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Cranial anatomy of Shunosaurus, a basal sauropod dinosaur from the Middle Jurassic of China


Abstract and Figures

Shunosaurus, from the Middle Jurassic of China, is probably the best-known basal sauropod and is represented by several complete skeletons. It is unique among sauropods in having a small, bony club at the end of its tail. New skull material provides critical information about its anatomy, brain morphology, tooth replacement pattern, feeding habits and phylogenetic relationships. The skull is akinetic and monimostylic. The brain is relatively small, narrow and primitively designed. The tooth replacement pattern exhibits back to front replacement waves in alternating tooth position. The teeth are spatulate, stout and show well-developed wear facets indicative of coarser plant food. Upper and lower tooth rows interdigitate and shear past each other. Tooth morphology, skull architecture, and neck posture indicate that Shunosaurus was adapted to ground feeding or low browsing. Shunosaurus exhibits the following cranial autapomorphies: emargination of the ventral margin of the jugal/quadratojugal bar behind the tooth row; postorbital contains a lateral pit; vomers do not participate in the formation of the choanae; pterygoid is extremely slender and small with a dorsal fossa; quadrate ramus of the pterygoid is forked; quadratojugal participates in the jaw articulation; tooth morphology is a combination of cylindrical and spatulate form; basipterygoid process is not wrapped by the caudal process of the pterygoid; trochlear nerve has two exits; occlusal level of the maxillary tooth row is convex downward, whereas that of the dentary is concave upward, acting like a pair of garden shears; dentary tooth count is 25 or more; and the replacing teeth invade the labial side of the functional teeth. Cranial characters among the basal sauropods are reviewed. As Shunosaurus is the earliest sauropod for which cranial remains are known, it occupies an important position phylogenetically, showing the modification of skull morphology from the prosauropod condition. Although the skull synapomorphies of Sauropoda are unknown at present, 27 cranial synapomorphies are known for the clade Eusauropoda. © 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145−169.
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Zoological Journal of the Linnean Society
, 2002,
, 145– 169. With 12 figures
© 2002 The Linnean Society of London,
Zoological Journal of the Linnean Society,
, 145–169
Blackwell Science, Ltd
Oxford, UK
ZOJZoological Journal of the Linnean Society
0024-4082The Linnean Society of London, 2002September 2002
Original Article
Correspondence. Sankar Chatterjee: E-mail:
Archosaurian anatomy and palaeontology. Essays in memory of
Alick D. Walker.
Edited by D. B. Norman and D. J. Gower
Cranial anatomy of
, a basal sauropod
dinosaur from the Middle Jurassic of China
Museum of Texas Tech University, Box 43191, Lubbock, TX 79409-3191, USA
, from the Middle Jurassic of China, is probably the best-known basal sauropod and is represented by
several complete skeletons. It is unique among sauropods in having a small, bony club at the end of its tail. New skull
material provides critical information about its anatomy, brain morphology, tooth replacement pattern, feeding hab-
its and phylogenetic relationships. The skull is akinetic and monimostylic. The brain is relatively small, narrow and
primitively designed. The tooth replacement pattern exhibits back to front replacement waves in alternating tooth
position. The teeth are spatulate, stout and show well-developed wear facets indicative of coarser plant food. Upper
and lower tooth rows interdigitate and shear past each other. Tooth morphology, skull architecture, and neck posture
indicate that
was adapted to ground feeding or low browsing.
exhibits the following cra-
nial autapomorphies: emargination of the ventral margin of the jugal/quadratojugal bar behind the tooth row; pos-
torbital contains a lateral pit; vomers do not participate in the formation of the choanae; pterygoid is extremely
slender and small with a dorsal fossa; quadrate ramus of the pterygoid is forked; quadratojugal participates in the
jaw articulation; tooth morphology is a combination of cylindrical and spatulate form; basipterygoid process is not
wrapped by the caudal process of the pterygoid; trochlear nerve has two exits; occlusal level of the maxillary tooth
row is convex downward, whereas that of the dentary is concave upward, acting like a pair of garden shears; dentary
tooth count is 25 or more; and the replacing teeth invade the labial side of the functional teeth. Cranial characters
among the basal sauropods are reviewed. As
is the earliest sauropod for which cranial remains are
known, it occupies an important position phylogenetically, showing the modification of skull morphology from the
prosauropod condition. Although the skull synapomorphies of Sauropoda are unknown at present, 27 cranial syn-
apomorphies are known for the clade Eusauropoda. © 2002 The Linnean Society of London,
Zoological Journal of
the Linnean Society
, 2002,
, 145
ADDITIONAL KEYWORDS: sauropod dinosaurs – China – skull anatomy – feeding habits – phylogenetic
Sauropods were undoubtedly the most spectacular of
all dinosaurs and the largest terrestrial animals that
have ever lived. They were the most successful clades
of herbivorous dinosaurs in terms of diversity, abun-
dance and longevity with a temporal span of 160 mil-
lion years (Myr). During the Jurassic and Cretaceous
periods, they were the dominant herbivores, but
their early history is poorly known. Recent discoveries
of early sauropods from China, Thailand, India and
Africa fill important temporal and morphological gaps
in their evolutionary history. The earliest known sau-
ropod is
from the Late Triassic in Thai-
land (Buffetaut
et al
., 2000). By the Early Jurassic,
several sauropod taxa are known from different parts
of Pangea:
from Zimbabwe (Raath, 1972),
from India (Jain
et al
., 1975),
from China (Dong, Zhou
& Zhang, 1983),
from Germany
(Wild, 1978), and several ichnotaxa from Poland and
Italy (Lockley, 1991). During the Late Jurassic, sauro-
pods were widespread globally in considerable abun-
dance and diversity as Pangea rifted and drifted
apart. During the Cretaceous, sauropods waned in
importance and diversity in Laurasia because of
competition with other ornithischians that possessed
© 2002 The Linnean Society of London,
Zoological Journal of the Linnean Society,
, 145–169
sophisticated masticatory apparatus, but they contin-
ued to play a significant role in the continental ecosys-
tems of Gondwana continents (Czerkas & Czerkas,
Sauropods are characterized by having the smallest
skulls and brains relative to their body mass of any of
the dinosaurs, with the lowest encephalization quo-
tients (EQ) (Hopson, 1980a). All have long necks and
tails, sturdy upright limbs and quadrupedal posture,
supported by slender manus and blunt and pendadac-
tyl pes. The best known were of gigantic proportions.
They range in size from about 7 m to as much as 40 m
in length and weighing perhaps as much as 70 tons.
Most, however, were probably in the range of 20–30
tons (McIntosh, 1990a, 1997).
The postmortem loss of the skull from the rest of
the skeleton is a common taphonomic theme in sauro-
pods. Sauropod skulls, which were lightly built and
fragile, often broke off before burial (McIntosh, 1997).
Moreover, because of a weak joint connecting the
occipital condyle with the atlantal ring, there was a
tendency for the light and gracile skull to separate
from the rest of the heavy skeleton and drift down-
stream in the rivers. Consequently, the skulls are
unknown in many genera. Traditionally, the vertebrae
and other postcranial bones have been used in the
description and classification of sauropods, resulting
in nearly 90 different genera and over 150 species
(Dodson, 1990a). The majority of these taxa are based
on fragmentary remains that are not diagnostic at the
generic level. Known sauropod skulls show a great
deal of variation in their shapes and sizes. Some sau-
ropods have long snouts, others are short-faced. Sev-
eral characters such as nature and position of the
external naris, tooth morphology, number of teeth,
wear pattern, and shape of the dental arcade are vari-
able among sauropod genera.
The deficiency of our knowledge of skull anatomy of
early sauropods is partly remedied by the discovery of
several excellent skulls of
from China.
Shunosaurus lii
et al.
, 1983) is known from the
Lower Shaximiao (Middle Jurassic) Formation in
Sichuan province of Central China. The monotypic
species was established on the basis of its unique post-
cranial skeleton, especially the development of an
armour-spiked tail club (Dong
et al.
, 1983).
was a medium-sized (11 m long), short-necked sau-
ropod with long, slender spatulate teeth (Fig. 1). There
are 12 short cervical vertebrae exhibiting opisthoc-
oelous centra with elongated depressions; their neural
spines are low with a weak dorsal notch on the poste-
rior cervicals. However, true spinal bifurcation as seen
is absent in
. There are 13 dorsals, where the centra are
platecoelous and lack pleurocoel; the neural spines are
high but the laminar structures on the neural arch
are atrophied. Since this preliminary description,
material, including several
complete skeletons, has been collected and described
in detail (Dong
et al
., 1983; Zhang, Yang & Peng, 1984;
Zhang, 1988; Zheng, 1991).
is restricted
to the Zigong Dashanpu quarry at the Zigong
Dinosaur Museum.
The skull (ZG65430) described here was collected
from the Dashanpu quarry, but it lacks an associated
postcranial skeleton. It is well preserved but some-
what disarticulated, providing a wealth of three-
dimensional anatomical information. An endocast was
Figure 1.
Skeletal reconstruction of
Shunosaurus lii
(modified from Wilson & Sereno, 1998).
© 2002 The Linnean Society of London,
Zoological Journal of the Linnean Society,
, 145–169
made to reconstruct the brain morphology and cranial
nerves, arteries and veins. The shape of the dental
arcade, tooth morphology and wear, nature of the jaw
articulation, and the neck posture shed new light on
the feeding strategy of
The Jurassic continental sediments of the Sichuan
Basin in central China contain thick successions of flu-
vial, lacustrine and flood plain deposits (Xia, Li & Yi,
1983) (Fig. 2A). The sediments are folded and faulted
and show a plunging anticline where the closure of
the fold is located near Dashanpu, just north-east of
Zigong (Fig. 2B). The Jurassic strata have yielded
a variety of spectacular dinosaurs. Most dinosaur
remains come from the Middle Jurassic Xiashaximiao
Lower Shaximiao) Formation and the Upper Juras-
sic Shangshaximiao (
Upper Shaximiao) Formation.
Dong (1979) recognized two sauropod biozones in
the Sichuan basin corresponding to the two lithos-
tratigraphic formations. In ascending order, they are:
(1) the
fauna of the Lower Shaximiao
Formation (Middle Jurassic); and (2) the
fauna of the Upper Shaximiao Formation
(Upper Jurassic). The Lower Shaximiao Formation is
difficult to correlate with global time scale because the
fauna is endemic. Several freshwater
invertebrate zone fossils such as bivalves (
fauna), conchostracans (
Euestheria zilin-
) and charaophytes (
) suggest a
Middle Jurassic (Bajocian) age (Chen
et al
., 1982).
Dong (1979) placed the
fauna in the
Late Jurassic (Kimmeridgian
Tithonian), comparable
with the fauna of the Morrison Formation of North
America and Tendaguru Beds of Tanzania (Fig. 3).
The skull of
(ZG65430) came from
Zigong County, about 250 km south-west of Chengdu,
the capital city of Sichuan Province (Fig. 2).
is the most abundant taxon in the Zigong Das-
hanpu quarry, representing 90% of the dinosaur
population. At least 10 skeletons of
been discovered at one site, indicating a catastrophic
event, such as a flash flood (Xia
et al.
, 1983). The
assortment of dinosaurs found along with the remains
includes the theropod
, the
, and the primitive stego-
which has unusual spikes and
plates for body armour. This locality is extremely pro-
ductive and has yielded more than 100 individual
dinosaurs along with fishes, amphibians, reptiles and
early mammals (Dong
et al
., 1983). The occurrence
of four sympatric sauropod genera in the Zigong
Dashanpu quarry such as
indicates the diversity
of this group of dinosaurs during the Middle Jurassic
ecosystem of China (Dong, 1992).
It is generally believed that the Jurassic dinosaurs
in China evolved in isolation (Russell & Zheng, 1993;
Russell, 1993; Upchurch, 1995). This is reflected by
the appearance of several diverse and endemic dino-
saur taxa in the Sichuan Basin. These are:
Sinraptor, Monolophosaurus
et al.
, 1984; He, Li & Cai, 1988).
material (ZG65430) was borrowed
from the Zigong Dinosaur Museum of Sichuan prov-
ince, China. It includes a nearly complete skull with
an atlantal ring. The right quadrate and the pterygoid
are disarticulated from the skull, allowing observation
of nearly every detail of the braincase, including var-
ious openings for the cranial nerves, arteries and veins
(Fig. 4). The inner architecture of the braincase is also
visible. Most of the gross preparation was made at the
Institute of Vertebrate Palaeontology and Palaeoan-
thropology (IVPP), Beijing, but the braincase was pre-
pared at the Museum of Texas Tech University. The
skull includes an intact braincase with well preserved
basioccipital, basisphenoid-parasphenoid, prootic, lat-
erosphenoid, orbitosphenoid, supraoccipital, exoccipi-
tal and opisthotic. The braincase, in turn, is fused with
the dorsal skull roof bones (frontal, parietal, squamo-
sal). In the palatal complex, two quadrates, left ptery-
goid, left palatine, left ectopterygoid and left vomer
are preserved. Separate elements of the left premax-
illa, two maxillae and two dentaries are also present.
Preparation was done mechanically using needles and
pneumatic grinders. In the preparation of some deli-
cate parts, especially in the braincase regions, weak
formic acid (5%) was employed.
A beautiful, disarticulated skull of
(DNM 28) from Dinosaur National Monument,
Utah, was available for comparison with that of
. At the Museum of Texas Tech University
collection, casts of several sauropod genera such as
, and
were used for comparative study. Several
braincases of Late Cretaceous Indian sauropods such
as cf.
(ISIR 199 and 467) and cf.
(ISIR 162) at the collection of the Indian Sta-
tistical Institute (ISI) were helpful for comparative
study. For outgroup comparison, a beautiful disartic-
ulated skull of
(IVPP 13246), collected
by the senior author and his field party from the
© 2002 The Linnean Society of London,
Zoological Journal of the Linnean Society,
, 145–169
Lufeng Group of Yunnan Province of China, was very
Zhang (1988) described and illustrated several
skulls of
of which two specimens are rel-
atively complete. One was ascribed to a juvenile indi-
vidual (T5401) and the other to an adult (T5403). In
addition to differences attributable to size, the shape
and snout differed considerably in these two skulls.
That of the former is narrow and pointed, whereas the
latter is rounded and blunt. In that sense, the latter
Figure 2.
A, Sketch map showing the fossil localities of Early and Middle Jurassic dinosaurs in China. B, Generalized geo-
logical map of Zigong area, Sichuan Province, China, showing the location of the Dashanpu quarry where several skeletons
were discovered (simplified from Dong, 1992).
© 2002 The Linnean Society of London,
Zoological Journal of the Linnean Society,
, 145–169
most closely resembles other sauropods, and the
former appears autapomorphic. The specimen in this
study (ZG65430) closely resembles the juvenile. The
disarticulated nature of many cranial elements and
the loss of the interdental plates indicate that the ani-
mal was not fully mature when it died.
Unfortunately, the existing description and diagno-
sis of
by Zhang (1988) is inadequate.
Specimen ZG64430 shares several characters with
described and figured by Zhang. Some
of the characters are used to define Eusauropoda
(Upchurch, 1995; Wilson & Sereno, 1998). These
include: posterodorsal migration of external nares;
snout with stepped anterior margin; absence of antor-
bital fossa; maxillary border of external naris long;
antorbital opening smaller than the external naris;
rostral process of the quadratojugal expands dors-
oventrally; quadrate with a caudal fossa; tooth crown
overlaps with precise occlusion (v-shaped wear facets);
tooth enamel with wrinkled texture; reduced external
mandibular fenestra. The inclusion of ZG64430 within
is based on the following characters: het-
erodont tooth morphology showing both cylindrical
and spatulate teeth; occlusal level of the maxillary
tooth row is convex downward, whereas that of the
dentary is concave upward; emargination of the ven-
tral margin of the maxilla/jugal bar behind the tooth
row; dentary tooth count 25 or more.
Figure 3.
Columnar section of Middle Jurassic to Upper Jurassic strata in Sichuan Basin, China (modified from Dong,
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
Figure 4. Skull of Shunosaurus lii (ZG65430). AC, braincase in lateral, dorsal and ventral views. D & E, left maxilla and
jugal in lateral and medial views. F &G, left dentary in lateral and medial views. H & I, right dentary in lateral and medial
views. J, right maxilla in lateral view. K, right premaxilla in medial view. L, right pterygoid in lateral view. M, right quad-
rate and quadratojugal in medial view.
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
Diagnosis. The following cranial features are unique
to Shunosaurus among sauropods: (1) tooth morphol-
ogy is a combination of cylindrical and spatulate form;
(2) pterygoid is extremely slender and small; (3) ptery-
goid has a fossa on the dorsal aspect; (4) quadrate
ramus of the pterygoid is forked; (5) external nares are
at the level of the orbit; (6) emargination of the ventral
margin of the maxilla/jugal bar behind the tooth row;
(7) vomers do not participate in the formation of the
choanae; (8) quadratojugal participates in the articu-
lation of the jaws; (9) the basisphenoid recess is
extremely deep; (10) trochlear (IV) nerve has two
exits; (11) basipterygoid process is not wrapped by the
caudal process of the pterygoid; (12) postorbital con-
tains a lateral pit; (13) occlusal level of the maxillary
tooth row is convex downward, whereas that of the
dentary is concave upward, like a pair of garden
shears; (14) replacing tooth invades on the labial side
of the functional tooth; (15) dentary tooth count 25 or
Type species. Shunosaurus lii Dong, Zhou & Zhang,
Horizon. Lower Shaximiao Formation, Middle
Jurassic (Fig. 3).
Holotype. V9065-1-23, partial vertebrae and pelvis, in
the collection of IVPP, Beijing, China.
Referred specimens. ZG65430, a nearly complete skull
including an intact braincase and lower jaw (Fig. 4);
T5401, a complete skeleton including skull, lower jaw,
most of the vertebrae, pelvic girdle, sacrum, left
humerus, right femur and a claw; T5403, a complete
skeleton; T5404, a complete postcranial skeleton.
Type locality. Dashanpu, near Zigong city 10 km
north-east (lat. 29°5N; long. 104°50E) of Sichuan
Province, China (Fig. 2).
Specific diagnosis. Same as for genus.
Institutional abbreviations
IVPP The Institute of Vertebrate Palaeontology and
Palaeoanthropology, Academic Sinica, Beijing,
T The Chongqing Natural History Museum,
Chongqing, China.
ZG Zigong Dinosaur Museum, Zigong, China.
DNM Dinosaur National Monument, Utah.
TTU Museum of Texas Tech University, Lubbock,
ISI Geology Museum of Indian Statistical Insti-
tute, Calcutta, India.
The following description and interpretation of the
cranial anatomy are primarily based on ZG65430. The
specimen is nearly complete, beautifully preserved,
but somewhat disarticulated. The main block contains
the caudal part of the skull roof, braincase and left
quadrate in articulation. The premaxillae, maxillae,
palatal bones, and dentaries remain separate. Missing
bones include nasals and caudal part of the lower
jaws. These are supplemented by T5401. The skull is
495 mm long, 170 mm wide and 218 mm high, with a
tapering snout. The assortment of disarticulated ele-
ments and fusion of some skull bones indicates that
the animal was a subadult individual (Fig. 5). See
Appendix for list of abbreviations used in the figures.
The skull is proportionately smaller and lighter in
construction than that of Camarasaurus. There are
five openings in the skull laterally; these are the exter-
nal naris, antorbital fenestra, orbit, upper temporal
fenestra and lower temporal fenestra (Fig. 5A). In
Neosauropoda (e.g. Diplodocus and Apatosaurus), an
additional preantorbital is seen (Wilson & Sereno,
1998). In Shunosaurus, the external naris is not ter-
minal, but has moved upward and backward to the
level of the orbit. Caudally, the aperture supported a
large nasal vestibule (Witmer, 1997). The antorbital
fenestra is relatively small, as in all sauropods, indi-
cating that the antorbital sinus was considerably
reduced. The fenestra is elongated dorsoventrally, but
telescoped between the large external naris in front
and the orbit behind. The orbit is roughly triangular
and laterally placed. The upper temporal fenestra is
fairly large relative to that of other sauropods. The
lower temporal opening is orientated vertically and is
bordered caudally by the squamosal-quadratojugal
The premaxilla (Figs 5A, 6A) forms the tooth-bearing
tip of the snout and borders the external nares. It sup-
ports four long teeth. The body is roughly quadrangu-
lar in lateral view, shorter than its depth. Its rostral
margin has a steplike configuration. The premaxilla
has a prominent nasal process that is directed upward
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
and backward, tapering caudally to contact the nasal
and enclose the external naris; the maxilla borders
this opening caudoventrally. In primitive prosauro-
pods such as Lufengosaurus, the naris is entirely encir-
cled by the premaxilla and the nasal; the maxilla is
excluded from the formation of the opening. The narial
opening is large, elliptical where the long axis is hor-
izontal. The fleshy nostril was situated at the front of
the bony opening (Witmer, 2001). The naris has a shal-
low narial fossa rostrally at the confluence of the pre-
maxilla and maxilla. The maxillary process is tall and
sinuous and articulates with the rostral edge of the
maxilla in a complex tongue and groove fashion to lock
the two elements together. The suture is interrupted
by a small subnarial foramen. The premaxilla is short-
ened considerably in rostrocaudal direction from the
prosauropod condition to give the appearance of a
blunt muzzle. The median symphyseal facet is trian-
gular and flat.
The maxilla (Figs 5A, 6A) is the largest bone in the
skull and has a curved alveolar margin. It has a broad
base and a narrow ascending nasal process that sep-
arates the external naris from the antorbital fenestra.
The cranial margin of the nasal process that forms
the caudal rim of the external naris is long as in all
sauropods (Wilson & Sereno, 1998). The long, slender
nasal process is directed backward and upward but
expands somewhat terminally to meet the nasal for-
ward and lacrimal backward. It forms the rostral rim
of the antorbital fenestra. The antorbital fenestra is
oval, elongated dorso-ventrally, but lacks the fossa. It
probably housed a paranasal air sac as in all archo-
saurs (Witmer, 1997). The lateral surface of the max-
illa is roughly convex outside. The rostral process
overlaps the medial surface of the premaxilla. Cau-
dally, it joins the jugal in vertical, serrated suture. The
alveolar margin is convex in lateral aspect. There are
about 21 teeth in the maxilla. Such a large number of
maxillary teeth is a plesiomorphic feature for sauro-
pods. The maxillary tooth row terminates just below
the middle of the antorbital fenestra and occupies
almost 85% of the ventral margin of the maxilla. There
is a row of neurovascular foramina just above the alve-
olar margin to transmit nerves and blood vessels to
the skin.
In medial view, the maxilla is concave longitudinally
and shows a prominent palatine shelf at the ventral
margin of the antorbital fenestra. A vascular foramen
is present at the rostral edge of the shelf. The medial
surface is gently concave longitudinally. Ventrally, the
maxilla forms a large component of the secondary pal-
ate, where it meets the vomer medially, and premax-
illa rostrally. It is interrupted caudally by a large
choana where it meets the palatine laterally to enclose
the opening.
Figure 5. Restoration of the skull of Shunosaurus lii
(ZG65430). A, lateral view. B, occipital view.
Figure 6. Restoration of the skull of Shunosaurus lii
(ZG65430). A, dorsal view. B, ventral view.
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
The nasal (Figs 5A, 6A) is a triangular, plate-like
bone, somewhat arched dorsally to bound the caudal
margin of the external naris. In dorsal aspect, two
nasals are united along the midline to form the roof
of the skull at the central region. Rostrally, each bone
tapers considerably to contact the premaxilla. Cau-
dally, the nasal contacts the frontal by a transverse
suture and meets the prefrontal at its lateral corner.
In lateral aspect, the nasal rides over the ascending
proess of the maxilla behind the external naris and
extends backward to meet the lacrimal.
The lacrimal is partially preserved in ZG65430. It is
represented by the lower end (Figs 4, 6BD), separat-
ing the antorbital opening from the orbit. It is a long,
narrow vertical strut that sits on the jugal and
extends dorsally to meet the maxilla, prefrontal and
nasal in complex articulations. At the nasal contact,
the lacrimal is pierced by a small lacrimal foramen,
passing from the orbit to the antorbital fenestra.
The prefrontal (Figs 5A, 6A) is a small, triangular
element on the skull roof with a ventral lacrimal pro-
cess. In dorsal aspect, the medial edge of the bone is
thin and slightly overlaps the frontal and the nasal.
Unlike the prosauropod condition, the rostral process
of the prefrontal is atrophied. This feature is unique to
sauropods and is regarded a synapomorphy for Eusau-
ropoda (Wilson & Sereno, 1998). In lateral aspect, it
forms the rugose dorsal rim of the orbit along with the
frontal. Ventrally, the bone extends vertically and
bears a depressed surface for wrapping around the
The two frontals (Figs 5A, 6A) are relatively short,
wide and fused along the midline. They form the wid-
est part of the skull roof over the orbit. The frontal
provides an important contribution to the orbital rim
where it passes caudally to receive the postorbital.
Dorsally, the frontals narrow, receive the prefrontal
rostrolaterally and underlap the nasal. Caudally, the
frontal meets the parietal by a transverse suture that
is obliterated by fusion. The dorsal roof of conjoined
frontals shows a concavity on each side of the mid-
line. Ventrally, there is a large olfactory recess,
bounded by two diverging ridges. Behind the olfac-
tory recess, a large foramen for the olfactory (I) nerve
is present.
The two parietals (Figs 6A, 7A) are fused along the
midline. They are considerably shorter than the fron-
tals and form the caudal part of the skull roof between
the two upper temporal openings. Each bone has a flat
roof that slopes backward at the occiput. The medial
part of the parietal surface is roughly flat and gently
slopes backward. The bone in this area is very thin; it
has broken and exposed some of the inner structure of
the roof. There is a distinctive parietal opening, about
12 mm in diameter, along the midline. The presence of
a parietal opening has been reported in the following
sauropod genera: Camarasaurus and Diplodocus
(McIntosh, 1990a; Madsen, McIntosh & Berman,
1995); Shunosaurus (T5401), Omeisaurus and
Mamenchisaurus (Zhang, 1988). However, in the holo-
type specimen of Omeisaurus tianfuensis, He (1988)
failed to identify this feature. Behind the supratempo-
ral opening, each parietal sends out caudolaterally a
long, wing-like process to contact the squamosal. Cau-
dally, the parietal has a vertical sheet of bone on the
occiput that wraps the outer edge of the supraoccipital
medially and contacts the squamosal laterally. On the
side wall of the braincase, the parietal is very thick
and articulates with the laterosphenoid, prootic and
supraoccipital in order, caudally.
The postorbital (Figs 5A, 6A) is a triradiate bone on
the lateral surface of the skull that forms the angle
between the two temporal openings and the orbit. Its
rostral arm tapers forward to articulate with the fron-
tal. Its caudal arm is short, wide and thin to fit into
a depression on the rostral end of the squamosal. A
small pit is present near this contact and may be an
autapomorphic character for Shunosaurus. The ven-
tral arm of the postorbital tapers gradually as it
descends and curves behind the orbit to meet the jugal
in a relatively long but simple lap joint. The bone
shows fine longitudinal ornamentation throughout the
body. Medially, it wraps around the laterosphenoid
process and extends farther to join the parietal around
the rostral fossa of the upper temporal opening.
The jugal (Figs 5A, 6A) is a very thin, broad bone
that forms the ventral margin of the orbit and the ros-
troventral corner of the lower temporal opening. It
is relatively large in Shunosaurus, similar to the
condition in prosauropods. The jugal is considerably
reduced in later sauropods where it is completely
excluded from the lower margin of the skull by a con-
tact between the maxilla and quadratojugal (Madsen
et al., 1995). It has three distinct branches: the rostral
branch is overlapped by the maxilla, where it contrib-
utes to the formation of the antorbital fenestra.
Behind this fenestra, the jugal is overlain by the ver-
tical bar of the lacrimal. The dorsal branch leads
between the orbit and the lower temporal opening and
is overlapped by the postorbital. The caudal branch is
broad dorsoventrally and abuts the quadratojugal.
The squamosal (Figs 5A, 6A) is a complex, tetrara-
diate bone that forms an isthmus between the upper
and lower temporal openings. The medial process
extends forward to overlap the occipital wing of the
parietal. Laterally, there are two processes, the rost-
rolateral and the caudolateral. The rostrolateral pro-
cess is forked to receive the caudal process of the
postorbital. The ventral process of the squamosal is
long and nearly reaches the midheight of the quadrate
to receive the quadratojugal as in other prosauropods.
In most sauropods, this connection is breached. The
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
absence of squamosalquadratojugal contact has been
incorrectly cited as a synapomorphy for Eurosau-
ropoda, a clade that includes Shunosaurus (Wilson &
Sereno, 1998). Unlike other sauropods, the caudal pro-
cess of the squamosal is well developed in Shunosau-
rus, a plesiomorphic feature common in prosauropods.
This process tapers backward with a slight curvature
and is covered medially by the paroccipital. The artic-
ulation of the squamosal with the quadrate is com-
plex. It has a ventral socket to receive the quadrate
The quadratojugal (Figs 5A, 6A) is an L-shaped bone
firmly attached to the lateral surface of quadrate and
participates in the jaw articulation. The vertical arm
of the quadratojugal is thin and slender and meets the
descending process of the squamosal. It overlaps the
quadrate behind. The horizontal, rostral ramus
extends forward to meet the jugal as in prosauropods.
In later sauropods, the rostral process extends farther
forward to meet the maxilla. However, the rostral
ramus of quadratojugal is elongate and distally
expanded as in other sauropods (Gauthier, 1986).
Our knowledge of the palatal structure of sauropods is
limited. In Shunosaurus, the right side of the palate is
very well preserved in ZG65430, but the elements are
disarticulated. The left quadrate is held in place in the
skull in normal position. The palate is highly vaulted
with large choanae that are separated by narrow
palatines. The vomers fail to enter the borders of the
choanae, a character unique to Shunosaurus. In Cama-
rasaurus and most archosaurs, on the other hand, the
choanae are separated by narrow vomers. In Shuno-
saurus, the choanae are broad and oval as seen ven-
trally. They are shifted caudally in relation to the
external nares, bringing a separate air passage well
back along the palate. Rostral to the choanae, both
premaxillae and maxillae contribute to form a broad
secondary palate. Two palatal foramina identified in
Camarasaurus, the subnarial foramen and rostral
maxillary foramen, are absent in Shunosaurus
(Berman & McIntosh, 1995). The epipterygoid is not
The two vomers (Fig. 6B) are narrow and elongate,
meeting along the midline to form a horizontal plate
behind the premaxillae. Laterally, they are bordered
by the premaxillae. Caudally, each bone attaches the
palatine laterally in a broad articulation. Both sides of
the vomer are smooth and flat, but become thick along
the symphysis.
The palatine (Fig. 6B) is a thin, curved plate with
two bifurcating processes directed rostrally and later-
ally to form the caudal rim of the choana. The lateral
process that abuts the maxilla does not expand trans-
versely and has a narrow contact. The rostral process
is long and narrow has a pronounced articular surface
at the tip for the vomer. The rostral rami of the
palatine form a bridge between the choanae. Caudally,
the palatines meet the palatal rami of the pterygoids
by a broad, V-shaped suture. The caudal half of the
palatine is broad and flat. It constricts backward to fit
into a crescentic fossa of the pterygoid. In Shunosau-
rus, the palatines are fairly large bones and meet
along the midline. In other sauropods they are sepa-
rated laterally by the intervening pterygoids.
The pterygoid (Fig. 6B) is extremely slender when
compared to that of other sauropods. It occupies about
one-third the length of the palate. It has three distinct
processes: a palatal process directed forward, an
ectopterygoid process directed forward and outward,
and a quadrate ramus directed backward and out-
ward. The palatal rami are narrow and closely
apposed along the midline for most of the length cre-
ating a small interpterygoid vacuity at the caudal end.
Rostrally, the conjoined palatal rami receive the
palatines in a crescentic fossa but they fail to meet the
vomers. Lateral to the palatine process, the ectoptery-
goid process is relatively robust with a prominent
articular surface that is overlain laterally by the
ectopterygoid. This process lies below the rostral rim
of the orbit as in other sauropods (Gauthier, 1986).
Behind the ectopterygoid process, the pterygoid
becomes narrow vertically and contains a distinctive
fossa on the medial wall. This character may be an
apomorphy for Shunosaurus. Behind this fossa, there
is a distinct socket to receive the basipterygoid process
of the basisphenoid. This socket is relatively shallow
and broad with a hook-like process that curves around
the distal end of the basipterygoid process. In derived
members of sauropods, this hook appears to be absent
(Wilson & Sereno, 1998). Behind the basipterygoid
articulation, the pterygoid becomes thinner and wider,
where the quadrate ramus bifurcates. This bifurcated
quadrate ramus appears to be a unique feature in
Shunosaurus. Both processes overlap the quadrate;
the ventral is short and rounded; the dorsal is long
and narrow.
The ectopterygoid (Fig. 6B) is small and rhombic, ori-
entated in two planes to form the medial and lateral
processes. The medial process is horizontal, relatively
thick and receives the pterygoid. The lateral process is
extremely thin and vertical and attaches to the jugal
to form the lateral support of the palate.
The quadrate (Figs 5B, 7B) is very tall, and distinc-
tive in that the head is twisted and curved dorsolat-
erally in relation to ventral expansion. It forms the
caudolateral margin of the skull and ventral articula-
tion with the mandible. The main shaft of the quad-
rate is a stout vertical plate, broad mediolaterally with
two flanges: the medial flange for the pterygoid and
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
the lateral flange for the squamosal and quadratoju-
gal. The attitude of the quadrate is highly variable
among sauropods where the ventral articular surface
slants considerably forward in advanced genera. The
dorsal head is somewhat expanded rostrocaudally
and fits into a socket of the squamosal. The head is
caudally directed to form an emargination for the
tympanic membrane. The lateral flange is small and
shows two articular surfaces: the dorsal surface is
overlapped by the squamosal, while the ventral sur-
face is overlapped by the quadratojugal. The medial
flange, the pterygoid ramus, extends forward and
inward as a vertical triangular sheet. It has two dis-
tinct facets for receiving the bifurcated quadrate
ramus of the pterygoid. The thickest part of the quad-
rate is located at the lower end which articulates with
the mandible. However, the mandibular articular sur-
face is complex and is partly formed by the quadrato-
jugal. The ventral surface is smooth, slightly convex
with an oval outline, but the condyles are not well dif-
ferentiated. In the back of the quadrate, there is a
large depression, the quadrate fossa, on the lateral
flange that extends longitudinally about two thirds of
the length of the quadrate body. The quadrate fossa is
well developed in Shunosaurus. It is probably a pneu-
matic recess, linked to the cervical system of pulmo-
nary air sacs.
The braincase of Shunosaurus ZG65430 (Fig. 7) is
complete and excellently preserved, and fused with
the dorsal skull roof. It includes the supraoc-
cipital, exoccipital-opisthotic complex, basioccipital,
basisphenoid-parasphenoid complex, laterosphenoid,
orbitosphenoid and prootic. The occipital condyle has a
transverse diameter of 45 mm and is caudoventrally
directed. The foramen magnum is circular with a max-
imum width of 23 mm. The distance between the exit
of the olfactory (I) nerve opening and the caudal mar-
gin of the occipital condyle is about 110 mm, indicat-
ing the length of the braincase. The braincase is short,
narrow and deep. The braincase depth is most notice-
able in the great ventral development of the basal
tuberal and the basipterygoid processes. The brain-
case is completely ossified as in all sauropods.
The occiput slopes downward and backward from
the parietal flange at an angle of about 60° and is of
considerable depth. The post-temporal fenestrae are
absent. The brain cavity slopes upward to terminate
against the frontals just caudal to the midpoint of the
orbit. The upward tilt of the braincase appears to be
characteristic of the sauropods (White, 1958) and car-
nosaurs (Romer, 1956). In contrast, some ornithis-
chian braincases are nearly parallel to the skull axis.
The braincase is primitively designed in basal archo-
saurian fashion and lacks tympanic pneumaticity and
refined otic capsules as in other sauropods.
The basioccipital (Figs 5A, 7A) has the same char-
acteristics seen in other sauropod specimens. The
occipital condyle, formed mostly by the basioccipital, is
clearly larger than the foramen magnum. It is some-
what kidney-shaped. The basioccipital is capped by
the exoccipitals dorsolaterally and shows a longitudi-
nal depression along the midline; it is constricted to a
neck and then flares again at the basal tubera. At the
ventral side of the condylar neck there is a small pit.
There is a vertical depression on each lateral wall of
the basioccipital.
Figure 7. Braincase of of Shunosaurus lii (ZG65430). A,
left lateral view. B, rostroventral view.
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
At the occiput, there are two openings, about 7 mm
apart, for cranial nerve XII, lateral to the base of the
foramen magnum. Both are located in the exoccipital.
At the sidewall of the braincase, at the junction of the
basioccipital and basisphenoid, there are two large
foramina at the otic capsule region, bounded rostrally
by the crista prootica (= otosphenoidal crest). These
two foramina are separated by a thin crest of the
opisthotic, the crista interfenestralis. The caudal
opening is about 10 mm wide and 15 mm high and
represents the metotic foramen that transmitted cra-
nial nerves IXXI, and the posterior branch of the jug-
ular vein. The rostral foramen, the fenestra ovalis, is
9 mm wide and 20 mm high, and would have received
the footplate of the stapes. A foramen for the perilym-
phatic duct is visible through the fenestra ovalis.
There is a short, tubular cochlear recess that extends
downward from the fenestra ovalis for housing the
The basisphenoid (Figs 4B, 7A) is the major element
in the floor of the braincase, extending longitudinally
in front of the basal tubera to the basipterygoid pro-
cess. It is fused indistinguishably to the parasphenoid,
so the two bones will be treated together here. The
parasphenoid rostrum is not complete, lacking the ros-
tral portion. Passing forward along the lateral wall of
the braincase, the basisphenoid contacts the caudal
ends of the prootic and laterosphenoid. At the contact
of the prootic and the basisphenoid, the rostral tym-
panic recess (RTR) is located just behind the orbito-
sphenoidal crest. In this recess, there is a large vidian
canal through which the internal carotid (= cerebral
carotid) artery entered into the pituitary fossa accom-
panied by the palatal ramus of the facialis (VII) nerve.
The stoutly built basipterygoid processes diverge
outward and downward. The angle between them in
Shunosaurus is about 90°, indicating a primitive fea-
ture. In most sauropods this angle is acute, ranging
from 45° to 20° (Salgado & Calvo, 1992). At the base,
each process is subcircular in section, and about half
as long as high in its dimensions. The outside surface
of the process is lapped by the pterygoid, but the ptery-
goidal socket is shallow. In Camarasaurus (DNM28),
this socket is very deep and conceals most of the basip-
terygoid process. Rostrally, the parasphenoid rostrum
extends farther forward. The length, shape, and atti-
tude of the basipterygoid processes are variable
among sauropods and may have taxonomic implica-
tions: Brachiosaurus, Apatosaurus, and Camarasaurus
show strong, robust, but short basipterygoid pro-
cesses, while Diplodocus, Barosaurus and Dicraeosau-
rus possess long, slender, attenuated processes
(Berman & McIntosh, 1978). Shunosaurus has a
unique feature that combines short, slender but very
weak processes (Fig. 7). In rostral aspect, the large
pituitary fossa (= sella turcica) is bounded by the dor-
sum sella of the basisphenoid. The fossa is about
13 mm wide, 20 mm long, and 13 mm deep. The pitu-
itary fossa is encased completely by bone in Camara-
saurus and Diplodocus, but in Shunosaurus there is a
rostral opening. The dorsum sella is penetrated by two
canals: the canal for the internal carotid arteries cau-
dodorsally, and the foramen for nerve VI rostrolater-
ally. At the base of the dorsum sella, there is a shallow
medial facet for the attachment of the rectus muscu-
lature of the eye. This facet is relatively smaller than
that of Camarasaurus (DNM28).
The exoccipital is fused with the opisthotic (Figs 5B,
7A) as in most archosaurs. The exoccipital caps the top
of the basioccipital from the upper part of the condyle.
A lateral component of the exoccipital is pierced by
two openings for the hypoglossal nerve (XII). The
opisthotic forms the stout paroccipital process. The
lateral extent of the paroccipital process is difficult
to estimate because of crushing. It is clearly directed
downward with a slightly dorsal curvature. In T5403,
two paroccipitals are beautifully preserved where the
end is slightly expanded, with a straight dista; mar-
gin. In later sauropods, the end of the paroccipital pro-
cess is not expanded, and has a rounded ‘tongue’-like
feature (Upchurch, 1998).
The supraoccipital (Fig. 7A) is very similar to that of
the prosauropods with a roughly pentagonal outline.
The median ridge is pronounced and reaches the fora-
men magnum caudally, but forks rostrally to receive
the parietal. Rostrolaterally, it articulates with the
occipital wing of the parietal with a long suture.
Caudally, it meets the exoccipital-opisthotic complex
and forms the upper margin of the foramen magnum.
The lateral process of the supraoccipital is relatively
broad and overlaps the prootic and opisthotic medially.
It forms the roof of the hindbrain. Dorsally, the
lateral process of the supraoccipital is covered by the
The prootic (Fig. 7A) is well exposed on the sidewall
of the otic capsule, where it forms the rostral margin of
the fenestra ovalis. The crista prootica (= otosphenoi-
dal crest) is an important landmark on the lateral wall
of the prootic, separating the orbital contents in front
from the otic capsule behind. Rostral to the fenestra
ovalis, two foramina are visible along the crista proot-
ica. The upper foramen, 2 mm in diameter, extends
caudolaterally, through which passed the hyomandib-
ular branch of the facialis nerve (VII). The lower fora-
men, about the same size, is directed ventrally for the
ramus communicans branch of the facialis (Galton,
1985). Dorsal to this foramen, the prootic expands
slightly as it rises, but is truncated abruptly. This part
of the bone is very thin, sheet-like, and attached at the
rostroventral margin of the supraoccipital. The prootic
extends forward as a vertical sheet below the skull
roof to meet the laterosphenoid. In its ventral portion,
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
the prootic borders the caudal margin of the trigemi-
nal (V) foramen, whereas the rostral margin is formed
by the laterosphenoid. This foramen is deep, large and
The laterosphenoid (Fig. 7A) is highly developed and
forms the rostral part of the sidewall of the braincase.
The rostral end of this bone is transversely expanded
with a prominent, laterally directed process that fits
into a depression of the postorbital. There is a promi-
nent ridge, the crista antotica, at the lateral end of the
process. The dorsal margin of the laterosphenoid is
firmly sutured with the frontal and parietal. In lateral
aspect, the laterosphenoid shows two additional
foramina around the trigeminal (V) nerve. The rostral
foramen is the opening for the ramus maxillaris (V2)
and the dorsal foramen is the emergence point of the
middle cerebral vein (= vena cerebralis media). The
laterosphenoid is strongly sutured to the basisphe-
noid in its lower portion, but has a wide contact with
the prootic caudally. Rostroventrally it meets the
In rostral aspect, near the suture with the orbito-
sphenoid, the laterosphenoid bears three openings
progressing caudally that serve as exits for the optic
(II), trochlear (IV) and occulomotor (III) nerves. The
opening for nerve III is extremely elongated and con-
nects both the forebrain and the pituitary canal. Two
openings for nerve IV are seen. Just above and rostral
to the trochlear foramen, there is a large aperture at
the junction of the laterosphenoid and orbitosphenoid
for the entrance of the ethmoidal artery.
The orbitosphenoid (Fig. 7B) is a thin triangular
bone that is broad rostrally but constricted caudally.
Two orbitosphenoid bones meet along the midline with
a large caudal opening for the optic (II) foramen. Ros-
trodorsally, it forms a V-shaped valley along the mid-
line to border the lower margin of the olfactory (I)
nerve opening and the floor of the forebrain. Dorsolat-
erally, it contacts the laterosphenoid and shows three
cranial nerve openings (IIIV) along the suture, as
discussed earlier. Two caudal processes of the orbito-
sphenoid extend to meet the dorsal sellae of the
The two rami of the lower jaw had been joined by a
weak symphyseal suture but were found separated
(Fig. 8). In later sauropods they are strongly fused. In
specimen ZG65430 only the rostral portion of each
lower jaw was preserved. Both jaws contain most of
the dentary, and the left includes the splenial. The
caudal part of the jaw has been restored from other
specimens (T5401, T5403 from Zhang et al., 1984;
Zhang, 1988). Proportionally, the lower jaw in
ZG65430 is extremely slender with no sign of dors-
oventral expansion at the back. The coronoid process
is not prominent as in other sauropods. The two den-
taries form a U-shaped outline in dorsal view. Each
ramus takes a sharp turn medially from the 7th tooth
position. Each dentary is 200 mm long and is 70 mm
deep at the symphysis, shallowing progressively back-
ward. The tooth row is relatively long compared to
other sauropods. The jaw articulation is below the
mandibular tooth row, as in other sauropods (Gauth-
ier, 1986). Several well-spaced nutrient foramina are
found on the lateral surface of the dentaries (Fig. 5A).
The mandibular fenestra is present in Shunosaurus. It
is relatively large, oval, about 10% of the length of the
mandible, and is bouded by the dentary, angular, and
surangular (Zhang, 1988). This fenestra is consider-
ably reduced or lost in later sauropods.
The dentary (Figs 5A, 8) forms nearly half the
length of the jaw. It has 26 alveoli, but most of the
Figure 8. Lower jaws of Shunosaurus lii (ZG65430). A,
lateral view of the left dentary. B, lateral view of the right
dentary. u, medial view of the right dentary. D, medial view
of the left dentary.
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
teeth are missing. The rostral end of the dentary
increases in depth and robustness as in other sauro-
pods (Upchurch, 1998). Caudally, the thin lateral
part of the dentary overlaps the angular ventrally
and the surangular dorsally. On the medial surface
of the dentary there is a longitudinal groove, the
Meckelian canal, which extends forward almost to the
tip of the symphysis. This canal becomes extremely
wide caudally and is overlapped by the splenial. The
caudal half of the dentary is sheathed by the splenial
The splenial (Figs 5A, 8C) is a thin medial sheet that
covers much of the dentary, and the ventral part of the
prearticular, while the tapering caudal part overlaps
the angular. The splenial forms part of the ventral
edge of the lower jaw, but remains hidden in lateral
view. Rostrally, it terminates caudal to the ventral
symphysysis. There is a foramen at mid-length of the
splenial in T5401 (Zhang, 1988).
To make the description of the lower jaw of Shuno-
saurus complete and comprehensive, the morphology
of the missing parts such as the coronoid, angular,
surangular, articular and prearticular is given here
(Fig. 5A), based on other specimens (T5401, T5403
from Zhang et al., 1984; Zhang, 1988).
The coronoid forms a low convex process behind the
tooth row. It is wedged between the surangular and
splenial in medial aspect and passes upward behind
the maxillary when the jaw is articulated with the
skull. The coronoid is wedged at the junction of the
surangular, splenial, and the dentary medially, and
only a small area of the upper part is exposed.
The angular forms the base of the caudal half of the
ramus. Most of this bone is overlapped by the splenial
rostrally and prearticular and surangular dorsally.
The surangular overlaps the angular ventrally and
forms most of the margin of the mandibular fenestra
medially and laterally. It is narrow in the caudal direc-
tion where it receives the articular.
The articular is a small compact bone, longer than
wide, with a shallow glenoid for the reception of the
quadrate. It is lapped by the surangular and angular
laterally, and prearticular rostrally.
The prearticular is visible in medial aspect where it
forms the caudal border of the mandibular fenestra.
There is a large fossa on the prearticular behind the
mandibular fenestra.
Tooth morphology
Sauropod teeth exhibit two distinct crown morpho-
types: (1) spoon shaped, (2) long, cylindrical and peg
shaped (Romer, 1956). The teeth of Shunosaurus are
somewhat intermediate between these two varieties:
cylindrical with a spoon-shaped crown. Among sauro-
pods, Shunosaurus possesses the largest number of
teeth. The dental formula is: [pm (4, 5) + m (20)]/d (25–
26), where pm indicates premaxillary, m, maxillary
and d, dentary teeth. There are four premaxillary
alveoli in Shunosaurus ZG65430. However, there is
some asymmetry in tooth count in specimen T5403
(Zhang, 1988); there are five teeth in the right side and
four in the left. The fifth tooth appears to have added
caudally in the right side. Most of the premaxillary
teeth are missing from this specimen except for the
fourth, a small eruptive tooth. The tooth crown is
sharp and triangular with radiating striations on the
medial side. The number of maxillary teeth varies
between 19 and 21 in Shunosaurus (Zhang et al., 1984;
Zhang, 1988); ZG65430 shows 20 teeth (Fig. 9). The
shape of the tooth crown varies from triangular spat-
ulate to oval spatulate and is more slender than other
sauropods. There is a strong constriction between the
tooth crown and the root. The crown appears triangu-
lar in the rostral portion of the maxillary teeth, from
the first to the tenth position. Behind this series, the
crown becomes oval from the 11th to the 20th position.
There is a gradual decrease of tooth size from front to
back, ranging from 80 mm to 10 mm. The root : crown
ratio also reduces from front to back; the largest is 3
(6th tooth), and the smallest is 0.5 (17th tooth). This
ratio is applicable to functional teeth only. There is
apparently no correlation between tooth size and wear
pattern. Wear facets are restricted to the lingual side
of the maxillary teeth. These facets occur on the ros-
tral, caudal and tip region of the crown in the rostral
half of the maxillary ramus. In the caudal portion, the
wear surface is found on the tip and caudal portion of
the crown.
The occlusal level of the upper tooth row forms a
convex arc downward to counteract the concave alve-
olar level of the dentary (see Zhang, 1988: fig. 19).
Together they form a cutting edge, similar to a garden
shear, to slice tough twigs and leaves. The maxillary
teeth are not vertical, but curve inward in the rostral
part of the tooth row; they are relatively straight
in the caudal half. The crowns are covered by
thick enamel and show axial striations that radiate
Only three dentary teeth are preserved in the left
mandible (Fig. 8D) and most of the alveoli are empty
with no trace of replacement teeth. There are 26 alve-
oli on the right side and 25 on the left. The alveoli of
the dentary are subcircular in section, and their size
gradually decreases from front to back. The number of
teeth in the lower jaw exceeds that of the upper. The
reverse is seen in most sauropods. The crowns of the
preserved mandibular teeth do not show spoon-shaped
morphology. They are laterally compressed where two
sharp edges bear a small serration. Moreover, there is
no constriction between the root and the crown. In this
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
respect the tooth morphology closely approaches that
of prosauropods.
An unusual feature in the dentary of Shunosaurus is
the relative depth of the labial wall compared to the
lingual wall. The lingual margin is extremely low rel-
ative to the labial wall. This feature is reminiscent of
the pleurodont type of tooth implantation seen in mod-
ern lizards. Obviously the lingual wall was strength-
ened by a series of overlapping interdental plates.
These plates are entirely missing in the dentary. In
the maxilla, two interdental plates are still in place,
but most are missing. These plates must have been
loosely held in life, and may have facilitated tooth
replacement when old teeth were shed. Upchurch
(1995: fig. 12) refered to the labial wall as the ‘lateral
plate’ and recognized this feature as a synapomorphy
of Eusauropoda.
Tooth replacement pattern
Edmund (1960, 1962, 1969) studied in detail the tooth
replacement phenomenon in reptiles. He observed
back to front replacement waves in alternating tooth
positions in most reptiles. A single front to back wave
of developing teeth is known as a Zahnreihen, and
numerous Zahnreihen pass along the jaw during the
life of a reptile. DeMar (1972) has shown that in most
reptiles the spacing between Zahnreihen (Z-spacing)
is between 1.56 and 2.80 tooth positions. Z-spacing of
2.0–3.0 results in the usual secondary pattern of back
to front replacement waves of alternate teeth. The
relationships between the number of teeth between
replacement waves (W), and the number of teeth
between Zahnreihen (Z, Z-spacing) can be expressed
by the following equation: W = R/(Z-R) + 1, where R is
2 if every other tooth is counted, or 3 if every third
tooth is counted (Osborn, 1970, 1973). As Z-spacing
increases, the length of the back to front replacement
waves decreases at 2.0, there is perfect alternation
and the waves are of infinite length; at 2.25 the waves
are nine alternating positions in length, at 2.5 five
positions and at 2.66 four positions. When the Z-spac-
ing is 3.0, the wavelength is three alternating tooth
positions, but it is the repeated pattern of the Zah-
nreihen (three sequential teeth decreasing in size cau-
dally) that becomes obvious.
Most tooth replacement data in dinosaurs come
from theropods (Edmund, 1960), prosauropods
(Galton, 1984, 1985) and ornithischians (Ostrom,
1961; Crompton & Charig, 1962; Galton, 1973, 1974;
Charig & Crompton, 1974; Gow, 1975; Hopson, 1975,
1980b). In sauropods, however, the teeth are often
found missing from the jaws. In rare instances such as
in Camarasaurus and Brachiosaurus, tooth replace-
ment pattern has been observed (White, 1958;
Edmund, 1960). In this context, both Shunosaurus
(ZG65430) and Camarasaurus (DNM28) retain an
intact maxillary tooth row that allows study of the
tooth replacement patterns.
Edmund (1960) recognized the following two criteria
for identifying the replacement patterns in reptiles:
(1) replacement teeth at varying developmental stages
lying within several pulp cavities in each jaw; (2) one
or more gaps (empty alveoli) in each jaw from which
teeth have fallen out. However, these criteria cannot
be applied in the sauropods since there is no gap or dif-
ference in length between the alternative tooth, and
the pulp cavities are difficult to visualize without X-
ray radiography, CT scanning, or serial sectioning.
Another way to study tooth replacement pattern in
sauropods as well as in Ornithschia is the use of dif-
ferent stages of the wear facets as an indicator of tooth
age (White, 1958; Crompton & Charig, 1962; Charig &
Crompton, 1974; DeMar 1972; Galton, 1973, 1974;
Hopson, 1975, 1980b).
Tooth wear pattern
In Shunosaurus (ZG65430), a full complement of teeth
is preserved in the maxilla (numbered herein 1–20
from front to back), showing different stages of wear
facets. These wear facets are essentially vertical along
the rostral and caudal ridge of the crown, indicating
that the jaw motion was orthal. The quadrate glenoid
articulation fits closely and precludes any propalinal
motion of the jaw. Upper and lower tooth rows inter-
digitate and shear past each other in such a fashion
that each lower tooth fits between two teeth of the
upper row along their lingual surface. In the maxilla,
wear facets are found on three regions of the crown:
the rostral edge, the caudal edge and at the tip of the
occlusal surface. There is an interesting sequential
pattern of tooth wear. In tooth #5, the wear facet is
restricted to the occlusal surface, the rostral and
medial edges are intact. Apparently this is the very
first sign of tooth wear. In tooth #7, both occlusal and
rostral facets can be seen, implying that the rostral
facet appeared after the occlusal facet. Finally, in
tooth #4, all three facets – occlusal, rostral and caudal
– are present. Thus the sequence of appearance of
wear facets is: occlusal Æ rostral Æ caudal (Fig. 9).
The rostral series of teeth (#110) have a slender
spatulate tooth crown and elongate root. Wear facets
are concentrated on the lingual side of the crown sur-
face. Tooth #1 bears three distinctive facets. The ros-
tral facet is very small and extends to the lower part of
the crown. The occlusal facet is more pronounced than
the rostral and exposes the dentine. The caudal facet
is also prominent and occupies the whole length of the
Tooth #2 is very important as it indicates the
sequence well. There are three disconnected facets at
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
the border of the lingual surface. The rostral facet is
weak and is restricted to the middle of the rostral rim;
it is very narrow and slit-like, tapering dorsoventrally.
Comparison with the wear pattern in tooth #1 sug-
gests that this facet and the caudal facet of tooth #1
were both formed by the shearing of the single lower
tooth. The occlusal wear is an incipient facet. The cau-
dal facet shows the same degree of wear as the rostral,
but is located closer to the lower part of the crown. All
three facets are weak and do not excavate the dentine
Tooth #3 lacks a wear facet. Tooth #4 shows exten-
sive wear facets at three edges, indicating it was
mature. The occlusal facet is prominent and connects
with the other two facets. All three facets expose the
Tooth #5, as discussed earlier, shows only the
occlusal facet. This condition indicates that the tooth
wear began with the crown tip, followed by the rostral
and caudal wear.
Tooth #6 also is another older generation tooth, and
shows a replacement tooth at the base. The occlusal
facet is continuous with the rostral and caudal facets.
The morphology and wear facet are nearly the same
as those of tooth #4. However, the occlusal facet is rel-
atively small, indicating that it is less old than tooth
Tooth #7 shows separate occlusal and rostral facets,
but lacks the caudal facet.
The wear pattern in the caudal half row (#11–20) of
ZG65430 is obviously different from the rostral series,
showing no rostral facet. Teeth #11 and 12 are not yet
functional and lack any wear facets. Teeth #13 and 15
show both occlusal and the caudal facets that exca-
vated the dentine, while #14, 16 and 18 exhibit only
the occlusal facets. However, wear has made the
occlusal surface a sharp cutting edge.
The wear pattern sheds new light on the tooth func-
tion of Shunosaurus. The geometry of the wear facets
clearly indicates that the movement of the lower jaws
was essentially orthal (i.e. simple open-and-shut),
without any propalinal or lateral motion. This orthal
motion of jaw action has been observed in many sau-
ropods (Upchurch & Barrett, 2000). Tooth wear began
at the crown tip. The tooth axis is curved outward from
the crown tip to the root. In the occlusal phase, the
opposing crown tips slide past each other, so the first
stage of tooth wear is occlusal wear. The location of the
replacing tooth at the labial margin is very unusual.
The new replacement tooth erupted at the base of the
Figure 9. Maxillary dentition and tooth wear of Shunosaurus lii (ZG65430). A, sagittal cross-section of a maxillary tooth
in alveolus showing the relative depth of the lingual and labial walls; the missing interdental plate gives a pleurodont-like
tooth implantation. B, the same jaw section when the interdental plate is restored in place, showing typical thecodont
implantation. C, lingual view of the left maxilla showing tooth wear facets, replacing teeth, and interdental plates.
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
alveolus lingually, as in other reptiles, but migrated
labially as it grew. When it reached the lower margin
of the maxilla, it occupied the labial side of the func-
tional tooth. As the replacement tooth grew further,
the tip of the functional tooth moved inward and
caused more surface of the rostral and caudal ridge to
be worn by the lower tooth. This caused the functional
tooth to reach the peak of its shearing effectiveness. At
this stage, the wear on both rostral and caudal edges
becomes dominant and is formed by the occlusion with
the alternative lower teeth.
Tooth replacement analysis
Because there is no obvious size gradation in alternate
tooth rows, the tooth replacement pattern can be
inferred from the tooth wear facet. The sequence of
replacement at each tooth position was apparently as
follows: (1) the tooth erupted on the caudo-lingual
side; (2) there is a window on the base of the lingual
side of the functional tooth; (3) the replacement tooth
tunnelled through the window and appeared at the
labial side; (4) the base of the tooth was completely
worn out leaving the functional tooth in a precarious
condition; and (5) the replacement tooth occupied the
position of the old tooth and grew. The labial migration
of the replacement tooth appears to be a unique fea-
ture in Shunosaurus.
A replacement tooth is recognized on the basis of its
small size, its location at the base of an old tooth, and
the lack of wear facets (Table 1). A functional tooth is
fully erupted with varying extent of wear facets. Five
stages of functional teeth (F1–F5) and three stages of
replacement teeth (R1R3) are recognized. The stages
are, in order of increasing age:
R1 Small incipient tooth showing the tip of the crown.
R2 Crown fully erupted.
R3 Crown reaches the labial margin of the maxilla.
Similarly, for functional teeth
F1 Tooth is fully developed with intact crown and
root, without any sign of wear.
F2 Crown tip begins to wear.
F3 Small wear facet present in the rostral and cau-
dal side.
F4 Wear facet is expanded.
F5 Occlusal facet is reduced but the rostral and cau-
dal facets become dominant and extend to the
crown base.
The next step is the measurement of Z-spacing (=
Zahnreihen spacing, DeMar 1972, 1973). Edmund
(1960, 1962, 1969) used the term Zahnreihen for each
member of a set of equally spaced caudally sloping
parallel lines that intersected a tooth at every tooth
position. In addition to Zahnreihen, he also recognized
tooth families (sets of lines approximately at right
angles to the horizontal tooth row), and replacement
waves (sets of lines with low slope, involving every
other tooth). With these three sets of parallel lines, he
believed he had completely described all of the possi-
bilities. DeMar (1972, 1973) modified the definition of
Zahnreihen to any member of a set of equally spaced
parallel lines drawn through the points representing
teeth of an Edmund diagram in such a way as to have
minimum caudal slope and intersecting a tooth at
every tooth position. This definition opens up an
infinite number of other possibilities that could be
confused with Zahnreihen. The arrangement of
Zahnreihen thus reflects the timing of tooth replace-
ment events, which is the phenomenon of interest
here. As a first approximation, this relative timing can
be expressed as distance (in tooth positions) between
Zahnreihen this is Z-spacing. For instance, in
Figure 10, Z-spacing is between 2 and 3.3. Z-spacing
can be determined in any dentition which is ordered
enough to reveal Zahnreihen. From these data an
Edmund diagram can be constructed.
Shunosaurus shows well organized replacement
activity; Z-spacing is determined by locating two teeth
in adjacent Zahnreihen that are in the same phase of
replacement, and measuring the distance in tooth
positions between them. The measurement of maxil-
lary teeth (Fig. 10) shows the range of observed vari-
ation in Z-spacing and some of the measurement
difficulties. The general strategy is always to measure
Table 1. Wear pattern in maxillary tooth row of Shuno-
saurus (+ present; – absent)
3– – – F2
4+++F5, R3
6+++F5, R3
8– – – F1
9+++F5, R2
10 F2, R1
11 – F1
12 F2, R1
13 ++F4, R3
14 + F3, R1
15 ++F4, R3
16 + F3. R1
17 – F2
18 + F3, R2
19 – F2
20 – F1
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
the number of tooth positions between teeth that are
at identical stages of development (White, 1958;
DeMar, 1972). This is done by measuring the distance,
parallel to the trend of the tooth row, between two
adjacent Zahnreihen. However, there are some varia-
tions in the illustrations that are probably caused by
three factors. First, the stages are objectively defined
with no quantitative meaning. Second, each stage’s
time interval is different. Based on the probability of
the number in each stage (Table 1), the stages F1, F2,
and F5 last a relatively short time (each 15%) com-
pared to stage F3 (30%), and F4 (25%) in Shunosaurus.
Third, the replacement tooth development period (R1–
R3) is definitely different than the functional tooth
(F1–F5) wear time. There is not enough evidence to
specify this data, and here it is treated as the same.
Based on the above study, the following generaliza-
tions about variation in Z-spacing are reached. First,
the Z-spacing varies between 2.0 and 3.3 in Shunosau-
rus. The average variation is in the range of most
reptiles, between 1.56 and 2.80 (DeMar, 1972, 1973).
These values indicate that the replacement wave
direction could only be back to front in Shunosaurus.
Second, the individual Zahnreihen seems to have a
consistent slope. Third, the Zahnreihen slope varies
from 1 to 3 in Shunosaurus.
1 The replacement series (odd and even) are from the
back to the front. The replacement wave direction,
independent of tooth morphology and number, relies
chiefly on the Z-spacing.
2 The total life of a tooth is relatively long so that the
time spent replacing it is relatively short. The pro-
portion of the replacing period (stages III and IV) is
20% of total tooth development.
3 The average Z-spacing is 2.66 and the individual or
whole Zahnreihen is highly organized. The slope
between them shows slight variation.
4 The jaw motion is exclusively orthal. The pattern of
wear facets suggests that the upper tooth row
shears past the labial side of the lower tooth series,
and they occlude in alternate fashion.
Sauropod endocasts are known from the following gen-
era: Antarctosaurus (Huene & Matley, 1933), Barosau-
rus (Janensch, 1935–36), Brachiosaurus (Janensch
(1935–36), Camarasaurus (Marsh, 1880, 1889; Osborn
& Mook, 1921; Ostrom & McIntosh, 1966), Cetiosaurus
(described as Megalosaurus bucklandi by Huene,
1906), Dicraeosaurus (Janensch (193536), and
Diplodocus (Marsh, 1884, 1896; Osborn, 1899; Hol-
land, 1906). It is generally believed that sauropods
had extraordinarily small brains relative to their body
size. Unlike birds and mammals, the brain of sauro-
pods did not fill the braincase completely. However, an
endocast provides a fairly superficial topography and
contour of the brain. The division of the brain into
three regions forebrain, midbrain, and hindbrain is
not well differentiated in the endocasts of sauropods.
Hopson (1980a) estimated the relative brain size of
two sauropod genera, Brachiosaurus (estimated brain
volume 174 mL; body weight 87 tons) and Diplodocus
(estimated brain volume 47 mL; body weight 11.7
tons), calculating their encaphalization quotient (EQ)
as 0.17 and 0.18, respectively. Both values lie in the
reptilian domain of brain/body size map (Hopson,
Latex casting was used to make the endocast of
Shunosaurus (ZG65430) and Camarasaurus (DNM28).
The endocranial cavity is much larger than the width
of the foramen magnum; as a result it is difficult to
remove the endocast from the cavity through the fora-
men magnum. It is impossible to fill the brain cavity
completely with latex. We designed a new method to
coat the endocranial cavity wall instead of the whole
brain cavity. First, we removed as much matrix as pos-
sible from the endocranial cavity. The cavity was then
washed using acetone before being coated with a very
thin layer of Butvar (B-76) which was allowed to dry
completely. A thin coat of latex was painted over the
endocranial cavity using flowing strokes. The brush
was kept full of latex at all times and was allowed to
flow onto the surface without any air bubbles. After
the first coat, a second was applied in the same man-
ner. Several additional coats of thicker latex were
applied, allowing each to dry before the next was
applied, until an opaque appearance was attained.
Then the entire latex endocast was ready to remove.
After removal, the endocast was trimmed to conform
to the contours.
The brain of Shunosaurus lii (Fig. 11) is relatively
small, shallow and narrow transversely with a prom-
inent pineal flexure and a subdued pontine flexure. It
is primitively designed in crocodilian fashion where
the forebrain, midbrian and hindbrain are narrow,
elongate and serially arranged. The brain is 11 cm
long, 2.5 cm wide and 4 cm high, excluding the ventral
Figure 10. Z-spacing diagrams of Shunosaurus lii
(ZG65430). (see Table 1 for raw data).
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
projection of the pituitary body. The shape of the olfac-
tory bulbs of Shunosaurus is reflected by depressions
on the undersurface of the frontal bones. They are
elongate, broad rostrally and join the cerebral region
by means of short, narrow peduncles. The cerebral
region widens gradually to a maximum width above
the trigeminal nerve (V). It rises well above the level of
the olfactory bulbs and terminates dorsally in a rugose
area below the parietal bone. This cerebral flexure is
not as prominent as in the endocast of Camarasaurus
(DNM28) and Diplodocus (Holland, 1906). Although
there is a parietal foramen in the skull roof of Shun-
osaurus, the endocast fails to show the distinct dorsal
projection indicating that the pineal organ may be
absent (Roth & Roth, 1980). The cerebrum is as long
as it is high, but is extremely narrow transversely. On
the rostroventral surface of the cerebrum, the optic
nerves (II) enter. Small eye muscle nerves such as ocu-
lomotor (III) and trochlear (IV) arise from the base of
the midbrain. Behind and below their exits, a promi-
nent ventral projection indicates the attitude of the
pituitary body which is directed backward and down-
ward. It is preserved beautifully in the endocast. It
extends considerably below the level of the floor of the
medulla. A large canal, presumably for the internal
carotid artery, passes caudodorsally from its base.
Halfway up its rostral face, the canal for the abducens
nerve (VI) enters the pituitary fossa.
The optic lobes are located behind the cerebrum.
However, they are not as clearly demarcated as in coe-
lurosaurs, birds and mammals. From the midbrain
region, the endocast of Shunosaurus slopes gradually
to the foramen magnum. The canal for the middle
cerebral vein projects above the trigeminal root. The
cerebrum is extremely narrow with a median dorsal
ridge. The ridge is somewhat curved dorsally, but in
other sauropods, this ridge slopes gently backward.
The small cerebellum sits at the caudal midline of
the brain. No lateral swelling for the floculus can be
demarcated. Because of quadrupedal posture, the
Figure 11. Endocast of Shunosaurus lii (ZG6543). A, ventral view. B, lateral view.
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
body weight was well distributed among four limbs in
sauropods. Consequently, the role played by cerebel-
lum and floculus for balance and coordination was rel-
atively small. In contrast, in bipedal theropods
(including birds), where balance, coordination and
proprioception are essential, both the cerebellum and
floculus are well developed in the form of large swell-
ings (Hopson, 1980a; Chatterjee, 1991).
The medulla of Shunosaurus is long compared to
that of Camarasaurus (DNM28) and Diplodocus
(Holland (1906). It lies below the optic lobes and
cerebellum and is the seat of many cranial nerves
(VXII). The rostral portion of this region is con-
stricted, but becomes broad caudally. The trigeminal
nerve (V) is large and emerges from the medulla below
the optic lobe. A small facial nerve (VII) lies caudoven-
tral to nerve V. The endocast reveals part of the inner
ear and canalicular system. It shows a ventral projec-
tion, the lagena, and the dorsal outgrowth that may
indicate part of the rostral and caudal vertical canals.
The fenestra ovalis lies at the base of the lagena.
Behind the inner ear cavity is a large ganglion that
represents several cranial nerves such as glossopha-
ryngeal (IX), vagus (X), accessory (XI) as well as the
internal jugular vein. Farther backward lies the hypo-
glossal nerve (XII). As it emerges from the braincase,
it divides into two major branches.
The skull bones of Shunosaurus are intimately fused
with no mobile units. The quadrate is firmly sutured
with the squamosal and the quadratojugal and is
entirely immobile (monimostylic). The fronto-parietal
suture is coalesced without any sign of mesokinesis.
Similarly, the braincase is fused with the skull roof
indicating that metakinesis is absent in the sauropod
skull. The two dentaries are immovably joined at
the symphysis. The lower jaw increases in depth and
robustness towards the symphysis to provide addi-
tional strength to the tooth-bearing positions of the
jaws for procuring food. The skull is entirely akinetic
and monimostylic as in other sauropods. The vertical
wear facets of the teeth, as well as a close fit between
the quadrate and articular, indicate that the jaw
action was essentially orthal. The quadrate is almost
vertical which would allow a very wide gape, enhanc-
ing a large intake of food. Lack of fleshy cheeks would
also facilitate a wider gape without any muscular
restriction (Upchurch & Barrett, 2000).
Sauropods were exclusively terrestrial herbivores,
but their specific diet is unclear. They exhibit two
distinct types of tooth morphology: the spatulate or
spoon-shaped crowns, and the long, slender, peg-like
teeth with cylindrical crowns (Romer, 1956; Dodson,
1990b). A considerable range of tooth variation occurs
among these two extreme morphotypes. Spatulate
teeth are known in Shunosaurus, Omeisaurus,
Mamenchisaurus, Jobaria, Camarasaurus and Brachio-
saurus. Peglike teeth, on the other hand, are common
in Nemegtosaurus, Quaesitosaurus., Diplodocus, Baro-
saurus, Apatosaurus, Dicraeosaurus, Amargasaurus,
Euhelopus, Antarctosaurus, and Malawisaurus.
The tooth morphology may give some clues as to the
feeding habit of Shunosaurus (Zheng, 1996; Chatterjee
& Zheng, 1997). Shunosaurus had a long muzzle, a
large lateral nasal opening, robust spatulate teeth,
high dental count and parabolic dental arcade. The
crowns are high, lingually curved, and spoon-shaped,
with a long slender root. The serration at the edge is
present only in the lower teeth. There is a prominent
constriction between the crown and the root. There are
different stages of wear on the maxillary teeth. These
wear facets are essentially vertical along the rostral
and caudal ridge of the crown indicating that the jaw
motion was orthal. Upper and lower tooth rows inter-
digitate and move past each other in such a fashion
that each lower tooth fits between two upper teeth
along their lingual surface, like a pair of garden
shears, indicating a modest degree of oral processing
of coarser plant material (Fig. 12). These shearing
teeth were very effective in cutting hard branches,
stems, seeds, and foliage of contemporary flora such as
conifers, ginkgoes, cycads, ferns, and horsetails.
The huge size of the sauropod body and dispropor-
tionately small head and simple teeth have prompted
considerable speculation concerning their diet and
mastication. The lack of cranial kinesis and the simple
orthal chewing style indicate that sauropod teeth
served as shearing or cropping devices in order to strip
plant leaves from branches and twigs. It is likely that
sauropods used gastroliths for processing food, similar
to modern birds; however, none have so far been
reported associated with any of the Shunosaurus skel-
etons. Unlike modern herbivorous mammals, sauro-
pods lack wide grinding or shearing check teeth for the
oral processing of food. Instead, the teeth are all uni-
form, conical and incisiform. Their only function was
to procure plant material by stripping, raking and
shearing foliage from branches and converting it to a
small size for swallowing. Sauropods lacked any chew-
ing or masticatory adaptation. Food items were simply
swallowed whole and passed to a powerful muscular
gizzard containing sharp, abrasive pebbles the gas-
tric mill (Brown, 1941; Galton, 1986; Farlow, 1987).
Stones were swallowed by these animals as a means of
mechanically breaking down the food. These stomach
stones, rather than teeth, were used to pulp the tough
plant tissues in readiness for digestion. Because well
preserved gastroliths, consisting of a concentrated
mass of small stones, have been found inside the stom-
ach region of some sauropods (Bakker, 1968, 1986;
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
Calvo, 1994; Gillette, 1994), the gastric mill hypothe-
sis appears to be logical. In addition to mechanical
breakdown of food, microbial fermentation might have
also played an important role in their digestive sys-
tem. The true stomachs of sauropods were probably
like huge fermentation tanks in which vast quantities
of plant matter were slowly broken down to release
their nutrients.
Ever since Seeley (1887) established the Order Sau-
rischia on the basis of pelvic structure, nearly all
authors have assumed that Sauropoda was a well-
defined, coherent group (Huene, 1927, 1932; Romer,
1956). Currently, there are about 90 genera of sauro-
pods known, but most taxa are based on fragmentary
postcranial material and are not diagnostic at lower
taxonomic levels. So far, cranial material is limited to
a few genera. Recent cladistic analyses (Gauthier,
1986; Russell & Zheng, 1993; Upchurch, 1995, 1998,
1999; Zheng, 1996; Salgado, Coria & Calvo, 1997;
Wilson & Sereno, 1998) confirm that the Sauropoda is
a monophyletic clade and shed new insights on the
interrelationships of several genera. However, there is
a great deal of disagreement about the placement of
the Chinese sauropod taxa. Upchurch (1998) grouped
Shunosaurus with Omeisaurus, Mamenchisaurus and
Euhelopus into a basal clade, Euhelopodidae. Wilson &
Sereno (1998), on the other hand, put Euhelopus as a
highly derived member, a sister taxon of Titanosauria.
Unfortunately, Sauropoda cannot be defined cladis-
tically on the basis of cranial characters (Wilson &
Sereno, 1998; Upchurch, 1998). The inclusion of Shun-
osaurus is based entirely on postcranial characters,
because the cranial anatomy of basal members of
sauropods such as Vulcanodon (Raath, 1972; Cooper,
1984) and Barapasaurus (Jain et al., 1975) is
unknown. As Shunosaurus is the earliest sauropod for
which cranial remains are known, it provides the crit-
ical morphology of the basal sauropods. It also pro-
vides insights about the evolution of the sauropod
skull from the prosauropod condition. Using prosau-
ropods as a sister clade to Sauropoda (Galton, 1990),
Shunosaurus shares the following synapomorphies
with Eusauropoda Upchurch 1995:
1. Premaxilla, stepped rostral margin (Wilson &
Sereno, 1998).
2. Maxillary border of external naris long (Gauthier,
3. External naris retracted upward and backward
(McIntosh, 1990b).
4. Antorbital fenestra smaller than external naris.
5. Antorbital fossa absent (Wilson & Sereno, 1998).
6. Quadratojugal, rostral process is expanded dors-
oventrally at its tip and is longer than the dorsal
process (Gauthier, 1986).
7. Quadrate, caudal fossa present (Wilson & Sereno,
8. External mandibular fenestra highly reduced
(Russell & Zheng, 1993).
9. dentary, rostral end increases in depth towards
symphysis (Upchurch, 1998).
10. Tooth crown lacks ridges.
11. Tooth enamel with wrinkled texture (Wilson &
Sereno, 1998).
12. Tooth row ends below the antorbital fenestra
(Gauthier, 1986).
13. Precise crown to crown occlusion (Wilson &
Sereno, 1998).
14. Rostral end of snout is broadly rounded in dorsal
view (Upchurch, 1998).
Figure 12. Diagrammatic view of the jaw occlusion of
Shunosaurus during feeding. A, comparison of dental mar-
gin of Shunosaurus with a pair of garden shears. In Shun-
osaurus, the occlusal level of the upper jaw is convex
downward, whereas that of the lower jaw is concave
upward. B, wear facets between the upper and lower tooth
rows. Upper and lower tooth rows interdigitate and shear
past each other so that each lower tooth fits between two
upper teeth along their lingual surfaces. C, one bite cycle
procedure for shearing stems and branches.
© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 145–169
15. Ventral border of external naris lies higher than
that of orbit (McIntosh, 1990b).
16. Lacrimal head slopes backward relative to its
17. Jugal length reduced so that antorbital and lat-
eral temporal fenestra are crowded and approach
each other (Upchurch, 1995).
18. Quadratojugal, ascending process fails to reach
the descending process of the squamosal
(Gauthier, 1986).
19. Frontal excluded from supratemporal fossa
(Wilson & Sereno, 1998).
20. Supratemporal fenestra, long axis orientated
transversely (Wilson & Sereno, 1998).
21. Supraoccipital platform prominent.
22. Premaxillary dental count 4.
23. Maxillary teeth less than 20 (Gauthier, 1986).
24. Tooth row terminates rostral to antorbital fenes-
tra (Gauthier, 1986).
25. Teeth are procumbent, sloping forward (Gauthier,
26. Constriction between tooth crown and root.
27. Palatine, lateral ramus narrow (Wilson & Sereno,
We dedicate this paper in memory of Alick D. Walker
who was a mentor of the senior author for more than
three decades. This paper resulted in part from Zhong
Zheng’s Ph.D thesis research conducted under the
supervision of S. Chatterjee at the Museum of Texas
Tech University. We thank David Gower for inviting
us to contribute this paper in the Walker volume. We
thank Ailin Sun and Zhiming Dong of the Institute of
Vertebrate Palaeontology and Palaeoanthropology
(IVPP) for loan of the skull of Shunosaurus, Dan
Chure of Dinosaur National Monument for the oppor-
tunity to study the beautiful skull of Camarasaurus,
and Ouyang Hui of Zigong Dinosaur Museum for pro-
viding information on the Dashanpu quarry. We thank
Michael W. Nickell for the original drawings, Kyle
McQuilkin for the computer rendition of the drawings,
Richard Porter for photography, Jack McIntosh for
helpful discussions, and Kyle McQuilkin for editorial
assistance. Jeff Wilson and Paul Upchurch critically
reviewed the manuscript and offered valuable sug-
gestions. The research was supported by Texas Tech
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a angular
aof antorbital fenestra
ar articular
bo basioccipital
bpt basipterygoid process
bs basisphenoid
bt basisphenoid tubera
cel cerebellum
cer cerebral hemispheres
c.w.f. caudal wear facet
ch choana
d dentary
ea ethmoidal artery
ec ectopterygoid
en external naris
eo exoccipital
eov foramen for external occipital vein
f frontal
fm foramen magnum
fo fenestra ovalis
ic internal carotid artery
idp interdental plate
int e internal ear region
IXII foramina for cranial nerves
iv interpterygoid vacuity
j jugal
l lacrimal
lfl lateral flange of pterygoid
ls laterosphenoid
m maxilla
mef metotic fenestra
mf mandibular fenestra
n nasal
o.w.f. occlusal wear facet
o orbit
ol t olfactory tract
op opisthotic
op l optic lobe
os orbitosphenoid
p parietal
papr paroccipital
pf parietal fenestra (pineal)
pit pituitary
pl palatine
pm premaxilla
po postorbital
popr paroccipal process
ppf post palatine fenestra
pra prearticular
prf prefrontal
pro prootic
ps parasphenoid
psf postfrontal
pt pterygoid
ptf posttemporal fenestra
ptr pterygoid ramus
q quadrate
qc quadrate condyle
qf quadrate fossa
qh quadrate head
qj quadratojugal
qr quadrate ramus
r.w.f. rostral wear facet
rf retractor fossa
sa surangular
saf surangular fenestra
snf subnasal fenestra
soc supraoccipital
sp splenial
sq squamosal
stf subtemporal fenestra
sym symphysis
utf upper temporal fenestra
v vomer
vcm vena capitis medius
... This would appear to link Diamantinasaurus with Sarmientosaurus; in the latter taxon, the presence of three discrete openings for CN V was regarded as autapomorphic (Martínez et al., 2016). All other titanosaurs are characterized by a single ossified opening for CN V (Martínez et al., 2016), and this appears to be the case in all eusauropods for which this can assessed, with the exception of Shunosaurus Dong et al., 1983, which also has two openings (Chatterjee & Zheng, 2002). We note that Phuwiangosaurus was described as having two openings for CN V (Suteethorn et al., 2009) but could not corroborate this interpretation based on the published figures (in which the upper CN V opening appears to be a fossa, and in which the exit for CN VII was not identified). ...
... However, we could not recognize borders between the adenohypophyseal and neurohypophyseal compartments on the hypophyseal endocast. The size and robustness of the hypophyseal extension suggests that the pituitary gland was hypertrophied in adult Diamantinasaurus, as in other sauropods, including the non-neosauropod eusauropods Shunosaurus (Chatterjee & Zheng, 2002) and Spinophorosaurus Remes et al., 2009(Knoll et al., 2012, diplodocoids (Janensch, 1935(Janensch, -1936Sereno et al., 2007;Witmer et al., 2008;Balanoff et al., 2010;Paulina Carabajal et al., 2014), the early-branching macronarians Camarasaurus (Witmer et al., 2008) and Giraffatitan (Janensch, 1935(Janensch, -1936Knoll & Schwarz-Wings, 2009) and titanosaurs Martínez et al., 2016). The hypophyseal chamber appears to have been connected to the pharynx via a canal that is externally exposed by the craniopharyngeal foramen sensu Edinger (1942) (Fig. 10A, B). ...
... Although these proportions are typical for sauropods, the trochlear canal is substantially thinner than the oculomotor canal in some taxa [e.g. Shunosaurus (Chatterjee & Zheng, 2002)]. In other cases, the two canals have similar diameters [e.g. ...
The titanosaurian sauropod dinosaur Diamantinasaurus matildae is represented by two individuals from the Cenomanian-lower Turonian 'upper' Winton Formation of central Queensland, northeastern Australia. The type specimen has been described in detail, whereas the referred specimen, which includes several elements not present in the type series (partial skull, atlas, axis and postaxial cervical vertebrae), has only been described briefly. Herein, we provide a comprehensive description of this referred specimen, including a thorough assessment of the external and internal anatomy of the braincase, and identify several new autapomorphies of D. matildae. Via an expanded data matrix consisting of 125 taxa scored for 552 characters, we recover a close, well-supported relationship between Diamantinasaurus and its contemporary, Savannasaurus elliottorum. Unlike previous iterations of this data matrix, under a parsimony framework we consistently recover Diamantinasaurus and Savannasaurus as early-diverging members of Titanosauria using both equal weighting and extended implied weighting, with the overall topology largely consistent between analyses. We erect a new clade, named Diamantinasauria herein, that also includes the contemporaneous Sarmientosaurus musacchioi from southern Argentina, which shares several cranial features with the referred Diamantinasaurus specimen. Thus, Diamantinasauria is represented in the mid-Cretaceous of both South America and Australia, supporting the hypothesis that some titanosaurians, in addition to megaraptoran theropods and possibly some ornithopods, were able to disperse between these two continents via Antarctica. Conversely, there is no evidence for rebbachisaurids in Australia, which might indicate that they were unable to expand into high latitudes before their extinction in the Cenomanian-Turonian. Likewise, there is no evidence for titanosaurs with procoelous caudal vertebrae in the mid-Cretaceous Australian record, despite scarce but compelling evidence for their presence in both Antarctica and New Zealand during the Campanian-Maastrichtian. These later titanosaurs presumably dispersed into these landmasses from South America before the Campanian (~85 Mya), when seafloor spreading between Zealandia and Australia commenced. Although Australian mid-Cretaceous dinosaur faunas appear to be cosmopolitan at higher taxonomic levels, closer affinities with South America at finer scales are becoming better supported for sauropods, theropods and ornithopods.
... However, further research is needed to confirm this conclusion. A large number of Shunosaurus individuals have been recovered to date, which makes the skeletal structure relatively clear (Zhang et al. 1984;Zhang 1988;Sankar and Zhong 2010). Recently, a new dinosaur fossil group (Li et al. 2021;Dai et al. 2020) was discovered in Yunyang, Chongqing, China, and many Shunosaurus fossils were found in the lower member of the Shaximiao Formation in this area ( Figure 1). ...
... Some characteristics of bones change with the development of the individual (Zhang et al. 1984;Zhang 1988;Peng et al. 2005;Sankar and Zhong 2010). Since CLGPR V00007 does not preserve the skull, it is impossible to compare with ZDM5006 and ZDM5008 in terms of the healing degree of the bone sutures of the skull, teeth development, the clarity of the articular surface of paroccipital process and occipital condylar. ...
... Shunosaurus is a eusauropod (Wilson and Upchurch 2009), and its body size is obviously smaller than that of the members of Mamenchisauridae (Zhang 1988;Sankar and Zhong 2010;Yang 2014). Its neck length, in particular, is much shorter. ...
Full-text available
Shunosaurus is a small eusauropod from China. It is characterised by solid cervical and dorsal vertebral centra without complicated pneumatic structures, platycoelous or amphicoelous middle and posterior vertebrae and a lack of pubic foramen in adult individuals. Although many Shunosaurus individuals have been discovered, the ontogenetic characteristics of its long bones and bone tissues are not very clear and the existing description of the postcranial skeleton is relatively rudimentary. The new well-preserved and the smallest Shunosaurus specimen discovered in Yunyang, Chongqing, China, provides good material for solving these problems. The radial distal breadth is more than twice the minimum midshaft breadth, and the fibular distal end is twice as wide as the midshaft, while these ratios are all smaller in adult specimens. The lateral trochanter is undeveloped. This individual does show a pubic foramen. The degree of vascularisation of the bone tissue in the juvenile bone tissue is higher for adults. There are no arrested growth lines and peripheral rest lines in the compact bone, indicating that body size still increases slowly after maturity. The discovery of this new material expands the distribution range of Shunosaurus in China.
... Sauropods were the dominant herbivores of the Mesozoic Era. They were truly magnificent giant reptiles and the largest terrestrial animal that ever lived (Chatterjee & Zheng 2002;Sander & Clauss 2008). All sauropods are characterized by having long necks and relatively small heads (Wilson 2002;Upchurch et al. 2004). ...
... Here we give the comparison of the crown with known Jurassic sauropods with spatulate teeth. (Chatterjee & Zheng 2002). Anchisaurus teeth are leaf-shaped and serrated (Galton 1976). ...
This study presents the dental record of a non-neosauropod eusauropod from the Middle Jurassic rocks of the Jaisalmer Basin, India. An isolated, fragmentary heart-shaped crown of a turiasaur has been recovered from the intraformational conglomerate of the Badabag Member (Bathonian) of the Jaisalmer Formation. The heart-shaped, asymmetrical crowns with a labio-lingually compressed apex and apicobasal shallow grooves in the root are exclusive to the Turiasauria clade. This is the first record of a turiasaur from India as well as from Asia. This study extends the geographic distribution of the Turiasauria clade and provides new data about the diversity in the Jurassic sauropod fauna of India. The presence of heart-shaped teeth, from both Laurasia and Gondwanaland, shows a much wider geographic and stratigraphic distribution of the Turiasauria clade. There was a possibility of faunal exchange between the two big landmasses at least during the Jurassic. This study is one more step in bringing out the diversity in the vertebrate assemblage of the Jaisalmer Basin during the Middle Jurassic.
... So far, only the Z-spacings of Yinlong, Chaoyangsaurus, and Liaoceratops are known in ceratopsians and more research on the Z-spacing of ceratopsians are required to make meaningful comparisons. In non-avian dinosaurs, all known Z-spacing values are greater than 2.0 (Chatterjee and Zheng, 2002;Weishampel et al., 2004;Wiersma and Sander, 2016;Hanai and Tsuihiji, 2019;Becerra et al., 2020). Hanai and Tsuihiji, 2019 examined some extant crocodiles such as Alligator mississippiensis and Crocodylus siamensis which present infrequent Z-spacing less than 2.0. ...
... Because few replacement teeth are preserved in Yinlong and Chaoyangsaurus, it is difficult to reconstruct the tooth replacement waves by applying the same replacement index used in Liaoceratops. Therefore, we reconstructed the Zahnreihen according to the degree of tooth wear and the location of replacement teeth, as used in Shunosaurus (Chatterjee and Zheng, 2002), as well as applying a new methodology that includes the developmental stage of the pulp cavity. We divided the functional teeth in Yinlong and Chaoyangsaurus into 4 stages: (F1) no or slight wear on marginal denticles with an open pulp cavity; (F2) wear on marginal denticles and a slightly concave lingual wear facet with a large pulp cavity; (F3) extensive wear on marginal denticles and a concave lingual wear facet with the depression on the lingual surface of the roots or a bud of the replacement tooth; (F4) polished and greatly worn marginal denticles and a highly concave lingual wear facet with a broken pulp cavity or the emergence of a replacement tooth. ...
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The dental system of ceratopsids is among the most specialized structure in Dinosauria by the presence of tooth batteries and high-angled wear surfaces. However, the origin of this unique dental system is poorly understood due to a lack of relative knowledge in early-diverging ceratopsians. Here we study the dental system of three earliest-diverging Chinese ceratopsians: Yinlong and Hualianceratops from the early Late Jurassic of Xinjiang and Chaoyangsaurus from the Late Jurassic of Liaoning Province. By micro-computed tomographic analyses, our study has revealed significant new information regarding the dental system, including no more than five replacement teeth in each jaw quadrant; at most one replacement tooth in each alveolus; nearly full resorption of the functional tooth root; and occlusion with low-angled, concave wear facets. Yinlong displays an increase in the number of maxillary alveoli and a decrease in the number of replacement teeth during ontogeny as well as the retention of functional tooth remnants in the largest individual. Chaoyangsaurus and Hualianceratops have slightly more replacement teeth than Yinlong. In general, early-diverging ceratopsians display a relatively slow tooth replacement rate and likely use gastroliths to triturate foodstuffs. The difference in dietary strategy might have influenced the tooth replacement pattern in later-diverging ceratopsians.
... In Camarasaurus, the premaxillary body is lateromedially expanded, so that the posterodorsal process is medially displaced, with the subnarial fossa being expanded and separating the posterodorsal process, placed medially, from the anteroventral margin of the external naris, which is laterally offset (Zheng, 1996;Wilson and Sereno, 1998). An analogous situation seems to be true for Shunosaurus (Zheng, 1996;Chatterjee and Zheng, 2002). The posterodorsal process is reported to be absent in Brachiosaurus and Diplodocus as illustrated in Wilson and Sereno (1998). ...
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Non-sauropod sauropodomorphs, also known as 'basal sauropodomorphs' or 'prosauropods', have been thoroughly studied in recent years. Several hypotheses on the interrelationships within this group have been proposed, ranging from a complete paraphyly, where the group represents a grade from basal saurischians to Sauropoda, to a group on its own. The grade-like hypothesis is the most accepted; however, the relationships between the different taxa are not consistent amongst the proposed scenarios. These inconsistencies have been attributed to missing data and unstable (i.e., poorly preserved) taxa, nevertheless, an extensive comparative cladistic analysis has found that these inconsistencies instead come from the character coding and character selection, plus the strategies on merging data sets. Furthermore, a detailed character analysis using information theory and mathematical topology as an approach for character delineation is explored here to operationalise characters and reduce the potential impact of missing data. This analysis also produced the largest and most comprehensive matrix after the reassessment and operationalisation of every character applied to this group far. Additionally, partition analyses performed on this data set have found consistencies in the interrelationships within non-sauropod Sauropodomorpha and has found strong support for smaller clades such as Plateosauridae, Riojasauridae, Anchisauridae, Massospondylinae and Lufengosarinae. The results of these analyses also highlight a different scenario on how quadrupedality evolved, independently originating twice within the group, and provide a better framework to understand the palaeo-biogeography and diversification rate of the first herbivore radiation of dinosaurs.
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The Upper Cretaceous Winton Formation of Queensland, Australia, has produced several partial sauropod skeletons, but cranial remains—including teeth—remain rare. Herein, we present the first description of sauropod teeth from this formation, based on specimens from three separate sites. An isolated tooth and a dentary fragment from the Diamantinasaurus matildae type locality are considered to be referable to that titanosaurian taxon. A single tooth from the D. matildae referred specimen site is similarly regarded as being part of that individual. Seventeen teeth from a new site that are morphologically uniform, and similar to the teeth from the two Diamantinasaurus sites, are assigned to Diamantinasauria. All sauropod teeth recovered from the Winton Formation to date are compressed-cone-chisel-shaped, have low slenderness index values (2.00–2.88), are lingually curved at their apices, mesiodistally convex on their lingual surfaces, and lack prominent carinae and denticles. They are markedly different from the chisel-like teeth of derived titanosaurs, more closely resembling the teeth of early branching members of the titanosauriform radiation. This provides further support for a ‘basal’ titanosaurian position for Diamantinasauria. Scanning electron microscope microwear analysis of the wear facets of several teeth reveals more scratches than pits, implying that diamantinasaurians were mid-height (1–10 m) feeders. With a view to assessing the spatio-temporal distribution of sauropod tooth morphotypes before and after deposition of the Winton Formation, we provide a comprehensive continent-by-continent review of the early titanosauriform global record (Early to early Late Cretaceous). This indicates that throughout the Early–early Late Cretaceous, sauropod faunas transitioned from being quite diverse at higher phylogenetic levels and encompassing a range of tooth morphologies at the start of the Berriasian, to faunas comprising solely titanosaurs with limited dental variability by the end-Turonian. Furthermore, this review highlights the different ways in which this transition unfolded on each continent, including the earliest records of titanosaurs with narrow-crowned teeth on each continent.
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Eusauropods are large-bodied and long-necked dinosaurs that dominated the role of large herbivores in terrestrial ecosystems since at least the late Early Jurassic (Pliensbachian–Toarcian). Their early diversification is best recorded in South America where the best-preserved eusauropods and close relatives from this period of time have been found. The earliest sauropod from the Jurassic of South America is Amygdalodon patagonicus from the Cerro Carnerero Formation (Pliensbachian–early Toarcian), and its fragmentary remains suggest a position at the base of Gravisauria or as closely related to this clade. The Cañadón Asfalto Formation (middle–late Toarcian) has provided three named sauropods, although a higher diversity of sauropods may have existed. These are the basal eusauropod Patagosaurus fariasi, known from multiple specimens, the much more incompletely known early sauropod Volkheimeria chubutensis, and Bagualia alba that is known from multiple specimens and includes fairly complete craniomandibular remains. These taxa provide the earliest evidence of ecological predominance by large-bodied sauropods and are therefore significant for understanding the rise and success of this group in the Jurassic Period. The current knowledge of these sauropods from the late Early Jurassic of South America indicates that the evolutionary radiation of Eusauropoda occurred at least by the mid-Toarcian, subsequent to a large-scale volcanic event in the Southern Hemisphere that has been linked to global climatic change and the rise of conifers as the predominant components of Jurassic seasonal forests.
A new sauropod-dominanted dinosaur fauna, including several new species of sauropods and stegosaurs, was recently excavated from the Lower Shaximiao Formation in Yunyang, northeastern Chongqing, southwestern China. Although this dinosaur fauna could possibly provide significant clues for a better understanding of the basal sauropod's evolution and diversification of basal sauropods in the Jurassic, many aspects of this fauna are not well defined constrained in terms of detailed stratigraphy, geochronology, and comparison with similar faunas in other regions of the Sichuan Basin. This paper reports on the petrological features of this locality, using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometry (EDS), with laser ablation-multi-collector-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb geochronology for a tuffaceous siltstone layer beneath the dinosaur fossil-bearing layers from the fossil site. The SEM and XRD results show a typical tuffaceous mineral composition of this layer. Zircon U-Pb data yielded a weighted average age of 166.0 ± 1.5 Ma, indicating a maximum burial age for this dinosaur fauna. This fauna is chronologically comparable with the Shunosaurus-Omeisaurus Fauna, which is primarily found in the central Sichuan Basin.
Sauropod dinosaurs achieved the largest body sizes and the most elongate necks and tails of any terrestrial vertebrate. Their necks and tails were held aloft as cantilevers, beams supported at one end and free at the other. Synovial joints between vertebrae provide mobility, and synchondrosial joints within vertebrae facilitate growth. These requirements come at a cost, as the joints are potential sites of dislocation, with deleterious consequences for the living animal. Morphological specializations of sauropod inter- and intravertebral joints provided stability without compromising other functional demands. Sauropod intervertebral joints were characterized by concavo-convex morphology, which has been hypothesized to confer greater flexibility or to stabilize joints against dislocation by translation. Examination of joint mobility in an extant analog, Alligator, reveals that concavo-convex joints do not confer greater flexibility than do planar joints, nor do they inherently limit mobility. Convexity is greatest in regions of the greatest shear, consistent with a stabilizing function. Sauropod intervertebral joints have a consistent polarity in which the concave articular surface faces the body (i.e., cervical opisthocoely, caudal procoely). Physical modeling reveals that this polarity is more stable than its opposite because it inhibits the convex articulation from rotating out of joint. The advantage of the sauropod polarity is enhanced by greater joint mobility, distal loading, and mechanically advantageous ligament insertion sites. This provided stabilization without compromising other functions. The intravertebral (i.e., neurocentral) joints of archosaurs such as sauropods remain unfused to a later age than in most other vertebrates, permitting rapid, sustained growth to large body sizes. The greater susceptibility of the joints to dislocation compared to fused bone may be compensated for by complex, interdigitated sutures that resist compression, rotation, and translation. In the sauropod Spinophorosaurus, variation in sutural complexity along the vertebral column is consistent with the expected stress distribution. Large-scale morphological structures in the sutures are oriented to resist specific regional stresses. The integration of fossil data with studies of extant taxa and model experiments provides a means to answer functional questions about extinct organisms. The results offer insights into skeletal biomechanics that are widely applicable to other vertebrates.
The holotypic materials of the sauropod ‘Morosaurus’ agilis—a partial skull, proatlases, and first three cervical vertebrae—have been a taxonomic and phylogenetic mystery since their initial description by O. C. Marsh in 1889 and redescription by C. W. Gilmore in 1907. Although most species of Morosaurus were subsumed into Camarasaurus in 1919, ‘M.’ agilis was left in the defunct genus without a proper taxonomic assignment. Similarities have been noted between ‘M.’ agilis and other Morrison Formation sauropod taxa, including Camarasaurus, Haplocanthosaurus, Diplodocus, and Brachiosauridae, but it had yet to be included in a phylogenetic analysis. Here we present new data following additional preparation and study that suggest ‘M.’ agilis is a basally diverging member of Dicraeosauridae, along with the recently described sauropod Kaatedocus. Based upon its recovery as a distinct taxon, we propose that ‘M.’ agilis receive the new generic name Smitanosaurus, yielding the new combination Smitanosaurus agilis.