266
]. After the faunal turnover between the early and middle Jurassic,
the diversity and number of rhomaleosaurids droped drastically [
267
]. Only two valid
rhomaleosaurid species are currently known from middle Jurassic: Maresaurus coccai
[
268
] and Borealonectes russelli [
148
]. Despite their decline in number, some middle
Jurassic rhomaleosaurids still retained large size. Assuming a same body proportion with
Rhomaleosaurus,M. coccai and “Trematospondylus macrocephalus” might reach similar
sizes with R. cramptoni and R. thorntoni ([
268
]: Fig. 1; [
269
]). B. russelli, on the other
hand, was much smaller with an estimated body length of 3 m [
148
]. The youngest
rhomaleosaurid fossils discovered so far are from Oxford Clay Formation, Callovian [
270
].
Their fossils, although fragmented, suggest large body size.
7.2.2 Pliosauridae
Early pliosaurids were small-headed and long-necked plesiosaurs. Thalassiodracon hawkinsi,
an 1.5
∼
2 m long Hettangian species, was one of the smallest plesiosaurs [
33
]. Most
pliosaurids from early Jurassic did not exceed 1 t in body mass, exemplified by Atten-
borosaurus conybeari (800 kg [
50
]), Hauffiosaurus zanoni (350 kg [
271
]) and H. tomis-
tomimus (920 kg [
53
]). H. longirostris (2.9 t), on the other hand, was larger than the
other two congeneric species [
272
,
22
]. Shortly after the early-middle Jurassic transi-
tion, relatively large pliosaurids (possibly thalassophoneans) appeared in Bajocian oceans
[
273
,
274
,
247
], probably to fill the apex predator niche left by rhomaleosaurids and
Temnodontosaurus [
267
]. These macrophagous pliosaurids coexisted with relic rhomale-
osaurids, which were similar to them in body size, till the extinction of Rhomaleosauridae
in the end of middle Jurassic, but they probably employed different feeding strategies
[275,276,138,247]
It was in the Callovian that gigantic thalassophoneans (e.g., the Stewartby pliosaur,
NHMUK PV R8322 [
88
]) first appeared. Body size diversity of pliosaurids also reached a
peak in this period. Smaller species like Liopleurodon ferox (largest individual NHMUK
PV R3536 around 8 m, 7.8 t [
277
]) and Peloneustes philarchus (holotype NHMUK PV
R3318 about 650 kg [
43
]) were also present. The 9
∼
10 m range was frequently reached by
late Jurassic pliosaurids (e.g., Megalneusaurus rex from Oxfordian [
251
,
134
]; Pliosaurus
kevani from Kimmeridgian [
193
]; P. funkei and P. rossicus from Tithonian [
187
,
278
]).
As discussed in the last section, some Jurassic pliosaurs might reach or exceed 20 t in life,
accompanied by relatively small species like P. brachyspondylus (1.65 t [248]).
It has been revealed that the Jurassic-Cretaceous transition severely affected the evolution
of Pliosauridae, and only one lineage, Brachaucheninae, crossed the boundary [
20
]. Little
is known about pliosaurids from the beginning of Cretaceous due to poor fossil sampling.
So far only two modest-sized brachauchenine species from Hauterivian have been erected:
Makhaira rossica (1.05 t [
279
]) and Luskhan itilensis (2.87 t [
216
,
75
]). Recent studies have
revealed the abundant pliosaurid fossil records in the Southern hemisphere, especially South
America [
280
,
74
,
246
] and Australia [
35
,
281
,
282
]. South American pliosaurids from Paja
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Formation were diverse in both size and morphology: from short-snouted Acostasaurus
pavachoquensis (4 m [
283
]), to piscivorous Stenorhynchosaurus munozi (8.65 m, 8.5 t
[
207
]), to giant Monquirasaurus boyacensis (14 t [
98
,
74
]) and Sachicasaurus vitae (17 t
[
39
]). Opinions on the taxonomic status of a bunch of pliosaurids from Toolebuc Formation
or Doncaster Formation, previously referred to Kronosaurus or Eiectus, haven’t reached
consensus (Espen Knutsen, pers. comm, 2022; Leslie Noè, pers. comm, 2022; and see [
282
]
for a review of this issue). Despite the conflicting taxonomy, these brachauchenines were un-
doubtfully among the largest Cretaceous pliosaurs [
35
], with a maximum 15.5 t body mass.
Pliosaurid fossils from Cenomanian are rare, most of which were referred to the “waste-
bascket” genus Polyptychodon [
284
]. The classification of Polyptychodon was generally
based on tooth morphologies and its research history is chaotic [
214
]. As argued by
McHenry [
35
], estimating body size from plesiosaur teeth is difficult and the results may
be unreliable since tooth size varies across species and their positions in the jaws. However,
a large tooth with 95 mm crown height (CAMSM B 75754) implies that giant pliosaurs
existed in this period [
284
]. Some vertebrae described by Owen ([
285
]: p. 22-23) and a
giant isolated cervical from Russia [
286
] also suggest that large pliosaurs around 10 m
were present in Cenomanian oceans. They probably went to extinction together with the
last ichthyosaurs [
286
]. Generally there is a consensus that pliosaurids went to extinction
in middle Turonian [287]. Although some younger teeth were classified to Polyptychodon
[
284
], the posibility that some “Polyptychodon” materials are actually polycotylids can
not be ruled out [
288
]. Turonian pliosaurs were relatively modest in size. The holotype of
Brachauchenius lucasi (USNM 4989) from Western Interior was 5.35 m, 2.2 t with SKL
around 1 m [
35
,
289
]. But some other materials referred to this genus indicate larger size.
Both MNA V9433 from the Western Interior [
290
] (often referred to B. cf. lucasi; e.g.,
[
23
]) and an individual from Morocco [
291
] have cranial materials suggesting 1.5 m skull
size, similar to the holotype of Megacephalosaurus eulerti. The paratype of M. eulerti,
USNM 50136, contains partail skull elements from a larger individual, of which the DCL
might be around 1.75 m in life [
292
]. It is the largest Turonian pliosaur discovered so far.
Using the body proportion of coeval B. lucasi, USNM 50136 was around 9 m and 9.2 t.
Therefore, some Turonian pliosaurids were still large in absolute size, although they were
dwarfed by some early relatives like Sachicasaurus vitae [39].
7.2.3 Microcleididae and basal Plesiosauroidea
Basal pleiosauroids were present in the beginning of Jurassic, represented by Eoplesiosaurus
antiquior, a 377 kg and long-necked species [
22
]. Small body size was retained in Ple-
siosaurus dolichodeirus from Sinemurian. NHMUK PV R1313 was around 3.5 m and 300
kg [
293
], and a humerus referred to P. cf. dolichodeirus by Dames indicates similar or
slightly lagrer body size ([
119
]: p.8-10). Westphaliasaurus simonsensii (holotype about 4.5
m, 670 kg [
294
]) and Eretmosaurus rugosus (neotype NHMUK PV OR 14435 about 460 kg
[
79
,
22
]) were relatively larger. Most known microcleidids were from Toarcian, represented
by Microcleidus spp. and Seeleyosaurus guilelmiimperatoris [
22
]. S. guilelmiimperatoris,
M. brachypterygius,M. melusinae and M. tournemirensis were small species, not exceeding
350 kg in body mass [
183
]. The type species M. homalospondylus was much larger, with
a maximum body length around 5 m [
79
]. In general, these basal plesiosauroids and
microcleidids from early Jurassic presented a low size disparity pattern, as discovred by
Benson et al [22].
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7.2.4 Cryptoclididae
Cryptoclidids occurred in Bajocian, shortly after the early-middle Jurassic transition [
267
].
The earliest known cryptoclidid material is a propodial (MNHNL BM782), which is smaller
than those of Muraenosaurus and Cryptoclidus ([
267
]: Fig. 10). Regressions based on
propodial lengths or chords suggest that it was from a 350∼630 kg individual.
Abundant cryptoclidids occurred in Callovian, reaching high disparity in both morphology
and body size [
38
]. From small Tricleidus seeleyi (266 kg) and Muraenosaurus beloclis
(adult about 2.5 m in length), to medium-sized Cryptoclidus eurymerus (neotype NHMUK
PV R2860 about 3.7 m and 440 kg; there exist materials 20% to 30% larger, which indicate
>
900 kg body mass [
41
]), to large Muraenosaurus leedsi (largest individual NHMUK PV
R2425 up to 1.47 t [41]).
The flourish of this clade continued into late Jurassic. Two valid cryptoclidid species,
Pantosaurus striatus and Tatenectes laramiensis, are known from Sundance Formation
of Oxfordian [
295
,
296
,
297
]. P. striatus was a medium-sized species, with adult mass
around 730 kg [
81
]. T. laramiensis was much smaller, with adult body mass just about
372 kg ([
102
]: Fig. 8). Some cryptoclidids travelled through the Carribean Seaway and
reached South America during this period (e.g., Muraenosaurus sp. that weighed 985 kg
[
298
]). Cryptoclidid fossils from Kimmeridgian Clay Formation are rather fragmented
and incomplete, hiding the disparity of this clade [
196
]. The holotype of Kimmerosaurus
langhami contains only a skull and several cervicals [
38
], thus it is difficult to estimate
its size. Colymbosaurus was very large comparing with cryptoclidids from Oxford Clay
Formation. For example, the syntypes of C. megadeirus (CAMSM J.29596etc) had a
1.7 m trunk indicative of 1.76 t body mass [
196
], and its body length was estimated as
5 m by Brown [
38
]. NHMUK PV OR 31787, the holotype of “C. trochanterius”, might
be 6.6 m in body length [
38
]. Thus the largest Colymbosaurus from Kimmeridgian Clay
Formation might reach 4 t in mass. “Plesiosaurus” manselii (NHMUK PV OR 40106),
estimated by Brown [
38
] to be 6.16 m in length, was within the size range of C. megadeirus.
Many cryptoclidid materials have been discovered in Slottsmøya Member of the Agardhf-
jellet Formation. As argued by Roberts et al [
150
], Cryptoclididae can be divided into
Colymbosaurinae and an unamed subclade. The later contains some Tithonian species
from Agardhfjellet Formation that possessed elongated necks. For example, Spitrasaurus
wensaasi has 60 cervicals [
299
], which is within the range of elasmosaurids [
48
,
300
]. The
holotype of S. wensaasi was around 720 kg, and S. larseni was 4% larger. Osteological
features indicative of immaturity explain the reason for their modest sizes. The cervical
series of Djupedalia engeri is incomplete, but its estimated cervical count is 54 [
301
]. The
holotype was also a juvenile at the time of death, weighing about 500 kg. The holotype of
Ophthalmothule cryostea is an adult and possesses 50 cervicals [
150
]. However, vertebral
dimensions suggest that it was just slightly larger than Spitrasaurus spp. at 790 kg.
Colymbosaurines were also discovered from this formation, represented by Colymbosaurus
svalbardensis. All individuals referred to this species were within the range of 1.3
∼
1.6 t
[
302
,
105
]. Besides the Agardhfjellet Formation, colymbosaurines are best known from
Volgian Russia, with a high size disparity [303].
Morphologies of late Jurassic cryptoclidids indicate a preference for deepwater envi-
ronments (e.g., O. cryostea [
150
]). This is consistant with the rise of global sea level during
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that period [
304
]. The sea level changed rapidly and drastically during the earliest Creta-
ceous [
305
], which might lead to the decline of Cryptoclididae [
306
]. Abyssosaurus nataliae,
a long-necked species from Hauterivian with adaptations for deep-diving [
307
,
135
], was
around 6.7 m, similar to the largest C. megadeirus. However, it was much thinner, weighing
only 1.42 t. The youngest occurance of cryptoclidids was Opallionectes andamookaensis
from Aptian of Australia, and the holotype was a 5 m juvenile [
308
]. In general, it appears
that relatively large cryptoclidids with body mass over a t already occurred in middle
Jurassic, and such a mass range evolved repeatedly in different species from late Jurassic
and early Cretaceous.
7.2.5 Elasmosauridae
This clade is known for extremely long necks (over 70 cervicals in some taxa [
171
,
55
]).
Gutarra et al [
4
] proved that large body sizes can compensate for extra drag caused by
long necks. Indeed, elasmosaurids were larger than other groups of long-necked plesiosaurs,
possibly able to reach 13 m in length (see below). But girdle dimensions suggest that
they were very slim animals. For example, width of coracoid at hinder angle of glenoid
in Elasmosaurus platyurus is less than that of a 4.3 m Peloneustes philarchus [
34
,
111
].
No elasmosaurid examined in this study exceeded 10 t, thus they are dwarfed by the
largest pliosaurs (
>
20 t). Welles [
36
,
34
,
214
] offered body length estimates for many
elasmosaurids, but most of them contained no intervertebral cartilages. Therefore, his
results should be used with caution.
There were only a few valid elasmosaurid species from the early Cretaceous, and their
fragmented fossils often preclude body size estimation. Jucha squalea, a Hauterivian
species with 1.65 t body mass, was modest in size for elasmosaurids [
309
]. Callawayasaurus
colombiensis from Paja Formation was around 8.2 m, 2.46 t [
214
]. Wapuskanectes bet-
synichollsae from Clearwater Formation was the largest early Cretaceous elasmosaurid
examined in this study [
52
]. Coracoid width suggests it was 3.18 t in body mass. Kear
[
147
] argued that Australian elasmosaurids require a broad revision on both taxonomy and
phylogeny, and many materials are undiagnostic to date [
282
]. Some extremely small juve-
nile elasmosaurids (body length
<
2 m) are known from early Cretaceous of Australia [
310
],
while larger individuals around 6
∼
7 m have also been discovered [
308
]. The holotype of
Eromangasaurus australis, QM F11050, was probably bitten and killed by a large pliosaur
[
311
]. It is difficult to estimate its body size due to its incompleteness. Generally speaking,
most elasmosaurids from early Cretaceous were modest in size, and they coexisted with
similar-sized relic cryptoclidids like Opallionectes andamookaensis [310].
Large elasmosaurids occurred in the earliest late Cretaceous. Thalassomedon haningtoni
from Cenomian weighed over 7 t thanks to its large trunk [36]. Libonectes morgani from
Cenomian-Turonian was much smaller (holotype around 8.3 m using the ratio of referred
individual SMNK-PAL 3978, which was 7.2 m in length [
312
,
313
]). Many elasmosaurid
species have been discovered from Campanian of the Western Interior Sea in North America.
These elasmosaurids are characterized by their elongated necks, and were once thought
by previous researchers to form a monophyletic clade, Elasmosaurinae (= “Styxosaurinae”
) [
212
,
149
]. However, it has been suggested that some characters supporting this clade
might be homoplastic [
309
]. Both Albertonectes fernandezi and Elasmosaurus platyurus
possessed over 70 cervicals [
46
,
55
] and were large in size (A. fernandezi about 12.1 m,
4.46 t; E. platyurus about 11.4 m, 4.43 t). One major taxon from the Western Interior
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Sea is Styxosaurus.Styxosaurus sp. SDSM 451 was selected as a basic model (Table 2).
Vertebral dimensions suggest that S. snowi KUVP 1301, S. browni AMNH 5835 and “Hy-
dralmosaurus serpentinus” AMNH 1495 were all within the range of 3.5
∼
4.5 t. Styxosaurus
rezaci from Cenomanian was previously referred to Thalassomedon haningtoni, and they
were similar in body proportions [
314
]. Using the model of T. haningtoni, the body mass
of S. rezaci was around 9 t [
315
]. It is the largest non-aristonectine elasmosaur to my
knowledge. Fragmented materials from other large elasmosaurids have also been discovered
from Niobrara Chalk. KUVP 1302 and KUVP 1312 (which was overestimated as
>
18 m by
Williston [
316
]) were both around 6 t in life [
34
]. Welles [
214
] argued that “Plesiosawrus
helmersenii” from Senonian is the largest elasmosaur, but vertebral sizes indicate a 6
∼
7 t
mass [
317
], which is exceeded by T. haningtoni. Using the ratio of Styxosaurus SDSM
451, “P. helmersenii” might reach or exceed 13 m in length. Elasmosaurids also existed in
North America during Maastrichtian, exemplified by Hydrotherosaurus alexandrae (8.8 m,
2.3 t [36]) and Nakonanectes bradti (5.1∼5.6 m, 1.6 t [318]).
Studies in the past decade have revealed the diversity and broad distribution of Weddel-
lonectia, the major elasmosaur clade during Maastrichtian [
212
,
107
]. Species of this clade
have been discovered in North America (e.g., 2.8
∼
4.6 t Morenosaurus stocki [
36
]), Asia
(2.2 t Futabasaurus suzukii [
106
]), New Zealand (
>
8 m Tuarangisaurus keyesi [
108
,
319
]),
South America (3.8
∼
4.2 m Kawanectes lafquenianum [
66
]) and Antarctica (6.2 m, 1.25 t
Vegasaurus molyi [300]).
Besides these modest-sized species, a derived subclade of Weddellonectia, Aristonectinae,
is characterized for shortening of the neck [
212
,
184
,
127
]. The body size estimation of
Aristonectes metrits a review. Most of our knowledge on postcranial elements of this genus
comes from A. quiriquinensis, of which the fossils are sufficient for constructing a mixed
model (Fig. 1A). It should be noted that attribution of this species to Aristonectes has
been revealed to be questionable, and more complete materials are needed to clarify its
relationship with the type species [
320
]. Otero et al [
109
] proposed a vertebral formula
which contains 40
∼
42 cervicals and 20 dorsals without mentioning sacrals or pectorals.
They estimated the body length of the holotype at 9 m. Otero et al [
127
] later suggested 43
cervicals, 3 pectorals, 23 or 24 dorsals, 2 or 3 sacrals and 35 caudals for this species. They
also calculated the forelimb as 3 m and trunk as 4 m, which were adopted in some recent
studies (e.g., [
4
]). However, it can be noticed that the body proportions summarized from
their calculation don’t match the silhouette reconstruction ([
127
]: Fig. 10). O’Gorman
et al [
149
] adopted the vertebral formula proposed by Otero et al [
127
], and re-estimated
the body length of holotype at 10.232 m. They also reported a larger individual MLP
89-III-3-1, which is referable to Aristonectes sp., and estimated a body mass over 10 t using
the Cryptoclidus model constructed by Henderson [
215
]. Trunk lengths of both individuals
were estimated as being much shorter than 4 m by them. A model for Aristonectes was
created under the criteria proposed in this paper and was based on A. quiriquinensis
(Table 2; see supplementary materials for detailed process). Here I also confirm that no
elasmosaur had the potential to possess a 4 m trunk among all the fossils reviewed in
this study. The body mass of MLP 89-III-3-1 is reduced to 9.2 t due to the thinner rib
cage comparing with Cryptoclidus, but it remains the largest elasmosaurid known to date,
followed by Styxosaurus rezaci USNM 50132. The holotype of Aristonectes parvidens,
MLP 40-XI-14-6 [
184
], was estimated here at 8.5 m by comparing its cervical dimensions
with A. quiriquinensis holotype.
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Besides giant A. quiriquinensis, a small taxon, Morturneria seymourensis, was also
discovered from the López de Bertodano Formation. SKL of the holotype was estimated
at only three fourths of that in adult A. quiriquinensis [
321
]. Although the holotype of
M. seymourensis is osteologically immature, another older specimen helps confirm the
small size of this species [
322
]. Kaiwhekea katiki is known from an articulated specimen
discovered in New Zealand [
323
]. Attempt to construct a model for this taxon is hampered
by the missing or poorly preserved girdle elements. Its trunk length indicates a 2.5 t body
mass.
7.2.6 Polycotylidae
The phylogeny and taxonomy of Polycotylidae has been revised very recently by Clark
et al [
176
], and I follow their taxonomic system despite the unstable topologies inside
this clade discovered in previous studies [
324
,
213
,
325
,
326
]. The earliest polycotylids
are known from Aptian of Australia [
327
]. Other materials from early Cretaceous include
Edgarosaurus muddi from North America (estimated at 3.2
∼
3.7 m by Drunkenmiller [
222
]),
the “Richmond pliosaur” from Australia and some indeterminate fossils [
310
,
328
]. The
key turning point of polycotylid evolution was the Cenomian-Turonian transition, which
marked the extinction of some basal lineages (e.g., Occultonectia [
326
], contra [
213
]) and
the rise of polycotylines. The body size disparity within Polycotylidae was also very high
during this period, caused by the existence of species from different lineages which varied
greatly in body mass: from the 400 kg Scalamagnus tropicensis [
329
], to 730 kg Palmula
quadratus and 865 kg Eopolycotylus rankini [
324
], to large Trinacromerum bentonianum
which weighed over 1.5 t [
70
]. Although polycotylids were traditionally considered as
representatives of “pliosauromorph”, some species during this period possessed relatively
long necks, exemplified by the 1.58 t Thililua longicollis from Morocco [
330
]. It has been
revealed that long necks evolved in polycotylids for several times [
213
], and this body plan
existed till end of Cretaceous (e.g., Serpentisuchops pfisterae [
331
]). A long neck was also
present in Polycotylus latipinnis which was 4.7 m, 1.15 t [
218
]. The most derived and
recently named subclade of Polycotylinae, Dolichorhynchia [
176
], contains some relatively
short necked species like Dolichorhynchops spp., which were 400
∼
500 kg in body mass
[
78
,
197
,
332
]. Unktaheela specta was tinier, possessing the smallest skull among all adult
polycotylids [
176
]. On the other hand, large dolichorhynchians like Martinectes bonneri
(4.5 m, 1.45 t) were also present in late Cretaceous [
333
,
190
]. Fossil records imply that
polycotylids declined significantly before the end of Maastrichtian, and the reason remains
unclear in the current stage [213].
7.2.7 Leptocleididae and freshwater plesiosaurs from China
In addition to their flourish in marine environments, multiple plesiosaur lineages entered
freshwater or marginal habitats and radiated there [
201
]. One clade of iconical freshwater
plesiosaurs, Leptocleididae, is characterized for their relatively short necks and small
body sizes [
334
]. This family was once regarded as a subclade of Pliosauroidea, but has
been revealed as a lineage of Xenopsaria in recent phylogenies [
20
,
213
]. The type genus,
Leptocleidus, was estimated to be 2.5
∼
3 m in body length in previous studies [
61
,
335
]. L.
clemai, with 400
∼
440 kg mass, was the largest species of this genus, followed by 277 kg
L. superstes. The smallest L. capensis was only around 200 kg. There exist some fossil
materials referable to Leptocleidus but undiagnostic at species level, and they all indicate
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small body sizes [
201
]. Besides freshwater species, leptocleidids are also known from marine
environments. Umoonasaurus demoscyllus was a small marine species (about 2.5 m) which
inhabited in a seasonally near-freezing region [
336
]. Another marine species, Nichollssaura
borealis, is known from a 2.6 m articulated skeleton discovered from Clearwater Formation
[337].
Fossils of other plesiosaur clades have also been found in freshwater or marginal en-
vironments (e.g., [
82
,
104
]). A review of these materials have been carried out by Bunker
et al [
201
]. Therefore, I don’t re-examine all these fossils but focus on body sizes of
freshwater plesiosaurs from China. Many of them were described using chinese in the last
century and have never been included in phylogenies, thus a broad revision is desirable
but beyond the scope of this study. The phylogenetic affinity of Bishanopliosaurus youngi
is unclear since the holotype is an osteologically immature individual which shows both
rhomaleosaurid and pliosaurid features [
338
]. Dong [
339
] offered a 4 m length estimate,
and vertebral dimensions indicate a 600 kg body mass. Another species, B. zigongensis,
is known from the type specimen which contains 20 articulated dorsals, 2 sacrals and
incomplete pelvic girdles and limbs [
340
]. The articulated dorsal series measures 770 mm,
which is slightly over one third of the trunk length in Rhomaleosaurus thorntoni [
67
]. All
species of Sinopliosaurus were established on very fragmented materials, making their
validity suspicious [
341
,
342
]. Buffetaut et al [
343
] reviewed these materials and argued
that the type species S. weiyuanensis is a nomen dubium and S. fusuiensis is actually
a spinosaurid. Another species, S. sheziensis, was established on extremely fragmented
materials including teeth, incomplete ribs and sacral vertebrae [
344
], which might be
undiagnostic as well. In general, the validity of Sinopliosaurus remains doubtful, and it
is difficult to carry out body size estimation due to the poor preservation of fossils. But
all current materials indicate small body sizes. An incomplete mandible is preserved in
the holotype of Yuzhoupliosaurus chengjiangensis, which represents the only plesiosaur
cranial material discovered in China so far. Zhang [
345
] reconstructed the complete length
of mandible at 540 mm and estimated total body length at 4 m. Dimensons of its dorsal
centra suggest a 663 kg body mass. Freshwater plesiosaur fossils were also discovered from
Xinhe Formation and Xintiangou Formation of China, most of which are undiagnostic
teeth or vertebrae [346,347].
7.3 Enlightment for future studies
This study is the first attemp to develop a uniform set of body reconstruction criteria for
the whole Plesiosauria with quantitative restoration of cross-sectional profiles. Zhao [
234
]
argued that careful reconstructions of body cross-sections are required in body mass esti-
mations since simply assuming elliptical or superelliptical approximations may incorporate
errors. Although the criteria proposed in this study are restricted to plesiosaurs, similar
methods can be developed for precise body reconstruction of other extinct vertebrates
including dinosaurs and other marine reptiles.
Body mass and surface area of an animal are linked to many of its biological prop-
erties. There is a growing trend in the past decade to use fluid dynamic methods like
Computational Fluid Dynamics (CFD) or experiments to study the hydrodynamic perfor-
mances of plesiosaurs (e.g., [
73
,
161
,
4
]). Results of these studies would be affected by body
shapes, thus rigorous reconstructions would help clarify many physiological features of
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plesiosaurs (e.g., optimal swimming speed) and lead to a better understanding of variation
in locomotory strategies applied by different clades.
This study also offers several scaling equations for quick body mass estimation, which
allows predicting body size from very fragmented fossils. Mass is a better proxy for body
size than length since plesiosaurs display diverse body proportions. After quick obtainment
of mass information, these results can be imported to subsequent morphometric studies
to control for allometry, which is necessary because body masses of plesiosaurs range
from several hundred kilograms to over 20 tonnes. These tools also enable the study of
plesiosaur body size evolution through deep time and help illuminate the whole picture of
their 135-million-year story.
8 Conclusions
Body mass estimations of plesiosaurs were scarce and were carried out under different
criteria in previous studies. Some published results were just rough estimates based on
models with questionable reliability. The burst of plesiosaur studies in the past two decades
has offered sufficient information for the development of an uniform set of reconstruction
protocols. During this process, some correlations between plesiosaur skeletal elements were
revealed.
Twenty two models were created under this set of criteria. Linear regressions based
on these models illuminate that there exist positive corrlations between plesiosaur skeletal
elements and body volume. The scaling equations can thus been applied for quick body
mass prediction using very fragmented materials.
The maximum body length of Plesiosauria was probably 13 m, which was reached by both
elasmosaurids and pliosaurids juding from fragmented materials. The heaviest plesiosaurs
examined in this study are some Jurassic pliosaurs with body masses over 20 t. On the
other hand, the smallest adult plesiosaur weighed less than 200 kg, revealing the high
body size disparity of this clade. This study is a staring point of a comprehensive research
on plesiosaur body size evolution.
9 Acknowledgements
I warmly thank Benedon Paratodus, Frank Fang, Emo Young, Shinya Noguchi, Frederick
Dakota, Yang Song, Y.-W. Fang, Devin LYu, Lingcheng Liu and Andy Thomas for
discussion and their constant support during the three-year preparation of this study. I
also thank Espen Knutsen, Richard Forrest and Leslie Noè, who offered comments and
suggestions on the plesiosaur reconstructions. Nikolay Zverkov, Jørn Hurum, Nigel Larkin,
Luis Spalletti, Zulma Gasparini, Anna Krahl, Carla Crook, Eric Buffetaut, Peggy Vincent,
Glenn Storrs, Bruce Schumacher and Eberhard Frey are thanked for kindly sharing their
knowledge or providing photos.
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References
[1]
Judy A Massare. Swimming capabilities of mesozoic marine reptiles: implications
for method of predation. Paleobiology, 14(2):187–205, 1988.
[2]
Ryosuke Motani. Swimming speed estimation of extinct marine reptiles: energetic
approach revisited. Paleobiology, 28(2):251–262, 2002.
[3]
Katsufumi Sato, Yutaka Watanuki, Akinori Takahashi, Patrick J.O Miller, Hideji
Tanaka, Ryo Kawabe, Paul J Ponganis, Yves Handrich, Tomonari Akamatsu, Yuuki
Watanabe, Yoko Mitani, Daniel P Costa, Charles-André Bost, Kagari Aoki, Masao
Amano, Phil Trathan, Ari Shapiro, and Yasuhiko Naito. Stroke frequency, but not
swimming speed, is related to body size in free-ranging seabirds, pinnipeds and
cetaceans. Proceedings of the Royal Society B: Biological Sciences, 274(1609):471–477,
dec 2006.
[4]
Susana Gutarra, Thomas L. Stubbs, Benjamin C. Moon, Colin Palmer, and Michael J.
Benton. Large size in aquatic tetrapods compensates for high drag caused by extreme
body proportions. Communications Biology, 5(1), apr 2022.
[5]
Jennifer A. Sheridan and David Bickford. Shrinking body size as an ecological
response to climate change. Nature Climate Change, 1(8):401–406, October 2011.
[6]
Annie Séguin, Éric Harvey, Philippe Archambault, Christian Nozais, and Dominique
Gravel. Body size as a predictor of species loss effect on ecosystem functioning.
Scientific Reports, 4(1), April 2014.
[7]
Mariko Nagano, Masaki Sakamoto, Kwang-Hyeon Chang, and Hideyuki Doi.
Predator-induced plasticity in relation to prey body size: A meta-analysis of daphnia
experiments. Freshwater Biology, 68(8):1293–1302, June 2023.
[8] WK Gregory. The weight of the Brontosaurus.Science, 22(566):572–572, 1905.
[9]
Donald M Henderson. Estimating the masses and centers of mass of extinct animals
by 3-d mathematical slicing. Paleobiology, 25(1):88–106, 1999.
[10]
Ryosuke Motani. Estimating body mass from silhouettes: testing the assumption of
elliptical body cross-sections. Paleobiology, 27(4):735–750, 2001.
[11]
Karl T Bates, Phillip L Manning, David Hodgetts, and William I Sellers. Estimating
mass properties of dinosaurs using laser imaging and 3d computer modelling. PloS
one, 4(2):e4532, 2009.
[12]
Ryosuke Motani. Paleomass for r—bracketing body volume of marine vertebrates
with 3d models. PeerJ, 11:e15957, aug 2023.
[13]
J. F. Anderson, A. Hall-Martin, and D. A. Russell. Long-bone circumference and
weight in mammals, birds and dinosaurs. Journal of Zoology, 207(1):53–61, sep 1985.
[14]
Kenneth E Campbell, Leslie Marcus, et al. The relationship of hindlimb bone
dimensions to body weight in birds. Natural History Museum of Los Angeles County
Science Series, 36(3):395–412, 1992.
47
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[15]
Nicolás E. Campione, David C. Evans, Caleb M. Brown, and Matthew T. Carrano.
Body mass estimation in non-avian bipeds using a theoretical conversion to quadruped
stylopodial proportions. Methods in Ecology and Evolution, 5(9):913–923, jul 2014.
[16]
Nicolás E. Campione and David C. Evans. The accuracy and precision of body mass
estimation in non-avian dinosaurs. Biological Reviews, 95(6):1759–1797, sep 2020.
[17]
François Therrien and Donald M. Henderson. My theropod is bigger than yours . . .
or not: estimating body size from skull length in theropods. Journal of Vertebrate
Paleontology, 27(1):108–115, 2007.
[18]
Eoin J. O’Gorman and David W. E. Hone. Body size distribution of the dinosaurs.
PLoS ONE, 7(12):e51925, December 2012.
[19]
Hilary F. Ketchum and Roger B. J. Benson. Global interrelationships of plesiosauria
(reptilia, sauropterygia) and the pivotal role of taxon sampling in determining the
outcome of phylogenetic analyses. Biological Reviews, 85(2):361–392, April 2010.
[20]
Roger B. J. Benson and Patrick S. Druckenmiller. Faunal turnover of marine
tetrapods during the jurassic-cretaceous transition. Biological Reviews, 89(1):1–23,
apr 2013.
[21]
Tanja Wintrich, Shoji Hayashi, Alexandra Houssaye, Yasuhisa Nakajima, and P. Mar-
tin Sander. A triassic plesiosaurian skeleton and bone histology inform on evolution
of a unique body plan. Science Advances, 3(12), December 2017.
[22]
Roger B. J. Benson, Mark Evans, and Patrick S. Druckenmiller. High diversity,
low disparity and small body size in plesiosaurs (reptilia, sauropterygia) from the
triassic–jurassic boundary. PLoS ONE, 7(3):e31838, mar 2012.
[23]
Daniel Madzia and Andrea Cau. Estimating the evolutionary rates in mosasauroids
and plesiosaurs: discussion of niche occupation in late cretaceous seas. PeerJ, 8:e8941,
April 2020.
[24]
Nathalie Bardet. Extinction events among mesozoic marine reptiles. Historical
Biology, 7(4):313–324, July 1994.
[25]
F Robin O’Keefe. The evolution of plesiosaur and pliosaur morphotypes in the
plesiosauria (reptilia: Sauropterygia). Paleobiology, 28(1):101–112, 2002.
[26]
Glenn W Storrs. Function and phylogeny in sauropterygian (diapsida) evolution.
American Journal of Science, 293(A):63, 1993.
[27]
Benjamin C Wilhelm. Novel anatomy of cryptoclidid plesiosaurs with comments on
axial locomotion, 2010.
[28]
Anna Krahl. The locomotory apparatus and paraxial swimming in fossil and living
marine reptiles: comparing nothosauroidea, plesiosauria, and chelonioidea. PalZ,
95(3):483–501, jun 2021.
[29]
F. R. O’Keefe and L. M. Chiappe. Viviparity and k-selected life history in a mesozoic
marine plesiosaur (reptilia, sauropterygia). Science, 333(6044):870–873, August 2011.
48
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[30]
A Yu Berezin. On viviparity in the plesiosaur Abyssosaurus nataliae.PALEOSTRAT-
2016, 2016.
[31]
Aurélien Bernard, Christophe Lécuyer, Peggy Vincent, Romain Amiot, Nathalie
Bardet, Eric Buffetaut, Gilles Cuny, François Fourel, François Martineau, Jean-
Michel Mazin, and Abel Prieur. Regulation of body temperature by some mesozoic
marine reptiles. Science, 328(5984):1379–1382, June 2010.
[32]
Corinna V. Fleischle, Tanja Wintrich, and P. Martin Sander. Quantitative histological
models suggest endothermy in plesiosaurs. PeerJ, 6:e4955, June 2018.
[33]
Glenn W Storrs and Michael A Taylor. Cranial anatomy of a new plesiosaur genus
from the lowermost lias (rhaetian/hettangian) of street, somerset, england. Journal
of Vertebrate Paleontology, 16(3):403–420, 1996.
[34]
Samuel Paul Welles. A review of the North American Cretaceous elasmosaurs.
University of California Press, 1952.
[35]
Colin Richard McHenry. Devourer of gods: the palaeoecology of the Cretaceous
pliosaur Kronosaurus queenslandicus. PhD thesis, University of Newcastle, 2009.
[36]
Samuel Paul Welles. Elasmosaurid plesiosaurs with description of new material from
california and colorado. Memoirs of the University of California, 13:125–254, 1943.
[37]
Alfred Sherwood Romer and Arnold D Lewis. A mounted skeleton of the giant
plesiosaur Kronosaurus.Brevoria Museum of Comparative Zoology, (112):1–15, 1959.
[38]
David Seymour Brown. The english upper jurassic plesiosauroidea (reptilia) and a
review of the phylogeny and classification of the plesiosauria. 1981.
[39]
María Eurídice Páramo-Fonseca, Cristian David Benavides-Cabra, and Ingry Esmirna
Gutiérrez. A new large pliosaurid from the barremian (lower cretaceous) of sáchica,
boyacá, colombia. Earth Sciences Research Journal, 22(4):223–238, 2018.
[40]
Adam S Smith and Peggy Vincent. A new genus of pliosaur (reptilia: Sauropterygia)
from the lower jurassic of holzmaden, germany. Palaeontology, 53(5):1049–1063,
2010.
[41]
Charles William Andrews. A descriptive catalogue of the marine reptiles of the
Oxford clay: Based on the Leeds Collection in the British Museum (Natural History),
London, volume 1. order of the Trustees, 1910.
[42]
Gregory S Paul. Restoring the true form of the gigantic blue whale for the first time,
and mass estimation. BioRxiv, pages 2022–08, 2022.
[43]
Charles William Andrews. A descriptive catalogue of the marine reptiles of the
Oxford clay: Based on the Leeds Collection in the British Museum (Natural History),
London, volume 2. order of the Trustees, 1913.
[44]
Harry G. Seeley. Note on some of the generic modifications of the plesiosaurian
pectoral arch. Quarterly Journal of the Geological Society, 30(1-4):436–449, feb 1874.
[45]
AS Romer. Osteology of the reptiles university of chicago press. Chicago, Illinois,
1956.
49
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[46]
Tai Kubo, Mark T. Mitchell, and Donald M. Henderson. Albertonectes vanderveldei,
a new elasmosaur (reptilia, sauropterygia) from the upper cretaceous of alberta.
Journal of Vertebrate Paleontology, 32(3):557–572, may 2012.
[47]
Eberhard Frey, Eric WA Mulder, Wolfgang Stinnesbeck, Héctor E Rivera-Sylva,
José Manuel Padilla-Gutiérrez, and Arturo Homero González-González. A new
polycotylid plesiosaur with extensive soft tissue preservation from the early late
cretaceous of northeast mexico. Boletín de la Sociedad Geológica Mexicana, 69(1):87–
134, 2017.
[48]
Kenneth Carpenter. Revision of north american elasmosaurs form the cretaceous of
the western interior. Paludicola, 2(2):148, 1999.
[49]
Tamaki Sato. Description of plesiosaurs (reptilia: Sauropterygia) from the bearpaw
formation (campanian-maastrichtian) and a phylogenetic analysis of the elasmosauri-
dae. 2003.
[50]
W. J. Sollas. On a new species of Plesiosaurus (P. conybeari) from the lower lias of
charmouth- with observations on P. megacephalus, stutchbury, and P. brachycephalus,
owen. Quarterly Journal of the Geological Society, 37(1-4):440–481, feb 1881.
[51]
Orville W Bonner. An osteological study of nyctosaurus and trinacromerum with a
description of a new species of nyctosaurus, 1964.
[52]
Patrick S Druckenmiller and Anthony P Russel. A new elasmosaurid plesiosaur
(reptilia: Sauropterygia) from the lower cretaceous clearwater formation, northeastern
alberta, canada. Paludicola, 5(4):184, 2006.
[53]
Roger B. J. Benson, Hilary F. Ketchum, Leslie F. Noè, and Marcela Gómez-Pérez.
New information on hauffiosaurus (reptilia, plesiosauria) based on a new species
from the alum shale member (lower toarcian: Lower jurassic) of yorkshire, UK.
Palaeontology, 54(3):547–571, may 2011.
[54]
Pernille V Troelsen. Mobility and hydrodynamic implications of the long neck in
plesiosaurs. Liverpool John Moores University (United Kingdom), 2019.
[55]
Sven Sachs, Benjamin P Kear, and Michael J Everhart. Revised vertebral count in
the “longest-necked vertebrate” elasmosaurus platyurus cope 1868, and clarification
of the cervical-dorsal transition in plesiosauria. PLoS One, 8(8):e70877, 2013.
[56]
SW Salisbury and E Frey. Crocodilian biology and evolution. Surrey Beatty & Sons,
2000.
[57]
Tanja Wintrich, René Jonas, Hans-Joachim Wilke, Lars Schmitz, and P. Martin
Sander. Neck mobility in the jurassic plesiosaur Cryptoclidus eurymerus: finite
element analysis as a new approach to understanding the cervical skeleton in fossil
vertebrates. PeerJ, 7:e7658, nov 2019.
[58]
Tanja Wintrich, Martin Scaal, Christine Böhmer, Rico Schellhorn, Ilja Kogan, Aaron
van der Reest, and P. Martin Sander. Palaeontological evidence reveals convergent
evolution of intervertebral joint types in amniotes. Scientific Reports, 10(1), aug
2020.
50
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[59]
O Rieppel. Osteology of simosaurus and the interrelationships of stem-group
sauropterygia (reptilia, diapsida). Fieldiana: Geology, New Series, 28:1–85, 1994.
[60]
Charles William Andrews. Description of a new plesiosaur from the weald clay of
berwick (sussex). Quarterly Journal of the Geological Society, 78(1-4):285–NP, 1922.
[61]
Benjamin P. Kear and Paul M. Barrett. Reassessment of the lower cretaceous
(barremian) pliosauroid Leptocleidus superstes andrews, 1922 and other plesiosaur
remains from the nonmarine wealden succession of southern england. Zoological
Journal of the Linnean Society, 161(3):663–691, jan 2011.
[62]
Samuel Paul Welles. A new elasmosaur from the eagle ford shale of texas: Systematic
description. Fondren Science Series, 1(1):1, 1949.
[63]
Ramon Nagesan. Neck mobility of the plesiosaur Nichollsaura borealis. Master’s
thesis, University of Calgary, 2017.
[64]
Maria Zammit, Christopher B. Daniels, and Benjamin P. Kear. Elasmosaur (reptilia:
Sauropterygia) neck flexibility: Implications for feeding strategies. Comparative
Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 150(2):124–
130, jun 2008.
[65]
José Patricio O’Gorman. Plesiosaurios del Cretácico Superior de Patagonia y la
península Antártica. PhD thesis, Universidad Nacional de La Plata, 2013.
[66]
José P. O’Gorman. The most complete specimen of Kawanectes lafquenianum
(sauropterygia, plesiosauria): New data on basicranial anatomy and possible sexual
dimorphism in elasmosaurids. Cretaceous Research, 125:104836, sep 2021.
[67]
AS Smith. Reassessment of ’Plesiosaurus’ megacephalus (sauropterygia: Plesiosauria)
from the triassic-jurassic boundary, UK. Palaeontologia Electronica, 2015.
[68]
Mark Evans. An investigation into the neck flexibility of two plesiosauroid plesiosaurs:
Cryptoclidus eurymerus and muraenosaurus leedsii, 1993.
[69]
B Zarnik. K etologiji plesiosaurija, sa primosima mehanici kralježnice u recentnih
sauropsida. Glasnik Nauč. Casopis Prirod. Društvo, 38(39):424–479, 1925.
[70]
John T Thurmond. A new polycotylid plesiosaur from the lake waco formation
(cenomanian) of texas. Journal of Paleontology, pages 1289–1296, 1968.
[71]
Jane Ann Robinson. The locomotion of plesiosaurs. Neues Jahrbuch für Geologie
und Paläontologie, Abhandlungen, 1975.
[72]
JUDY A Massare, L Maddock, Q Bone, and JMV Rayner. Swimming capabilities of
mesozoic marine reptiles: a review. Mechanics and physiology of animal swimming,
pages 133–149, 1994.
[73]
Shiqiu Liu, Adam S Smith, Yuting Gu, Jie Tan, C Karen Liu, and Greg Turk.
Computer simulations imply forelimb-dominated underwater flight in plesiosaurs.
PLoS Computational Biology, 11(12):e1004605, 2015.
51
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[74]
Leslie F. Noè and Marcela Gómez-Pérez. Giant pliosaurids (sauropterygia; ple-
siosauria) from the lower cretaceous peri-gondwanan seas of colombia and australia.
Cretaceous Research, 132:105122, apr 2022.
[75]
Valentin Fischer, Roger BJ Benson, Nikolay G Zverkov, Maxim S Arkhangelsky,
Ilya M Stenshin, Gleb N Uspensky, and Natalya E Prilepskaya. Anatomy and
relationships of the bizarre early cretaceous pliosaurid luskhan itilensis. Zoological
Journal of the Linnean Society, 198(1):220–256, 2023.
[76]
Adam Stuart Smith. Anatomy and systematics of the Rhomaleosauridae (Sauroptery-
gia: Plesiosauria). PhD thesis, 2007.
[77]
Peggy Vincent, Rémi Allemand, Paul D. Taylor, Guillaume Suan, and Erin E.
Maxwell. New insights on the systematics, palaeoecology and palaeobiology of a
plesiosaurian with soft tissue preservation from the toarcian of holzmaden, germany.
The Science of Nature, 104(5-6), jun 2017.
[78]
Samuel Wendell Williston. North American plesiosaurs: Part I, volume 2. Field
Columbian Museum, 1907.
[79]
Owen. Monograph of the fossil reptilia of the liassic formations. part first. sauroptery-
gia. Monographs of the Palaeontographical Society, 17(75):1–40, 1865.
[80]
Adam S Smith. Morphology of the caudal vertebrae in rhomaleosaurus zetlandicus
and a review of the evidence for a tail fin in plesiosauria. Paludicola, 9(3):144–158,
2013.
[81]
Benjamin C. Wilhelm and F. Robin O’Keefe. A new partial skeleton of a cryptoclei-
doid plesiosaur from the upper jurassic sundance formation of wyoming. Journal of
Vertebrate Paleontology, 30(6):1736–1742, dec 2010.
[82]
JosÉ Patricio O’Gorman. First record of Kawanectes lafquenianum (plesiosauria,
elasmosauridae) from the la colonia formation of argentina, with comments on the
mandibular morphology of elasmosaurids. Alcheringa: An Australasian Journal of
Palaeontology, 44(1):176–193, nov 2019.
[83]
Rodrigo A. Otero, Sergio Soto-Acuña, Alexander O. Vargas, and David Rubilar-
Rogers. A new postcranial skeleton of an elasmosaurid plesiosaur from the upper
cretaceous of central chile and reassessment of cimoliasaurus andium deecke. Creta-
ceous Research, 50:318–331, jul 2014.
[84]
Rodrigo A. Otero, Sergio Soto-Acuña, and David Rubilar-Rogers. A postcranial
skeleton of an elasmosaurid plesiosaur from the maastrichtian of central chile, with
comments on the affinities of late cretaceous plesiosauroids from the weddellian
biogeographic province. Cretaceous Research, 37:89–99, oct 2012.
[85] A. G. Sennikov. Peculiarities of the structure and locomotor function of the tail in
sauropterygia. Biology Bulletin, 46(7):751–762, dec 2019.
[86]
Leslie F Noè, Michael A Taylor, and Marcela Gómez-Pérez. An integrated approach
to understanding the role of the long neck in plesiosaurs. Acta Palaeontologica
Polonica, 62(1):137–162, 2017.
52
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[87]
Adam S. Smith and Roger B.J. Benson. Osteology of Rhomaleosaurus thorntoni
(sauropterygia: Rhomaleosauridae) from the lower jurassic (toarcian) of northamp-
tonshire, england. Monographs of the Palaeontographical Society, 168(642):1–40, oct
2014.
[88]
B Newman and LBH Tarlo. A giant marine reptile from bedfordshire. Animals,
10(2):61–63, 1967.
[89]
Jane Ann Robinson. Intracorporal force transmission in plesiosaurs. Neues Jahrbuch
fur Mineralogie Geologie und Palaontologie, Abhandlungen, 153:86–128, 1977.
[90]
Courtney D Richards. Plesiosaur body shape and its impact on hydrodynamic
properties, 2011.
[91]
Thomas Hawkins. The book of the great sea-dragons, Ichthyosauri and Plesiosauri,
gedolim taninim, of Moses. Extinct monsters of the ancient earth. W. Pickering„
1840.
[92]
A. Augusta and Z. Burian. Les animaux préhistoriques. Editions La Farandole,
Paris, 1959.
[93]
W. D. CONYBEARE. On the discovery of an almost perfect skeleton of the
Plesiosaurus.Transactions of the Geological Society of London, 1(2):381–389, jan
1824.
[94]
Sven Sachs, Jahn J. Hornung, and Benjamin P. Kear. Reappraisal of europe’s most
complete early cretaceous plesiosaurian: Brancasaurus brancai wegner, 1914 from
the “wealden facies” of germany. PeerJ, 4:e2813, dec 2016.
[95]
Gregory S Paul. The Princeton Field Guide to Mesozoic Sea Reptiles. Princeton
University Press, 2022.
[96]
Anna Krahl, Adam S. Smith, and Ingmar Werneburg. Historically transposed flipper
pairs in a mounted plesiosaurian skeleton. PalZ, 96(4):805–813, April 2022.
[97]
Heber A. Longman. Restoration of Kronosaurus queenslandicus.Memoirs of the
Queensland Museum, 1932.
[98]
Oliver Hampe. Ein großwüchsiger pliosauride (reptilia: Plesiosauria) aus-der un-
terkreide (oberes aptium) von kolumbien. 1992.
[99]
E. Frey and J. Riess. Considerations concerning plesiosaur locomotion. Neues
Jahrbuch für Geologie und Paläontologie - Abhandlungen, 164(1-2):193–194, dec
1982.
[100]
Stephen J. Godfrey. Plesiosaur subaqueous locomotion - a reappraisal. Neues
Jahrbuch für Geologie und Paläontologie - Monatshefte, 1984(11):661–672, dec 1984.
[101]
Lambert Beverly Tarlo. A review of the Upper Jurassic pliosaurs. British Museum
(Natural History), 1960.
53
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[102]
F. Robin O’Keefe, Hallie P. Street, Benjamin C. Wilhelm, Courtney D. Richards,
and Helen Zhu. A new skeleton of the cryptoclidid plesiosaur Tatenectes laramiensis
reveals a novel body shape among plesiosaurs. Journal of Vertebrate Paleontology,
31(2):330–339, mar 2011.
[103]
Tamaki Sato. A new polycotylid plesiosaur (reptilia: sauropterygia) from the upper
cretaceous bearpaw formation in saskatchewan, canada. Journal of Paleontology,
79(5):969–980, 2005.
[104]
James A. Campbell, Mark T. Mitchell, Michael J. Ryan, and Jason S. Anderson.
A new elasmosaurid (sauropterygia: Plesiosauria) from the non-marine to paralic
dinosaur park formation of southern alberta, canada. PeerJ, 9:e10720, feb 2021.
[105]
Aubrey J. Roberts, Patrick S. Druckenmiller, Lene L. Delsett, and Jørn H. Hurum.
Osteology and relationships of Colymbosaurus seeley, 1874, based on new material
of C. svalbardensis from the slottsmøya member, agardhfjellet formation of central
spitsbergen. Journal of Vertebrate Paleontology, 37(1):e1278381, January 2017.
[106]
Tamaki Sato, Yoshikazu Hasegawa, and Makoto Manabe. A new elasmosaurid
plesiosaur from the upper cretaceous of fukushima, japan. Palaeontology, 49(3):467–
484, 2006.
[107]
Jose P O’Gorman. Elasmosaurid phylogeny and paleobiogeography, with a reappraisal
of aphrosaurus furlongi from the maastrichtian of the moreno formation. Journal of
Vertebrate Paleontology, 39(5):e1692025, 2019.
[108]
Norton Hiller, Al A Mannering, Craig M Jones, and Arthur RI Cruickshank. The na-
ture of Mauisaurus haasti hector, 1874 (reptilia: Plesiosauria). Journal of Vertebrate
Paleontology, 25(3):588–601, 2005.
[109]
Rodrigo A. Otero, Sergio Soto-Acuña, Frank Robin O'Keefe, José P. O’Gorman,
Wolfgang Stinnesbeck, Mario E. Suárez, David Rubilar-Rogers, Christian Salazar,
and Luis Arturo Quinzio-Sinn. Aristonectes quiriquinensis, sp. nov., a new highly
derived elasmosaurid from the upper maastrichtian of central chile. Journal of
Vertebrate Paleontology, 34(1):100–125, jan 2014.
[110]
LB Tarlo. Stretosaurus gen. nov., a giant pliosaur from the kimmeridge clay.
Palaeontology, 2(1):39–55, 1959.
[111]
Hermann Linder. Beiträge zur kenntnis der plesiosaurier-gattungen Peloneustes und
Pliosaurus.Geologische und Palaeontologishe Abhandlungen, 1913.
[112]
Hilary F. Ketchum and Roger B. J. Benson. A new pliosaurid (sauropterygia, ple-
siosauria) from the oxford clay formation (middle jurassic, callovian) of england:
evidence for a gracile, longirostrine grade of early–middle jurassic pliosaurids. BAR-
RETT, P.M. and MILNER, A.R. (eds.) Studies on Fossil Tetrapods. Speicial Papers
in Palaeontology, (86):109–129, 2011.
[113]
Hilary Ketchum and Roger Benson. A new pliosaurid from the oxford clay formation
of oxfordshire, UK. Acta Palaeontologica Polonica, 67, 2022.
[114]
Susana Gutarra and Imran A. Rahman. The locomotion of extinct secondarily
aquatic tetrapods. Biological Reviews, 97(1):67–98, sep 2021.
54
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[115] Luke Muscutt. The hydrodynamics of plesiosaurs. PhD thesis, 2017.
[116]
Anna Krahl and Ingmar Werneburg. Deep-time invention and hydrodynamic conver-
gences through amniote flipper evolution. The Anatomical Record, 306(6):1323–1355,
dec 2022.
[117]
Laura Austin Sydes. Identifying behavioural and functional groups using a geometric
morphometric analysis of plesiosaur flipper morphology, 2022.
[118]
Lisa Noelle Cooper, Annalisa Berta, Susan D. Dawson, and Joy S. Reidenberg. Evo-
lution of hyperphalangy and digit reduction in the cetacean manus. The Anatomical
Record, 290(6):654–672, may 2007.
[119] Wilhelm Dames. Die plesiosaurier der süddeutschen liasformation. 1895.
[120]
F von Huene. Ein neuer plesiosaurier aus dem oberen lias württembergs. Jahreshefte
des Vereins für vaterländische Naturkunde in Württemberg, 79:3–23, 1923.
[121]
DMS Watson et al. The upper liassic reptilia. part iii. microcleidus macropterus
(seeley) and the limbs of microcleidus homalospondylus (owen). Manchester Memoirs,
4(17):1–9, 1911.
[122]
M. J. Moore, A. L. Bogomolni, S. E. Dennison, G. Early, M. M. Garner, B. A.
Hayward, B. J. Lentell, and D. S. Rotstein. Gas bubbles in seals, dolphins, and
porpoises entangled and drowned at depth in gillnets. Veterinary Pathology, 46(3):536–
547, January 2009.
[123]
Stefan Huggenberger, Helmut A Oelschläger, and Bruno Cozzi. Atlas of the Anatomy
of Dolphins and Whales. Academic Press, 2018.
[124]
Marina Ivančić, Mauricio Solano, and Cynthia R Smith. Computed tomography
and cross-sectional anatomy of the thorax of the live bottlenose dolphin (Tursiops
truncatus). The Anatomical Record, 297(5):901–915, 2014.
[125]
Benjamin P Kear. A new elasmosaurid plesiosaur from the lower cretaceous of
queensland, australia. Journal of Vertebrate Paleontology, 25(4):792–805, 2005.
[126]
Peggy Vincent, Nathalie Bardet, Xabier Pereda Suberbiola, Baâdi Bouya, Mbarek
Amaghzaz, and Saïd Meslouh. Zarafasaura oceanis, a new elasmosaurid (reptilia:
Sauropterygia) from the maastrichtian phosphates of morocco and the palaeobio-
geography of latest cretaceous plesiosaurs. Gondwana Research, 19(4):1062–1073,
2011.
[127]
Rodrigo A. Otero, Sergio Soto-Acuña, and Frank R. O'keefe. Osteology of Aris-
tonectes quiriquinensis (elasmosauridae, aristonectinae) from the upper maastrichtian
of central chile. Journal of Vertebrate Paleontology, 38(1):e1408638, jan 2018.
[128]
Megan F. McKenna, Ted W. Cranford, Annalisa Berta, and Nicholas D. Pyenson.
Morphology of the odontocete melon and its implications for acoustic function.
Marine Mammal Science, 28(4):690–713, September 2011.
55
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[129]
Maya Yamato, Darlene R. Ketten, Julie Arruda, Scott Cramer, and Kathleen
Moore. The auditory anatomy of the minke whale (Balaenoptera acutorostrata): a
potential fatty sound reception pathway in a baleen whale. The Anatomical Record,
295(6):991–998, April 2012.
[130]
Sarah S. Kienle, Roxanne D. Cuthbertson, and Joy S. Reidenberg. Comparative
examination of pinniped craniofacial musculature and its role in aquatic feeding.
Journal of Anatomy, 240(2):226–252, October 2021.
[131]
GH Schumacher. Comparative functional anatomy of jaw muscles in reptiles and
mammals. Fortschritte der Zoologie, 30:203–212, 1985.
[132]
Jeanette Wyneken. The external morphology, musculoskeletal system, and neuro-
anatomy of sea turtles. The Biology of Sea Turtles; Lohmann, KJ, Musick, JA,
Wyneken, J., Eds, pages 39–78, 2003.
[133]
John Davenport, John Fraher, Edward Fitzgerald, Patrick McLaughlin, Tom Doyle,
Luke Harman, and Tracy Cuffe. Fat head: an analysis of head and neck insulation
in the leatherback turtle (Dermochelys coriacea). Journal of Experimental Biology,
212(17):2753–2759, September 2009.
[134]
Robert E Weems and Robert B Blodgett. The pliosaurid Megalneusaurus: a newly
recognized occurrence in the upper jurassic naknek formation of the alaska peninsula.
Geologic Studies in Alaska by the US Geological Survey During..., page 169, 1996.
[135]
A. Yu. Berezin. Craniology of the plesiosaur Abyssosaurus nataliae berezin
(sauropterygia, plesiosauria) from the lower cretaceous of the central russian platform.
Paleontological Journal, 52(3):328–341, May 2018.
[136]
Pernille V Troelsen, David M Wilkinson, Mehdi Seddighi, David R Allanson, and
Peter L Falkingham. Functional morphology and hydrodynamics of plesiosaur necks:
does size matter? Journal of Vertebrate Paleontology, 39(2):e1594850, 2019.
[137]
Nikolai Iordansky. Jaw muscles of the crocodiles: structure, synonymy, and some
implications on homology and functions. Russ J Herpetol, 7, 10 2011.
[138]
Michael A. Taylor and A. R. I. Cruickshank. Cranial anatomy and functional
morphology of Pliosaurus brachyspondylus (reptilia: Plesiosauria) from the upper
jurassic of westbury, wiltshire. Philosophical Transactions of the Royal Society of
London. Series B: Biological Sciences, 341(1298):399–418, sep 1993.
[139]
Davide Foffa, Andrew R. Cuff, Judyth Sassoon, Emily J. Rayfield, Mark N. Mavro-
gordato, and Michael J. Benton. Functional anatomy and feeding biomechanics of a
giant upper jurassic pliosaur (reptilia: Sauropterygia) from weymouth bay, dorset,
uk. Journal of Anatomy, 225(2):209–219, June 2014.
[140]
Bruce M Rothschild and Glenn W Storrs. Decompression syndrome in plesiosaurs
(sauropterygia: Reptilia). Journal of Vertebrate Paleontology, 23(2):324–328, 2003.
[141] Daniel G. Blackburn and Christian A. Sidor. Evolution of viviparous reproduction
in paleozoic and mesozoic reptiles. The International Journal of Developmental
Biology, 58(10-11-12):935–948, 2014.
56
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[142]
Graham A. J. Worthy and Elizabeth F. Edwards. Morphometric and biochemical
factors affecting heat loss in a small temperate cetacean (phocoena phocoena) and a
small tropical cetacean (stenella attenuata). Physiological Zoology, 63(2):432–442,
March 1990.
[143]
Robin C. Dunkin, William A. McLellan, James E. Blum, and D. Ann Pabst. The
ontogenetic changes in the thermal properties of blubber from atlantic bottlenose
dolphin Tursiops truncatus.Journal of Experimental Biology, 208(8):1469–1480,
April 2005.
[144]
Heather E. M. Liwanag, Annalisa Berta, Daniel P. Costa, Suzanne M. Budge, and
Terrie M. Williams. Morphological and thermal properties of mammalian insulation:
the evolutionary transition to blubber in pinnipeds: Evolution of blubber in pinnipeds.
Biological Journal of the Linnean Society, 107(4):774–787, September 2012.
[145]
Xianyuan Zeng, Junhua Ji, Yujiang Hao, and Ding Wang. Topographical distribution
of blubber in finless porpoises (Neophocaena asiaeorientalis sunameri): a result from
adapting to living in coastal waters. Zoological Studies, 54(1), February 2015.
[146]
ER Nadel. Energy exchanges in water. Undersea biomedical research, 11(2):149–158,
1984.
[147]
Benjamin P Kear. Cretaceous marine reptiles of australia: a review of taxonomy
and distribution. Cretaceous Research, 24(3):277–303, 2003.
[148]
Tamaki Sato and Xiao chun Wu. A new jurassic pliosaur from melville island,
canadian arctic archipelago. Canadian Journal of Earth Sciences, 45(3):303–320,
mar 2008.
[149]
J.P. O’Gorman, S. Santillana, R. Otero, and M. Reguero. A giant elasmosaurid
(sauropterygia; plesiosauria) from antarctica: New information on elasmosaurid
body size diversity and aristonectine evolutionary scenarios. Cretaceous Research,
102:37–58, October 2019.
[150]
Aubrey Jane Roberts, Patrick S. Druckenmiller, Benoit Cordonnier, Lene L. Delsett,
and Jørn H. Hurum. A new plesiosaurian from the jurassic–cretaceous transitional
interval of the slottsmøya member (volgian), with insights into the cranial anatomy
of cryptoclidids using computed tomography. PeerJ, 8:e8652, March 2020.
[151]
RPM Topper, J Trabucho Alexandre, E Tuenter, and P Th Meijer. A regional ocean
circulation model for the mid-cretaceous north atlantic basin: implications for black
shale formation. Climate of the Past, 7(1):277–297, 2011.
[152]
W. A. McLellan, H. N. Koopman, S. A. Rommel, A. J. Read, C. W. Potter,
J. R. Nicolas, A. J. Westgate, and D. A. Pabst. Ontogenetic allometry and body
composition of harbour porpoises (Phocoena phocoena, l.) from the western north
atlantic. Journal of Zoology, 257(4):457–471, August 2002.
[153]
Bin Tang, Yujiang Hao, Chaoqun Wang, Zhengyu Deng, Guilin Shu, Kexiong
Wang, and Ding Wang. Variation of blubber thickness of the yangtze finless por-
poise (Neophocaena asiaeorientalis asiaeorientalis) in human care: Adaptation to
environmental temperature. Water Biology and Security, 2(4):100200, October 2023.
57
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[154]
M. A. Castellini, S. J. Trumble, T. L. Mau, P. K. Yochem, B. S. Stewart, and M. A.
Koski. Body and blubber relationships in antarctic pack ice seals: implications
for blubber depth patterns. Physiological and Biochemical Zoology, 82(2):113–120,
March 2009.
[155]
D. M. S. Watson. The elasmosaurid shoulder-girdle and fore-limb. Proceedings of
the Zoological Society of London, 94(3):885–917, September 1924.
[156]
LB Tarlo. The scapula of pliosaurus macromerus phillips. Palaeontology, 1(3):193–
199, 1957.
[157]
LB Halstead. Plesiosaur locomotion. Journal of the Geological Society, 146(1):37–40,
1989.
[158]
Kenneth Carpenter and Frank Sanders. Plesiosaur swimming as interpreted from
skeletal analysis and experimental results. Transactions of the Kansas Academy of
Science, 113(1/2):1, apr 2010.
[159]
R Araujo and F Correia. Soft-tissue anatomy of the plesiosaur pectoral girdle inferred
from basal eosauropterygia taxa and the extant phylogenetic bracket. Palaeontologia
Electronica, 2015.
[160]
Anna Krahl and Ulrich Witzel. Foreflipper and hindflipper muscle reconstructions
of Cryptoclidus eurymerus in comparison to functional analogues: introduction of a
myological mechanism for flipper twisting. PeerJ, 9:e12537, December 2021.
[161]
Luke E. Muscutt, Gareth Dyke, Gabriel D. Weymouth, Darren Naish, Colin Palmer,
and Bharathram Ganapathisubramani. The four-flipper swimming method of ple-
siosaurs enabled efficient and effective locomotion. Proceedings of the Royal Society
B: Biological Sciences, 284(1861):20170951, August 2017.
[162]
Ali Pourfarzan and Jaime G. Wong. Constraining optimum swimming strategies
in plesiosaurs: The effect of amplitude ratio on tandem pitching foils. Physics of
Fluids, 34(5), May 2022.
[163]
Brian L Bostrom, T Todd Jones, Mervin Hastings, and David R Jones. Behaviour
and physiology: the thermal strategy of leatherback turtles. PLoS One, 5(11):e13925,
2010.
[164]
Tomoko Narazaki, Katsufumi Sato, Kyler J Abernathy, Greg J Marshall, and
Nobuyuki Miyazaki. Loggerhead turtles (caretta caretta) use vision to forage on
gelatinous prey in mid-water. PLoS One, 8(6):e66043, 2013.
[165]
Chihiro Kinoshita, Takuya Fukuoka, Tomoko Narazaki, Yasuaki Niizuma, and
Katsufumi Sato. Analysis of why sea turtles swim slowly: a metabolic and mechanical
approach. Journal of Experimental Biology, 224(4), feb 2021.
[166]
P. T. Plotkin, M. K. Wicksten, and A. F. Amos. Feeding ecology of the loggerhead sea
turtle Caretta caretta in the northwestern gulf of mexico. Marine Biology, 115(1):1–5,
January 1993.
58
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[167]
DF Amorocho and RD Reina. Feeding ecology of the east pacific green sea turtle
Chelonia mydas agassiziiat gorgona national park, colombia. Endangered Species
Research, 3:43–51, 2007.
[168] Katsufumi Sato, Kozue Shiomi, Yuuki Watanabe, Yutaka Watanuki, Akinori Taka-
hashi, and Paul J. Ponganis. Scaling of swim speed and stroke frequency in ge-
ometrically similar penguins: they swim optimally to minimize cost of transport.
Proceedings of the Royal Society B: Biological Sciences, 277(1682):707–714, November
2009.
[169]
Natsuki Harada, Takuma Oura, Masateru Maeda, Yayi Shen, Dale M. Kikuchi, and
Hiroto Tanaka. Kinematics and hydrodynamics analyses of swimming penguins:
wing bending improves propulsion performance. Journal of Experimental Biology,
224(21), November 2021.
[170]
R. Bannasch. Functional anatomy of the ‘flight’ apparatus in penguins. pages
163–192, September 1994.
[171]
Carolin Kuhn and Eberhard Frey. Walking like caterpillars, flying like bats—pinniped
locomotion. Palaeobiodiversity and Palaeoenvironments, 92(2):197–210, April 2012.
[172]
David P. Hocking, Felix G. Marx, Shibo Wang, David Burton, Mark Thomp-
son, Travis Park, Ben Burville, Hazel L. Richards, Renae Sattler, James Robbins,
Roberto Portela Miguez, Erich M.G. Fitzgerald, David J. Slip, and Alistair R. Evans.
Convergent evolution of forelimb-propelled swimming in seals. Current Biology,
31(11):2404–2409.e2, June 2021.
[173]
Arthur Wm. English. Limb movements and locomotor function in the california sea
lion (Zalophus californianus). Journal of Zoology, 178(3):341–364, March 1976.
[174]
Arthur W. M. English. Structural correlates of forelimb function in fur seals and sea
lions. Journal of Morphology, 151(3):325–352, March 1977.
[175]
Caitlin E. Kielhorn, Richard M. Dillaman, Stephen T. Kinsey, William A. McLellan,
D. Mark Gay, Jennifer L. Dearolf, and D. Ann Pabst. Locomotor muscle profile of a
deep (Kogia breviceps) versus shallow (Tursiops truncatus) diving cetacean. Journal
of Morphology, 274(6):663–675, January 2013.
[176]
Robert O. Clark, F. Robin O’Keefe, and Sara E. Slack. A new genus of small
polycotylid plesiosaur from the upper cretaceous of the western interior seaway and
a clarification of the genus Dolichorhynchops.Cretaceous Research, page 105812,
December 2023.
[177]
R_T Bakker. Plesiosaur extinction cycles—events that mark the beginning, middle
and end of the cretaceous. Evolution of the western interior basin. Geological
Association of Canada, Special Paper, 39:641–664, 1993.
[178]
Theagarten Lingham-Soliar. Plesiosaur locomotion: Is the four-wing problem real or
merely an atheoretical exercise? Neues Jahrbuch für Geologie und Paläontologie -
Abhandlungen, 217(1):45–87, jul 2000.
[179]
Mark Cruz DeBlois. Quantitative reconstruction and two-dimensional, steady flow
hydrodynamics of the plesiosaur flipper, 2013.
59
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[180]
Alfred Sherwood Romer, Larry W Price, and Llewellyn Ivor Price. Review of the
Pelycosauria, volume 28. Geological Society of America, 1940.
[181]
ARI Cruickshank, DM Martill, and LF Noe. A pliosaur (reptilia, sauropterygia)
exhibiting pachyostosis from the middle jurassic of england. Journal of the Geological
Society, 153(6):873–879, 1996.
[182]
F Robin O’keefe. Preliminary description and phylogenetic position of a new
plesiosaur (reptilia: Sauropterygia) from the toarcian of holzmaden, germany. Journal
of Paleontology, 78(5):973–988, 2004.
[183]
FRANZISKA GROßMANN. The taxonomic and phylogenetic position of the ple-
siosauroidea from the lower jurassic posidonia shale of south-west germany. Palaeon-
tology, 50(3):545–564, may 2007.
[184]
José P O’Gorman. New insights on the Aristonectes parvidens (plesiosauria, elas-
mosauridae) holotype: news on an old specimen. Ameghiniana, 53(4):397–417,
2016.
[185]
Tamaki Sato, Xiao-Chun Wu, Alex Tirabasso, and Paul Bloskie. Braincase of
a polycotylid plesiosaur (reptilia: Sauropterygia) from the upper cretaceous of
manitoba, canada. Journal of Vertebrate Paleontology, 31(2):313–329, March 2011.
[186]
Marcela Gómez-Pérez. The palaeobiology of an exceptionally preserved colombian
pliosaur (sauropterygia: Plesiosauria), 2009.
[187]
Espen M Knutsen, Patric S Druckenmiller, and Jørn H Hurum. A new species
of Pliosaurus (sauropterygia: Plesiosauria) from the middle volgian of central
spitsbergen, norway. Norwegian Journal of Geology, 92:235–258, 2012.
[188]
Richard Owen. Monographs on the British Fossil Reptilia from the Kimmeridge Clay.
No. III, Containing Pliosaurus Grandis, Pl. Trochanterius, and Pl. Portlandicus.
Pages 1–12; Plates I—IV, volume 22. Taylor & Francis, 1869.
[189]
Adam S. Smith and Gareth J. Dyke. The skull of the giant predatory pliosaur Rhoma-
leosaurus cramptoni : implications for plesiosaur phylogenetics. Naturwissenschaften,
95(10):975–980, jun 2008.
[190]
F Robin O’keefe. Cranial anatomy and taxonomy of Dolichorhynchops bonneri new
combination, a polycotylid (sauropterygia: Plesiosauria) from the pierre shale of
wyoming and south dakota. Journal of Vertebrate Paleontology, 28(3):664–676, 2008.
[191]
Bruce A. Schumacher, Kenneth Carpenter, and Michael J. Everhart. A new cretaceous
pliosaurid (reptilia, plesiosauria) from the carlile shale (middle turonian) of russell
county, kansas. Journal of Vertebrate Paleontology, 33(3):613–628, may 2013.
[192]
Judyth Sassoon, Leslie F. Noè, and Micheal J. Benton. Cranial anatomy, tax-
onomic implications and palaeopathology of an upper jurassic pliosaur (reptilia:
Sauropterygia) from westbury, wiltshire, UK. Palaeontology, 55(4):743–773, may
2012.
60
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[193]
Roger B. J. Benson, Mark Evans, Adam S. Smith, Judyth Sassoon, Scott Moore-Faye,
Hilary F. Ketchum, and Richard Forrest. A giant pliosaurid skull from the late
jurassic of england. PLoS ONE, 8(5):e65989, may 2013.
[194]
Adam S. Smith and Ricardo Araújo. Thaumatodracon wiedenrothi, a morphome-
trically and stratigraphically intermediate new rhomaleosaurid plesiosaurian from
the lower jurassic (sinemurian) of lyme regis. Palaeontographica Abteilung A, 308(4-
6):89–125, jul 2017.
[195]
Hilary F. Ketchum and Adam S. Smith. The anatomy and taxonomy of Macroplata
tenuiceps (sauropterygia, plesiosauria) from the hettangian (lower jurassic) of war-
wickshire, united kingdom. Journal of Vertebrate Paleontology, 30(4):1069–1081, jul
2010.
[196]
Roger B. J. Benson and Timothy Bowdler. Anatomy of Colymbosaurus megadeirus
(reptilia, plesiosauria) from the kimmeridge clay formation of the u.k., and high
diversity among late jurassic plesiosauroids. Journal of Vertebrate Paleontology,
34(5):1053–1071, July 2014.
[197]
Orville Bonner. An osteological study of Nyctosaurus and Trinacromerum with
a description of a new species of Nyctosaurus. Master’s thesis, Fort Hays State
University, 1964.
[198]
Yves Lepage, Eric Buffetaut, and Gilles Lepage. Qu’est-ce que Tapinosaurus? lennier,
rabeck et les grands sauroptérygiens du kimméridgien supérieur de la région havraise
(normandie, france). Bulletin de la Société géologique de Normandie et des amis du
Muséum du Havre (2004), 96(1):27–59, 2009.
[199]
Johan Iraeus, Karin Brolin, and Bengt Pipkorn. Generic finite element models of
human ribs, developed and validated for stiffness and strain prediction–to be used
in rib fracture risk evaluation for the human population in vehicle crashes. Journal
of the Mechanical Behavior of Biomedical Materials, 106:103742, 2020.
[200]
CW Andrews. On the development of the shoulder-girdle of a plesiosaur (Cryptoclidus
oxoniensis, phillips, sp.) from the oxford clay. Journal of Natural History, 15(88):333–
346, 1895.
[201]
Georgina Bunker, David M. Martill, Roy E. Smith, Samir Zouhri, and Nick Longrich.
Plesiosaurs from the fluvial kem kem group (mid-cretaceous) of eastern morocco and
a review of non-marine plesiosaurs. Cretaceous Research, 140:105310, dec 2022.
[202]
Ricardo Araújo and Adam S Smith. Recognising and quantifying the evolution of
skeletal paedomorphosis in plesiosauria. Fossil Record, 26(1):85–101, 2023.
[203]
F Robin O’Keefe and Norton Hiller. Morphologic and ontogenetic patterns in
elasmosaur neck length, with comments on the taxonomic utility of neck length
variables. 2006.
[204]
Arthur S. Brum, Tiago R. Simões, Geovane A. Souza, André E. P. Pinheiro, Ro-
drigo G. Figueiredo, Michael W. Caldwell, Juliana M. Sayão, and Alexander W. A.
Kellner. Ontogeny and evolution of the elasmosaurid neck highlight greater diversity
of antarctic plesiosaurians. Palaeontology, 65(2), mar 2022.
61
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[205]
F R O’Keefe, P M Sander, T Wintrich, and S Werning. Ontogeny of polycotylid
long bone microanatomy and histology. Integrative Organismal Biology, 1(1), jan
2019.
[206]
Christina Joanne Byrd. Ontogenetic state of a juvenile polycotylid plesiosaur
(sauropterygia: Plesiosauria) and its implications for plesiosaur growth. 2013.
[207]
María E. Páramo-Fonseca, Cristian D. Benavides-Cabra, and Ingry E. Gutiérrez.
A new specimen of Stenorhynchosaurus munozi páramo-fonseca et al., 2016 (ple-
siosauria, pliosauridae), from the barremian of colombia: new morphological features
and ontogenetic implications. Journal of Vertebrate Paleontology, 39(4):e1663426,
July 2019.
[208]
R. Araújo, M.J. Polcyn, J. Lindgren, L.L. Jacobs, A.S. Schulp, O. Mateus, A. Olímpio
Gonçalves, and M.-L. Morais. New aristonectine elasmosaurid plesiosaur specimens
from the early maastrichtian of angola and comments on paedomorphism in ple-
siosaurs. Netherlands Journal of Geosciences - Geologie en Mijnbouw, 94(1):93–108,
feb 2015.
[209]
Marie-Céline Buchy, Eberhard Frey, Wolfgang Stinnesbeck, and José Guadalupe
López-Oliva. First occurrence of a gigantic pliosaurid plesiosaur in the late jurassic
(kimmeridgian) of mexico. Bulletin de la Société géologique de France, 174(3):271–278,
2003.
[210]
Per Ove Persson. A revision of the classification of the Plesiosauria with a synopsis
of the stratigraphical and geographical distribution of the group. Gleerup, 1963.
[211]
F Robin O’Keefe. A cladistic analysis and taxonomic revision of the plesiosauria
(reptilia: Sauropterygia). Acta Zoologica Fennica, 213:1–63, 2001.
[212]
Rodrigo A. Otero. Taxonomic reassessment of Hydralmosaurus as Styxosaurus:
new insights on the elasmosaurid neck evolution throughout the cretaceous. PeerJ,
4:e1777, mar 2016.
[213]
V. Fischer, R. B. J. Benson, P. S. Druckenmiller, H. F. Ketchum, and N. Bardet.
The evolutionary history of polycotylid plesiosaurians. Royal Society Open Science,
5(3):172177, March 2018.
[214]
Samuel Paul Welles. A new species of elasmosaur from the aptian of colombia and
a review of the cretaceous plesiosaurs. University of California publications in the
Geological sciences, 44:1–96, 1962.
[215]
Donald M. Henderson. Floating point: a computational study of buoyancy, equilib-
rium, and gastroliths in plesiosaurs. Lethaia, 39(3):227–244, sep 2006.
[216]
Valentin Fischer, Roger BJ Benson, Nikolay G Zverkov, Laura C Soul, Maxim S
Arkhangelsky, Olivier Lambert, Ilya M Stenshin, Gleb N Uspensky, and Patrick S
Druckenmiller. Plasticity and convergence in the evolution of short-necked plesiosaurs.
Current Biology, 27(11):1667–1676, 2017.
[217]
Valentin Fischer, Jamie A. MacLaren, Laura C. Soul, Rebecca F. Bennion, Patrick S.
Druckenmiller, and Roger B. J. Benson. The macroevolutionary landscape of short-
necked plesiosaurians. Scientific Reports, 10(1), October 2020.
62
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[218]
Bruce A Schumacher and James E Martin. Polycotylus latipinnis cope (plesiosauria,
polycotylidae), a nearly complete skeleton from the niobrara formation (early
campanian) of southwestern south dakota. Journal of Vertebrate Paleontology,
36(1):e1031341, 2016.
[219]
Samuel Paul Welles and James D Bump. Alzadasaurus pembertoni, a new elasmosaur
from the upper cretaceous of south dakota. Journal of Paleontology, pages 521–535,
1949.
[220]
Peggy Vincent, Robert Weis, Guy Kronz, and Dominique Delsate. Microcleidus
melusinae, a new plesiosaurian (reptilia, plesiosauria) from the toarcian of luxembourg.
Geological Magazine, 156(1):99–116, 2019.
[221]
Theodore Elmer White. On the skull of kronosaurus queenslandicus longman.
Occasional Papers of the Boston Society of Natural History, 1935.
[222]
Pat S Druckenmiller. Osteology of a new plesiosaur from the lower cretaceous (albian)
thermopolis shale of montana. Journal of Vertebrate Paleontology, 22(1):29–42, 2002.
[223]
David M. Martill, Megan L. Jacobs, and Roy E. Smith. A truly gigantic pliosaur
(reptilia, sauropterygia) from the kimmeridge clay formation (upper jurassic, kim-
meridgian) of england. Proceedings of the Geologists’ Association, 134(3):361–373,
June 2023.
[224] G. A. F. Seber and C. J. Wild. Nonlinear Regression. Wiley, February 1989.
[225]
Lois S. Russell. A plesiosaur from the upper cretaceous of manitoba. Journal of
Paleontology, 1935.
[226]
Hanns-Christian Gunga, Tim Suthau, Anke Bellmann, Stefan Stoinski, Andreas
Friedrich, Tobias Trippel, Karl Kirsch, and Olaf Hellwich. A new body mass
estimation of Brachiosaurus brancai janensch, 1914 mounted and exhibited at the
museum of natural history (berlin, germany). Fossil Record, 11(1):33–38, February
2008.
[227]
Marina A. Piscitelli, William A. McLellan, Sentiel A. Rommel, James E. Blum,
Susan G. Barco, and D. Ann Pabst. Lung size and thoracic morphology in shallow-
and deep-diving cetaceans. Journal of Morphology, 271(6):654–673, January 2010.
[228]
Sam H. Ridgway and Red Howard. Dolphin lung collapse and intramuscular circula-
tion during free diving: evidence from nitrogen washout. Science, 206(4423):1182–
1183, December 1979.
[229]
Konrad J. Falke, Roger D. Hill, Jesper Qvist, Robert C. Schneider, Michael Guppy,
Graham C. Liggins, Peter W. Hochachka, Richard E. Elliott, and Warren M. Zapol.
Seal lungs collapse during free diving: evidence from arterial nitrogen tensions.
Science, 229(4713):556–558, August 1985.
[230]
Peter L. Tyack, Mark Johnson, Natacha Aguilar Soto, Albert Sturlese, and Peter T.
Madsen. Extreme diving of beaked whales. Journal of Experimental Biology,
209(21):4238–4253, November 2006.
63
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[231]
Harold Berkson. Physiological adjustments to deep diving in the pacific green turtle
(chelonia mydas agassizii). Comp. Biochem. Physiol, 21(3):507–524, 1967.
[232]
Colm Murphy, Denis Kelliher, and John Davenport. Shape and material characteris-
tics of the trachea in the leatherback sea turtle promote progressive collapse and
reinflation during dives. Journal of Experimental Biology, January 2012.
[233]
John Davenport, T. Todd Jones, Thierry M. Work, and George H. Balazs. Unique
characteristics of the trachea of the juvenile leatherback turtle facilitate feeding,
diving and endothermy. Journal of Experimental Marine Biology and Ecology,
450:40–46, January 2014.
[234]
Ruizhe Jackevan Zhao. Estimating body volumes and surface areas of animals from
cross-sections. bioRxiv, pages 2023–10, 2023.
[235]
R Core Team. R: A Language and Environment for Statistical Computing. R
Foundation for Statistical Computing, Vienna, Austria, 2022.
[236]
E. K. Baken, M. L. Collyer, A. Kaliontzopoulou, and D. C. Adams. geomorph v4.0
and gmshiny: enhanced analytics and a new graphical interface for a comprehensive
morphometric experience. Methods in Ecology and Evolution, 12:2355–2363, 2021.
[237]
D. C. Adams, M. L. Collyer, A. Kaliontzopoulou, and E. K. Baken. Geomorph:
Software for geometric morphometric analyses. r package version 4.0.6, 2023.
[238]
C. Ritz, F. Baty, J. C. Streibig, and D. Gerhard. Dose-response analysis using r.
PLOS ONE, 10(e0146021), 2015.
[239]
Hadley Wickham. ggplot2: Elegant Graphics for Data Analysis. Springer-Verlag
New York, 2016.
[240]
Alboukadel Kassambara. ggpubr: ’ggplot2’ Based Publication Ready Plots, 2023. R
package version 0.6.0.
[241]
Claus O. Wilke. cowplot: Streamlined Plot Theme and Plot Annotations for ’ggplot2’,
2024. R package version 1.1.3.
[242]
Richard Owen. Report on British Fossil Reptiles..., volume 2. R. and JE Taylor,
1841.
[243] Richard Ellis. Sea dragons: predators of the prehistoric oceans. University Press of
Kansas, 2003.
[244]
Tim Haines. Walking with dinosaurs: A natural history. DK Publishing (Dorling
Kindersley), 1999.
[245]
Daniel Madzia, Sven Sachs, and Johan Lindgren. Morphological and phylogenetic
aspects of the dentition of Megacephalosaurus eulerti, a pliosaurid from the turonian
of kansas, usa, with remarks on the cranial anatomy of the taxon. Geological
Magazine, 156(07):1201–1216, July 2018.
64
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[246]
María Eurídice Páramo-Fonseca, Marcela Gómez-Pérez, Leslie F. Noè, and Fernando
Etayo-Serna. Stenorhynchosaurus munozi, gen. et sp. nov. a new pliosaurid from the
upper barremian (lower cretaceous) of villa de leiva, colombia, south america. Revista
de la Academia Colombiana de Ciencias Exactas, Físicas y Naturales, 40(154):84,
March 2016.
[247]
Sven Sachs, Daniel Madzia, Ben Thuy, and Benjamin P. Kear. The rise of macro-
predatory pliosaurids near the early-middle jurassic transition. Scientific Reports,
13(1), October 2023.
[248]
LB Tarlo. Pliosaurus brachyspondylus (owen) from the kimmeridge clay. Palaeontol-
ogy, 1(4):283–291, 1959.
[249]
J. L. GILBERT. The geologists’ association. Nature, 200(4913):1271–1272, December
1963.
[250]
Femke M. Holwerda, Mark Evans, and Jeff J. Liston. Additional sauropod dinosaur
material from the callovian oxford clay formation, peterborough, uk: evidence for
higher sauropod diversity. PeerJ, 7:e6404, February 2019.
[251]
WC Knight. A new jurassic plesiosaur from wyoming. Science, 2(40):449–449, 1895.
[252]
W. C. Knight. Some new jurassic vertebrates from wyoming. American Journal of
Science, 4:378–381, 1898.
[253]
William Wahl, Mike Ross, and Judy Massare. Rediscovery of wilbur knight’s
megalneusaurus rex site: new material from an old pit. Paludicola, 6:94–104, 07
2007.
[254] JB Delair. The mesozoic reptiles of Dorset, volume 79. 1958.
[255]
John Phillips. Geology of Oxford and the Valley of the Thames. Clarendon Press,
1871.
[256]
Henri Émile Sauvage. Prodrome des Plésiosauriens et des Elasinosauriens des
formations Jurassiques supéricures de Boulogne-sur-Mer. 1878.
[257]
Henri Émile Sauvage. Mémoire sur les dinosauriens et les crocodiliens des terrains
jurassiques de Boulogne-sur-Mer. Société géologique de France, 1874.
[258]
Henri Emile Sauvage. Les dinosauriens du terrain jurassique supérieur du boulonnais.
Number 22. 1895.
[259]
G Lennier. Description des fossiles du cap de la hève. Bulletin de la Société Géologique
de Normandie, Le Harve, 12:17–98, 1887.
[260]
Geo Rabeck. Notes sur la découverte d’ossements de dinosaurien dans les argi-
lessupérieures kimméridgiennes du cap de la hève (octeville-sur-mer). Bulletin de
laSociété Géologique de ormandie, 34:72–74, 1925.
[261]
Marie-Céline Buchy, Eberhard Frey, Wolfgang Stinnesbeck, and José Guadalupe
López-Oliva. An annotated catalogue of the upper jurassic (kimmeridgian and
tithonian) marine reptiles in the collections of the universidad autónoma de nuevo
león, facultad de ciencias de la tierra, linares, mexico. Oryctos, 6:1–18, 2006.
65
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[262]
Luis A Spalletti, Zulma Gasparini, Gonzalo Veiga, Ernesto Schwarz, Marta Fernandez,
and Sergio Matheos. Facies anóxicas, procesos deposicionales y herpetofauna de
la rampa marina titoniano-berriasiana en la cuenca neuquina (yesera del tromen),
neuquén, argentina. Revista geológica de Chile, 26(1):109–123, 1999.
[263]
Joseph Prestwich. Geology: chemical, physical, and stratigraphical, volume 2. Claren-
don Press, 1888.
[264]
Espen M Knutsen. A taxonomic revision of the genus pliosaurus (owen, 1841a) owen,
1841b. Norwegian Journal of Geology, 92:259–276, 2012.
[265]
Peggy Vincent and Glenn W. Storrs. Lindwurmia, a new genus of plesiosauria
(reptilia: Sauropterygia) from the earliest jurassic of halberstadt, northwest germany.
The Science of Nature, 106(1-2), jan 2019.
[266]
Richard Forrest. A large rhomaleosaurid pliosaur from the upper lias of rutland.
Mercian Geologist, 15(1):37–40, 2000.
[267]
Valentin Fischer, Robert Weis, and Ben Thuy. Refining the marine reptile turnover
at the early–middle jurassic transition. PeerJ, 9:e10647, feb 2021.
[268]
Zulma Gasparini et al. A new pliosaur from the bajocian of the neuquen basin,
argentina. Palaeontology, 40(1):135–148, 1997.
[269]
Sven Sachs, Pascal Abel, and Daniel Madzia. A ‘long-forgotten’ plesiosaur provides
evidence of large-bodied rhomaleosaurids in the middle jurassic of germany. Journal
of Vertebrate Paleontology, 42(5), November 2022.
[270]
Roger Benson, Nikolay Zverkov, and Maxim Arkhangelsky. Youngest occurrences of
rhomaleosaurid plesiosaurs indicate survival of an archaic marine reptile clade at
high palaeolatitudes. Acta Palaeontologica Polonica, 2015.
[271]
Peggy Vincent. A re-examination of Hauffiosaurus zanoni, a pliosauroid from the
toarcian (early jurassic) of germany. Journal of Vertebrate Paleontology, 31(2):340–
351, mar 2011.
[272]
Theodore E White. Holotype of Plesiosaurus longirostris blake and classification of
the plesiosaurs. Journal of Paleontology, pages 451–467, 1940.
[273]
Benjamin P. Kear. A revision of australia’s jurassic plesiosaurs. Palaeontology,
55(5):1125–1138, August 2012.
[274]
Sven Sachs, Christian Klug, and Benjamin P. Kear. Rare evidence of a giant
pliosaurid-like plesiosaur from the middle jurassic (lower bajocian) of switzerland.
Swiss Journal of Palaeontology, 138(2):337–342, November 2019.
[275]
Judy A. Massare. Tooth morphology and prey preference of mesozoic marine reptiles.
Journal of Vertebrate Paleontology, 7(2):121–137, June 1987.
[276]
Michael Alan Taylor. Functional anatomy of the head of the large aquatic predator
Rhomaleosaurus zetlandicus (plesiosauria, reptilia) from the toarcian (lower jurassic)
of yorkshire, england. Philosophical Transactions of the Royal Society of London.
Series B: Biological Sciences, 335(1274):247–280, feb 1992.
66
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[277]
Leslie F. Noè, Jeff Liston, and Mark Evans. The first relatively complete exoccipital-
opisthotic from the braincase of the callovian pliosaur, Liopleurodon.Geological
Magazine, 140(4):479–486, July 2003.
[278]
L BEVERLY Halstead. Liopleurodon rossicus (novozhilov)- a pliosaur from the
lower volgian of the moscow basin. Palaeontology, 14(4):566–570, 1971.
[279]
Valentin Fischer, Maxim S. Arkhangelsky, Ilya M. Stenshin, Gleb N. Uspensky,
Nikolay G. Zverkov, and Roger B. J. Benson. Peculiar macrophagous adaptations in
a new cretaceous pliosaurid. Royal Society Open Science, 2(12):150552, December
2015.
[280]
LF Noè and M Gómez-Pérez. Plesiosaurs, palaeoenvironments, and the paja forma-
tion lagerstätte of central colombia: an overview. The geology of Colombia, 2:441–483,
2020.
[281]
Timothy Holland. The mandible of Kronosaurus queenslandicus longman, 1924
(pliosauridae, brachaucheniinae), from the lower cretaceous of northwest queensland,
australia. Journal of Vertebrate Paleontology, 38(5):e1511569, September 2018.
[282]
Stephen F Poropat, Phil R Bell, Lachlan J Hart, Steven W Salisbury, and Benjamin P
Kear. An annotated checklist of australian mesozoic tetrapods. Alcheringa: An
Australasian Journal of Palaeontology, 47(2):129–205, 2023.
[283]
Marcela Gómez-Pérez and Leslie F Noè. Cranial anatomy of a new pliosaurid
Acostasaurus pavachoquensis from the lower cretaceous of colombia, south america.
Palaeontographica Abteilung A, 310(1-2):5–42, 2017.
[284]
Daniel Madzia. A reappraisal of Polyptychodon (plesiosauria) from the cretaceous of
england. PeerJ, 4:e1998, May 2016.
[285]
Owen. A monograph on the fossil reptilia of the cretaceous formations. supplement no.
iii. pterosauria (Pterodactylus) and sauropterygia (Polyptychodon). London:Printed
for the Palæontographical society, pages 1–25, 1861.
[286]
Nikolay G. Zverkov and Evgeny M. Pervushov. A gigantic pliosaurid from the
cenomanian (upper cretaceous) of the volga region, russia. Cretaceous Research,
110:104419, June 2020.
[287]
Michael J Everhart. Oceans of Kansas: a natural history of the Western Interior
Sea. Indiana University Press, 2017.
[288]
Sven Sachs, Markus Wilmsen, Joschua Knueppe, Jahn J Hornung, and Benjamin P
Kear. Cenomanian-turonian marine amniote remains from the saxonian cretaceous
basin of germany. Geological Magazine, 154(2):237–246, 2017.
[289]
Bruce A Schumacher, Kenneth Carpenter, and Michael J Everhart. A new cretaceous
pliosaurid (reptilia, plesiosauria) from the carlile shale (middle turonian) of russell
county, kansas. Journal of Vertebrate Paleontology, 33(3):613–628, 2013.
[290]
L Barry Albright, David D Gillette, and Alan L Titus. Plesiosaurs from the upper
cretaceous (cenomanian-turonian) tropic shale of southern utah, part 1: new records
of the pliosaur Brachauchenius lucasi.Journal of Vertebrate Paleontology, 27(1):31–
40, 2007.
67
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[291]
D Angst and N Bardet. A new record of the pliosaur brachauchenius lucasi williston,
1903 (reptilia: Sauropterygia) of turonian (late cretaceous) age, morocco. Geological
Magazine, 153(3):449–459, 2016.
[292]
Bruce A. Schumacher. On the skull of a pliosaur (plesiosauria; pliosauridae) from the
upper cretaceous (early turonian) of the north american western interior. Transactions
of the Kansas Academy of Science, 111:203–218, September 2008.
[293]
Glenn W Storrs. Morphological and taxonomic clarification of the genus plesiosaurus.
In Ancient marine reptiles, pages 145–190. Elsevier, 1997.
[294]
Leonie Schwermann. Osteologie und phylogenie von Westphaliasaurus simonsensii :
Ein neuer plesiosauride (sauropterygia) aus dem unteren jura (pliensbachium) von
sommersell (kreis höxter), nordrhein-westfalen, deutschland. Geol. Paläont. Westf.,
79:1, 2011.
[295]
Othniel Charles Marsh. Geological horizons as determined by vertebrate fossils.
American Journal of Science (1880-1910), 42(250):336, 1891.
[296]
F Robin O’Keefe and William Wahl Jr. Preliminary report on the osteology and
relationships of a new aberrant cryptocleidoid plesiosaur from the sundance formation,
wyoming. 2003.
[297]
F Robin O’Keefe and William Wahl Jr. Current taxonomic status of the plesiosaur
pantasaurus striatus from the upper jurassic sundance formation, wyoming. 2003.
[298]
Rodrigo A. Otero, Jhonatan Alarcón-Muñoz, Sergio Soto-Acuña, Jennyfer Rojas, Os-
valdo Rojas, and Héctor Ortíz. Cryptoclidid plesiosaurs (sauropterygia, plesiosauria)
from the upper jurassic of the atacama desert. Journal of Vertebrate Paleontology,
40(1):e1764573, January 2020.
[299]
Espen M Knutsen, Patric S Druckenmiller, and Jørn H Hurum. Two new species of
long-necked plesiosaurians (reptilia: Sauropterygia) from the upper jurassic (middle
volgian) agardhfjellet formation of central spitsbergen. Norwegian Journal of Geology,
92:187–212, 2012.
[300]
José P. O’Gorman, Leonardo Salgado, Eduardo B. Olivero, and Sergio A. Marenssi.
Vegasaurus molyi, gen. et sp. nov. (plesiosauria, elasmosauridae), from the cape lamb
member (lower maastrichtian) of the snow hill island formation, vega island, antarc-
tica, and remarks on wedellian elasmosauridae. Journal of Vertebrate Paleontology,
35(3), May 2015.
[301]
Espen M Knutsen, Patrick S Druckenmiller, and Jørn H Hurum. A new plesiosauroid
(reptilia: Sauropterygia) from the agardhfjellet formation (middle volgian) of central
spitsbergen, norway. Norwegian Journal of Geology, 92:213–234, 2012.
[302]
Espen M Knutsen, Patric S Druckenmiller, and Jørn H Hurum. Redescription and
taxonomic clarification of ‘Tricleidus’ svalbardensis based on new material from the
agardhfjellet formation (middle volgian). Norwegian Journal of Geology, 92:175–186,
2012.
68
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[303]
Maxim S. Arkhangelsky, Nikolay G. Zverkov, Mikhail A. Rogov, Ilya M. Stenshin, and
Evgeniya M. Baykina. Colymbosaurines from the upper jurassic of european russia
and their implication for palaeobiogeography of marine reptiles. Palaeobiodiversity
and Palaeoenvironments, 100(1):197–218, September 2019.
[304]
A. Hallam. A review of the broad pattern of jurassic sea-level changes and their
possible causes in the light of current knowledge. Palaeogeography, Palaeoclimatology,
Palaeoecology, 167(1-2):23–37, March 2001.
[305]
Heather M. Stoll and Daniel P. Schrag. Evidence for glacial control of rapid sea level
changes in the early cretaceous. Science, 272(5269):1771–1774, 1996.
[306]
A. Yu. Berezin. Early cretaceous plesiosaurs colymbosaurinae (cryptoclididae, ple-
siosauridae): Structural features and paleogeographic distribution. The Cretaceous
System of Russia and Neighboring Countries: Problems of Stratigraphy and Paleo-
geography, 2022.
[307]
A. Yu. Berezin. A new plesiosaur of the family aristonectidae from the early
cretaceous of the center of the russian platform. Paleontological Journal, 45(6):648–
660, November 2011.
[308]
Benjamin P Kear. Marine reptiles from the lower cretaceous of south australia:
elements of a high-latitude cold-water assemblage. Palaeontology, 49(4):837–856,
July 2006.
[309]
Valentin Fischer, Nikolay G Zverkov, Maxim S Arkhangelsky, Ilya M Stenshin, Ivan V
Blagovetshensky, and Gleb N Uspensky. A new elasmosaurid plesiosaurian from the
early cretaceous of russia marks an early attempt at neck elongation. Zoological
Journal of the Linnean Society, 192(4):1167–1194, October 2020.
[310]
Benjamin P Kear. Cretaceous marine amniotes of australia: perspectives on a decade
of new research. Memoirs of Museum Victoria, 74:17–28, 2016.
[311]
Tony Thulborn and Susan Turner. An elasmosaur bitten by a pliosaur. Modern
Geology, 18:489–501, 01 1993.
[312]
M.-C. Buchy. An elasmosaur (reptilia: Sauropterygia) from the turanian (upper
cretaceous) of morocco. Carolinea, 63:5–28, 2005.
[313]
Sven Sachs and Benjamin P. Kear. Redescription of the elasmosaurid plesiosaurian
libonectes atlasense from the upper cretaceous of morocco. Cretaceous Research,
74:205–222, jun 2017.
[314]
Elliott Armour Smith and F Robin O’Keefe. Occurrence of Styxosaurus (sauroptery-
gia: Plesiosauria) in the cenomanian: implications for relationships of elasmosaurids
of the western interior seaway. Journal of Systematic Palaeontology, 21(1):2242846,
2023.
[315]
SP Welles. The longest neck in the ocean. University of Nebraska News, Museum
Notes, 1970.
[316]
Samuel Wendell Williston. North ameriacan plesiosaurs, Elasmosaurus,Cimo-
liasaurus, and Polycotylus.American Journal of Science, 21:221–236, 1906.
69
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[317]
W Kiprijanoff. Studien uber die fossilien reptilien russland. ii theil. gattung ple-
siosaurus conybeare aus dem severischen sandstein oder osteolith der kreide-gruppe.
Mem. L’Academie Imperiale des Sciences de St.-Petersburg, 7, 1882.
[318]
Danielle J. Serratos, Patrick Druckenmiller, and Roger B. J. Benson. A new elas-
mosaurid (sauropterygia, plesiosauria) from the bearpaw shale (late cretaceous,
maastrichtian) of montana demonstrates multiple evolutionary reductions of neck
length within elasmosauridae. Journal of Vertebrate Paleontology, 37(2):e1278608,
March 2017.
[319]
Norton Hiller, José P. O’Gorman, Rodrigo A. Otero, and Al A. Mannering. A
reappraisal of the late cretaceous weddellian plesiosaur genus Mauisaurus hector,
1874. New Zealand Journal of Geology and Geophysics, 60(2):112–128, February
2017.
[320]
Rodrigo A. Otero. Reappraisal of the first historical record of Aristonectes cabrera,
1941 (elasmosauridae, aristonectinae) from the upper cretaceous of central chile.
Cretaceous Research, 156:105797, April 2024.
[321]
F. Robin O’Keefe, Rodrigo A. Otero, Sergio Soto-Acuña, Jose P. O’gorman, Stephen J.
Godfrey, and Sankar Chatterjee. Cranial anatomy of Morturneria seymourensis
from antarctica, and the evolution of filter feeding in plesiosaurs of the austral late
cretaceous. Journal of Vertebrate Paleontology, 37(4):e1347570, July 2017.
[322]
Elizabeth Lester. Description and histology of a small-bodied elasmosaur and
discription of mortuneria seyemourensis postcranium. 2019.
[323]
Arthur RI Cruickshank and R Ewan Fordyce. A new marine reptile (sauroptery-
gia) from new zealand: further evidence for a late cretaceous austral radiation of
cryptoclidid plesiosaurs. Palaeontology, 45(3):557–575, 2002.
[324]
L Barry Albright III, David D Gillette, and Alan L Titus. Plesiosaurs from the
upper cretaceous (cenomanian–turonian) tropic shale of southern utah, part 2:
polycotylidae. Journal of Vertebrate Paleontology, 27(1):41–58, 2007.
[325]
Donald J. Morgan and F. Robin O’Keefe. The cranial osteology of two specimens
of dolichorhynchops bonneri (plesiosauria, polycotylidae) from the campanian of
south dakota, and a cladistic analysis of the polycotylidae. Cretaceous Research,
96:149–171, April 2019.
[326]
José P. O’Gorman. Polycotylidae (sauropterygia, plesiosauria) from the la colo-
nia formation, patagonia, argentina: Phylogenetic affinities of Sulcusuchus erraini
and the late cretaceous circum-pacific polycotylid diversity. Cretaceous Research,
140:105339, December 2022.
[327]
Benjamin P Kear. Marine reptiles from the lower cretaceous (aptian) deposits
of white cliffs, southeastern australia: implications of a high latitude, cold water
assemblage. Cretaceous Research, 26(5):769–782, 2005.
[328]
Patrick S Druckenmiller and Anthony P Russell. Earliest north american occurrence
of polycotylidae (sauropterygia: Plesiosauria) from the lower cretaceous (albian)
clearwater formation, alberta, canada. Journal of Paleontology, 83(6):981–989, 2009.
70
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[329]
Rebecca Schmeisser McKean. A new species of polycotylid plesiosaur (reptilia:
Sauropterygia) from the lower turonian of utah: Extending the stratigraphic range
of dolichorhynchops. Cretaceous Research, 34:184–199, April 2012.
[330]
Nathalie Bardet, Xabier Pereda Suberbiola, and Nour-Eddine Jalil. A new polycotylid
plesiosaur from the late cretaceous (turonian) of morocco. Comptes Rendus Palevol,
2(5):307–315, 2003.
[331]
Walter Scott Persons, Hallie P. Street, and Amanda Kelley. A long-snouted and
long-necked polycotylid plesiosaur from the late cretaceous of north america. iScience,
25(10):105033, October 2022.
[332]
Kenneth Carpenter. A review of short-necked plesiosaurs from the cretaceous of the
western interior, north america. Neues Jahrbuch für Geologie und Paläontologie-
Abhandlungen, pages 259–287, 1996.
[333]
Dawn A Adams. Trinacromerum bonneri, new species, last and fastest pliosaur of
the western interior seaway. Texas Journal of Science, 49(3):179, 1997.
[334]
Roger BJ Benson, Hilary F Ketchum, Darren Naish, and Langan E Turner. A new
leptocleidid (sauropterygia, plesiosauria) from the vectis formation (early barremian–
early aptian; early cretaceous) of the isle of wight and the evolution of leptocleididae,
a controversial clade. Journal of Systematic Palaeontology, 11(2):233–250, 2013.
[335]
Arthur RI Cruickshank and John A Long. A new species of pliosaurid reptile from the
early cretaceous birdrong sandstone of western australia. Records-Western Australian
Museum, 18:263–276, 1997.
[336]
Benjamin P Kear, Natalie I Schroeder, and Michael S.Y Lee. An archaic crested
plesiosaur in opal from the lower cretaceous high-latitude deposits of australia.
Biology Letters, 2(4):615–619, June 2006.
[337]
Patrick S. Druckenmiller and Anthony P. Russell. Skeletal anatomy of an excep-
tionally complete specimen of a new genus of plesiosaur from the early cretaceous
(early albian) of northeastern alberta, canada. Palaeontographica Abteilung A,
283(1-3):1–33, March 2008.
[338]
Tamaki Sato, Chun Li, and XiaoChun Wu. Restudy of Bishanopliosaurus youngi
dong 1980, a fresh-water plesiosaurian from the jurassic of chongqing. Vertebrata
PalAsiatica, 41(01):17–33, 2003.
[339]
ZM Dong. A new plesiosauria from the lias of sichuan basin. Vertebrata PalAsiatica,
18(3):191, 1980.
[340]
Yuhui Gao, Yong Ye, and Shan Jiang. A new species of Bishanopliosaurus from
middle jurassic of zigong, sichuan. Vertebrata Palasiatica, 42(2):162–165, 2004.
[341]
CC Young. Fossil vertebrates from kuangyuan, n. szechuan, china. Bulletin of the
Geological Society of China, 22(3-4):293–309, 1942.
[342]
CC Young. On the reptilian remains from weiyuan, szechuan, china. Bulletin of the
Geological Society of China, 24(3-4):187–209, 1944.
71
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
[343]
Eric Buffetaut, Varavudh Suteethorn, Haiyan Tong, and Romain Amiot. An early cre-
taceous spinosaurid theropod from southern china. Geological Magazine, 145(5):745–
748, 2008.
[344]
YW Xiao, KH Chen, and DB Cao. First discovery of sinopliosaurus fossil from the
upper triassic in central yunan. Regional Geology of China, 2:180–182, 1991.
[345]
YH Zhang. A new plesiosaur from middle jurassic of sichuan basin. Vertebrata
palasiatica, 23(3):235, 1985.
[346]
Ting Gao, Da-Qing Li, Long-Feng Li, and Jing-Tao Yang. The first record of
freshwater plesiosaurian from the middle jurassic of gansu, nw china, with its
implications to the local palaeobiogeography. Journal of Palaeogeography, 8(1),
August 2019.
[347]
Feng Zhang, Hai-Dong Yu, Can Xiong, Zhao-Ying Wei, Guang-Zhao Peng, and Xue-
Fang Wei. New freshwater plesiosaurian materials from the middle jurassic xintiangou
formation of the sichuan basin, southwestern china. Journal of Palaeogeography,
9(1), August 2020.
72
.CC-BY-NC-ND 4.0 International licenseperpetuity. It is made available under a
preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in
The copyright holder for thisthis version posted February 19, 2024. ; https://doi.org/10.1101/2024.02.15.578844doi: bioRxiv preprint
