A new specimen of Plesiopterys wildi reveals the diversification of cryptoclidian precursors and possible endemism within European Early Jurassic plesiosaur assemblages

Abstract

Background
A virtually complete and articulated plesiosaur skeleton (MH 7) is described from the Lower Jurassic (Toarcian) Posidonienschiefer Formation near Holzmaden in southern Germany. Plesiosaur remains are rare in this rock unit compared to those of other marine reptiles, such as ichthyosaurs and thalattosuchian crocodylomorphs. The new specimen offers an opportunity to assess the biodiversity of Early Jurassic plesiosaurs documented from what is now Central Europe.

Methods
The osteology of MH 7 is described and compared with other Early Jurassic plesiosaurs based on first-hand observations. Phylogenetic analyses using both equal weighting and weighted parsimony determined phylogenetic placement within Plesiosauria.

Results
Plesiopterys wildi is an early-diverging plesiosauroid and a sister taxon to Franconiasaurus brevispinus and Cryptoclidia. MH 7 represents a subadult individual, providing an updated character state diagnosis of Plesiopterys wildi , which has hitherto only been known from the osteologically immature holotype SMNS 16812. The presence of multiple regionally distinct plesiosaur genera and species within the European epicontinental marine basins suggests possible paleobiogeographical segregation during the Toarcian.

Full text

A new specimen of Plesiopterys wildi
reveals the diversification of cryptoclidian
precursors and possible endemism within
European Early Jurassic plesiosaur
assemblages
Miguel Marx
1
, Sven Sachs
2
, Benjamin P. Kear
3
, Mats E. Eriksson
1
,
Klaus Nilkens
4
and Johan Lindgren
1
1Department of Geology, Lund University, Lund, Sweden
2Abteilung Geowissenschaften, Naturkunde-Museum Bielefeld, Bielefeld, Germany
3The Museum of Evolution, Uppsala University, Uppsala, Sweden
4Urwelt-Museum Hauff, Holzmaden, Germany
ABSTRACT
Background: A virtually complete and articulated plesiosaur skeleton (MH 7) is
described from the Lower Jurassic (Toarcian) Posidonienschiefer Formation near
Holzmaden in southern Germany. Plesiosaur remains are rare in this rock unit
compared to those of other marine reptiles, such as ichthyosaurs and thalattosuchian
crocodylomorphs. The new specimen offers an opportunity to assess the biodiversity
of Early Jurassic plesiosaurs documented from what is now Central Europe.
Methods: The osteology of MH 7 is described and compared with other Early
Jurassic plesiosaurs based on first-hand observations. Phylogenetic analyses using
both equal weighting and weighted parsimony determined phylogenetic placement
within Plesiosauria.
Results: Plesiopterys wildi is an early-diverging plesiosauroid and a sister taxon to
Franconiasaurus brevispinus and Cryptoclidia. MH 7 represents a subadult
individual, providing an updated character state diagnosis of Plesiopterys wildi, which
has hitherto only been known from the osteologically immature holotype SMNS
16812. The presence of multiple regionally distinct plesiosaur genera and species
within the European epicontinental marine basins suggests possible
paleobiogeographical segregation during the Toarcian.
Subjects Paleontology, Evolutionary Studies
Keywords Biodiversity, Diversification, Endemism, Jurassic, Plesiosaur
INTRODUCTION
From a global perspective, the Early Jurassic was characterized by the steady break-up of
Pangea and associated climatic fluctuations that produced alternating greenhouse and
icehouse conditions (Ruebsam et al., 2020;Nordt, Breecker & White, 2022). These
paleoenvironmental changes coincided with the radiation of various reptile groups,
including Plesiosauria (Brusatte et al., 2010;Benson, Evans & Druckenmiller, 2012;Toljagić
& Butler, 2013;Puértolas-Pascual et al., 2021). The Lower Jurassic fossil record of
How to cite this article Marx M, Sachs S, Kear BP, Eriksson ME, Nilkens K, Lindgren J. 2025. A new specimen of Plesiopterys wildi reveals
the diversification of cryptoclidian precursors and possible endemism within European Early Jurassic plesiosaur assemblages.
PeerJ 13:e18960 DOI 10.7717/peerj.18960
Submitted 13 September 2024
Accepted 19 January 2025
Published 31 March 2025
Corresponding author
Miguel Marx,
miguel.marx@geol.lu.se
Academic editor
David Hone
Additional Information and
Declarations can be found on
page 28
DOI 10.7717/peerj.18960
Copyright
2025 Marx et al.
Distributed under
Creative Commons CC-BY 4.0

plesiosaurians is especially diverse, with members of Plesiosauroidea Gray, 1825,
Pliosauridae Seeley, 1874 and Rhomaleosauridae von Nopcsa, 1928 represented by
numerous taxa from Europe, particularly in Germany and England (Benson, Evans &
Druckenmiller, 2012;Benson & Druckenmiller, 2014).
The early Toarcian Posidonienschiefer Formation (Posidonia Shale) in southwestern
Germany has been crucial for understanding the early evolution of Plesiosauria
(Groβmann, 2007;Benson, Evans & Druckenmiller, 2012). A total of thirteen largely
complete and articulated skeletons have been described to date (Sachs et al., 2025). These
constitute five distinct genera and species: the plesiosauroids Seeleyosaurus
guilelmiimperatoris (Dames, 1895), Microcleidus brachypterygius (von Huene, 1923),
Plesiopterys wildi O’Keefe, 2004; the pliosaurid Hauffiosaurus zanoni O’Keefe, 2001; and the
rhomaleosaurid Meyerasaurus victor (Fraas, 1910). However, the taxonomic status of some
of these taxa remains contentious.
Groβmann (2007) assigned plesiosauroid plesiosaurs from the Posidonienschiefer
Formation to two genera and species: S.guilelmiimperatoris and M.brachypterygius.
O’Keefe (2004) alternatively established P.wildi based on an osteologically immature
individual (SMNS 16812) that was subsequently assigned to S.guilelmiimperatoris by
Groβmann (2007).Benson & Druckenmiller (2014) recovered P.wildi as the sister taxon of
Cryptoclidia, and more derived than Microcleididae, thus rejecting this proposed
synonymy. Vincent (2010) and Vincent et al. (2017a) also described osteologically
immature individuals (SMNS 51143 and SMNS 51945) that they report as likely
representing new taxa. Although, SMNS 51141 was referred to M.brachypterygius by
Groβmann (2007).
Resolving the taxonomic status of the Posidonienschiefer Formation plesiosauroids
is vital for reconstructing marine vertebrate biodiversity and paleobiogeography within
the European Epicontinental Sea (which yields the most complete Lower Jurassic
plesiosaur record globally; Tutin & Butler, 2017). In particular, the Toarcian plesiosaurs
in Germany are distinguishable from coeval assemblages occuring in the Yorkshire Basin
of England (Groβmann, 2007). The English Toarcian taxa include the rhomaleosaurids
Rhomaleosaurus cramptoni (Carte & Baily, 1863), Rhomaleosaurus thorntoni Andrews,
1922,Rhomaleosaurus zetlandicus (Phillips in Annual report of the Yorkshire
Philosophical Society, 1854), and Sthenarosaurus dawkinsi Watson, 1909;the
microcleidids Microcleidus homalospondylus (Owen, 1865), and Microcleidus
macropterus (Seeley, 1865), and the pliosaurids Hauffiosaurus tomistomimus Benson
et al., 2011a,andHauffiosaurus longirostis (Tate & Blake, 1876)(=Macroplata White,
1940:Benson et al., 2011a). These populations were paleogeographically separated in part
by the extensive London-Brabant Massif, which may have instigated faunal endemism
(Godefroit, 1994;Maisch, 1999;Maisch & Ansorge, 2004).
However, the marine reptile assemblages between these two regions should feasibly be
similar, as is evident in their coeval actinopterygian fossil record (Wretman, Blom & Kear,
2016). Likewise, the Toarcian ichthyosaur fossil record across the European epicontinental
basins is broadly similar with conspecific taxa present across different “zones”of the
European Epicontinental Sea (Fischer, Guiomar & Godefroit, 2011).
Marx et al. (2025), PeerJ, DOI 10.7717/peerj.18960 2/33

Microcleidus has further been reported in other parts of the Central European Basin
(CEB), including the area that is today Luxembourg (Microcleidus melusinae Vincent et al.,
2017b) and the more southern Aquitarian Basin (Microcleidus tournemirensis (Bardet,
Godefroit & Sciau, 1999)). These species variations can potentially be explained by
temporal differences rather than paleobiogeographic segregation (Benson et al., 2011a),
since M.tournemirensis derives from the late Toarcian and is therefore younger than M.
melusinae (Vincent et al., 2017b). In addition, plausible remains of S. guilelmiimperatoris
(GPIH unregistered) and M.victor (GPIH 4851), two plesiosaurs previously known only
from the Southwestern German Basin, were reported also in Fennoscandian deposits
(Sachs et al., 2016), suggesting a possible dispersal of these taxa northwards toward what is
today Scandinavia. Sachs et al. (2025) determined that GPIH (unregistered) is instead only
referrable to the family rank, Microcleididae. Finally, Toarcian ichthyosaur and plesiosaur
genera long thought to be endemic to the CEB have been found in Siberia, indicating
further range extensions into the polar regions (Zverkov, Grigoriev & Danilov, 2021). It is
therefore surprising that plesiosaur taxa between the Yorkshire and Germanic basins are so
disparate. In this article, we describe a new plesiosaur skeleton from the Posidonienschiefer
Formation and establish its implications for plesiosaur assemblage segregation vs.
synonymy within the Early Jurassic basins of Europe.
Geological Setting
The Posidonienschiefer Formation has been extensively studied for over a century (see
Röhl et al., 2001;Röhl & Schmid-Röhl, 2005; and Muscente et al., 2023 for an overview of
the stratigraphy). The sediments of the Posidonienschiefer Konservat-Lagerstätte near
Holzmaden, Germany, accumulated within the Southwestern German Basin as part of the
shallow-marine epicontinental shelf that bordered the Tethys Ocean in the southeast
during the Early Jurassic (Röhl et al., 2001;Röhl & Schmid-Röhl, 2005;Sinha et al., 2021).
The marine environment was inundated with terrestrial clays and carbonates, leading to
the formation of marls and marly shales (Röhl et al., 2001). The Posidonienschiefer
Formation is divided into three ammonite zones, with the most complete plesiosaur
remains being found at the top of the Dactylioceras tenuicostatum zone (εII
1
), and in both
the lower and upper parts of the Harpoceras serpentinum zone (εII
3,
εII
4
&εII
9
)
(Groβmann, 2006,2007;Smith & Vincent, 2010;Vincent, 2010;Vincent et al., 2017a); (H.
serpentinum zone = Harpocerus falciferum zone: Williams, Benton & Ross, 2015).
MATERIALS AND METHODS
MH 7 was originally excavated near the village of Holzmaden from ɛII
6C
(Harpoceras
serpentinum zone) in 1940. In 1970, MH 7 was considered for exhibition at
Urwelt-Museum Hauff, but another plesiosaur (MH 8) was chosen instead and therefore
took its place. MH 7 would remain in storage until it was decided by Rolf Hauff and Klaus
Nilkens during the Covid-19 pandemic to prepare the skeleton. MH 7 was re-assembled
from multiple shale slabs using historical notes on handwritten index cards as references.
The fossil was prepared solely by Klaus Nilkens from Urwelt-Museum Hauff, mainly from
the ventral side of the body, which originally faced downwards into the seafloor. Saws were
Marx et al. (2025), PeerJ, DOI 10.7717/peerj.18960 3/33

used to cut the larger slabs into smaller portions and to reduce the thickness of the matrix.
The cyanoacrylate, Kö-Kleber C2 (Kömmerling Chemische Fabrik GmbH, Pirmasens,
Germany), was used in addition to epoxy during preparation. To remove the encasing rock
matrix, hammers, chisels and small hand-held tools were used in addition to pneumatic air
scribes and sandblasting with iron powder. The torso of MH 7 was enclosed in a hard
carbonate concretion that needed to be approached with pneumatic chipping hammers
and pneumatic air scribes. Incomplete and fractured bones were reconstructed with
synthetic resin, epoxy clay, or with shale matrix.
Comparison of MH 7 with other Jurassic plesiosaurians is based on collection visits by
M.M., B.P.K., S.S., and J.L. to Urwelt-Museum Hauff, Naturkundemuseum Stuttgart,
Institut für Geowissenschaften der Universität Tübingen, Tübingen, Germany, and
Museum für Naturkunde in Berlin, Germany. Phylogenetic analyses were performed using
the matrix of Sachs, Eggmaier & Madzia (2024) (270 characters, 130 taxa) with parsimony
analyses of equal and unequal weights and the addition of MH 7. Characters 25, 138, 139,
153, and 248 were previously modified by Madzia & Cau (2020) and thus differ from those
provided by Benson & Druckenmiller (2014). For the Sachs, Eggmaier & Madzia (2024)
matrix, 67 character states are ordered as per Madzia, Sachs & Lindgren (2019). Following
Sachs, Eggmaier & Madzia (2024),Neusticosaurus pusillus (Fraas, 1881) was designated as
the outgroup. TNT version 1.6 (Goloboff & Morales, 2023) was used to conduct the
analyses (see the phylogenetic analysis section for specific search parameters). The STATS.
RUN script calculated the consistency index, retention index, and recombined consistency
index. This script comes with downloading TNT version 1.6. Bremer indices were
calculated directly from the TNT v.1.6 interface (Trees/Bremer Supports). For the
weighted analyses, symmetric resampling was used to assess node support using the same
parameters as Sachs, Eggmaier & Madzia (2024) with symmetric resampling using the
settings: traditional search, 1,000 replicates, default change probability (33), default
settings for the cutoff (collapse groups below 1) and frequency differences (GC) as output.
Eight character changes were made to the holotype of Plesiopterys wildi (SMNS 16812)
based on first-hand study of the specimen (characters 55, 83, 151, 153, 192, 215, 257, and
260). See the supplemental for updates to these character changes.
RESULTS
SYSTEMATIC PALEONTOLOGY
Sauropterygia Owen, 1860
Plesiosauria de Blainville, 1835
Plesiosauroidea Gray, 1825
Plesiopterys O’Keefe, 2004
Plesiopterys wildi O’Keefe, 2004
Marx et al. (2025), PeerJ, DOI 10.7717/peerj.18960 4/33

Holotype. SMNS 16812; an osteologically immature skeleton (O’Keefe, 2004). Many of the
vertebral neural spines, centra and ribs have been reconstructed, as have sections of the
pectoral and pelvic girdles, and phalanges.
Referred specimen. MH 7, A ~3-m long plesiosauroid skeleton (Fig. 1) lacking only the
distal phalanges along with some braincase elements and parts of the craniofacial skeleton,
which are either missing or still embedded in matrix. The following cranial elements are
missing: anterior ramus of the right squamosal, one or both postorbitals, the postfrontals,
prefrontals, one of the jugals, the anterior portion of the right pterygoid, left pterygoid,
vomers may or may not be present, one of the ectopterygoids, one or both of the palatines,
the left hyoid, the right prootic and right exoccipital-opisthotic.
Type locality and horizon. εII
4
in the lower Toarcian Unterer Schiefer (Harpoceras
serpentinum zone) from the Posidonia Shale Formation near Holzmaden, Germany, as
reported by O’Keefe (2004).
Locality and horizon of referred specimen. MH 7 was excavated from εII
6C
of the lower
Toarcian Posidonienschiefer Formation. The provenance quarry was located in a forest
near Holzmaden, between the villages of Schlierbach and Ohmden.
Revised diagnosis. A plesiosauroid plesiosaurian with the following autapomorphies: An
elongated groove for the internal carotid artery on the dorsal surface of the quadrate ramus
of the pterygoid (present in MH 7); a short medial flange of the pterygoid separating the
posterior interpterygoid vacuity from the anterior interpterygoid vacuity (present in MH 7
and SMNS 16812); star-shaped interclavicle formed by a deeply concave anterior margin,
two anterolateral processes, a second pair of processes oriented laterally, and one
posteriorly oriented process (present in MH 7 and SMNS 16812). The following
combination of characters further differentiates the taxon from all other plesiosaurians:
exposure of the cultriform process of the parasphenoid to the posterior margin of the
anterior interpterygoid vacuity (present in SMNS 16812 and inferred in MH 7), possession
of a narrow and straight quadrate ramus of the pterygoid (present in both specimens); tall
dorsal ramus of the maxilla, demarcating the anterior margin of the orbit (present in both
specimens); lack of supernumerary elements in the limb (present in both specimens); lack
of a proximal flange on the radius (present in both specimens); posterior-half of caudal
neural spines are inflected posteriorly (present in both specimens).
Description
Skull
Craniofacial skeleton and palate. Much of the craniofacial skeleton is disarticulated and
dispersed throughout the matrix at the anterior end, while the mandible is intact (Fig. 2A).
Isolated teeth are also dispersed throughout the matrix, but many remain in their original
position in the mandible (Fig. 2B). The right premaxilla has been distorted by dorsoventral
flattening; thus, its original shape cannot be accurately determined. However, as preserved,
the premaxilla is broad anteriorly and then narrows posteriorly. Medially, this element
exhibits an elongated and straight contact with its counterpart (Figs. 3A,3B). The external
surface is marked by indentations on the bone.
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A dorsal ramus from the maxilla demarcates the anterior margin of the orbit (Figs. 3A,
3B). The bone surface of the maxilla is pitted by foramina. Posteriorly, the maxilla tapers to
form a low bar. Sixteen dental alveoli are visible in the left maxilla (Fig. 4A). Medially to
these, replacement dental lamina foramina are apparent. In lateral view, a sheet of bone is
present on the left maxilla that rises dorsal to the upper tooth row.
A jugal is identified among the many dispersed cranial elements in the matrix, as it
preserves part of the orbital rim (Fig. 2A). Part of the left temporal bar is preserved; it is
mediolaterally compressed with a facet for the postorbital present anteriorly (Fig. 4A). A
pair of plausible frontals is exposed and would have formed a smooth surface across the
skull roof between the orbits (Fig. 4A). The dorsal ramus of the right squamosal is visible
and compressed anteroposteriorly (Fig. 4B); its apex is low and flat. A medially placed
groove extends along almost the entire length of the ramus. Part of the left squamosal is
also visible where it articulates with the quadrate and partly contributes to the temporal bar
(Fig. 4C). The entire contribution of the squamosal to the temporal bar cannot be assessed;
nonetheless, the squamosal makes a small contribution to the ventral margin of the
temporal bar with an interdigitating suture visible laterally (Fig. 4C).
The condyles of the quadrate are exposed distally. The medial condyle is well-rounded
while the lateral one is flatter (Fig. 4B). Along the medial surface of the bone, a facet to
receive the quadrate process of the pterygoid is evident (Fig. 4B).
From the few observable palatal bones, the posterior portion of the right pterygoid is
preserved and exposed in dorsal view (Fig. 5A). An elongated groove for the internal
carotid artery extends along the dorsal surface of the narrow and straight quadrate flange.
The latter forms an extensive contact along its articulating surface with the quadrate
(Fig. 4B). A short and rounded medial flange from the pterygoid is apparent where the
posterior interpterygoid vacuity would have terminated anteriorly (Fig. 5A).
Figure 1 Skeleton of MH 7 exposed in ventral view. Full-size
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In MH 7, the anterior portion of the pterygoid narrows following this flange. Thus, the
anterior-most portion of the parasphenoid would have been close to the posterior opening
of the anterior interpterygoid vacuity, likely contributing to its posterior margin.
Additional palatal elements include an ectopterygoid with a ventral boss (Fig. 5B), and a
possible palatine (Figs. 2A,3B).
Braincase and hyoid. A parabasisphenoid is visible in ventral view and has a smooth
ventral surface that is concave (Fig. 4A). The parasphenoid covers most of the ventral
surface without the presence of medial foramen exposing the basisphenoid. A foramen for
the passage of the carotid artery could not be confidently identified and is likely obscured
by matrix. The basipterygoid process forms a buttress with a smooth ventral surface and a
broad lateral facet for the pterygoid (Fig. 4A). Posteriorly, the parabasisphenoid broadens
significantly to receive the basioccipital.
The basioccipital exhibits a rounded occipital condyle outlined by a ridge that contacts
the facets for the exoccipital-opisthotics (Fig. 5C). Matrix covers part of the occipital
condyle, thus obscuring this surface. Facets for the exoccipital-opisthotics are separated
and have ellipsoidal impressions on the dorsal surface that are slightly greater than half the
length of the basioccipital (Fig. 5C). An eminence on the dorsal surface has a triangular
outline and tapers posteriorly to terminate almost half-way along the paired exoccipital-
opisthotic facets. Most of the anterior surface of the basioccipital is not visible; however, a
sulcus located medially may delineate the notochordal pit.
Figure 2 Skull bones and dentition of MH 7. (A) Map of skull bone elements. Bone map key: 1, basioccipital; 2, left quadrate; 3, partial left
squamosal; 4, left postorbital?; 5, unidentified element; 6, atlas-axis complex; 7, unidentified element; 8, left temporal bar (partial left squamosal); 9,
parabasisphenoid; 10, unidentified element; 11, left maxilla; 12, anterior cervical vertebra; 13, frontals; 14, partial left maxilla and perhaps part of the
palate; 15, unidentified element; 16, vomers?; 17, mandible; 18, right premaxilla; 19, right maxilla; 20, possible palatine or jugal; 21, unidentified
element; 22, supraoccipital; 23, ectopterygoid; 24, right quadrate flange of the pterygoid; 25, right quadrate; 26, right squamosal suspensorium; 27,
parietals; 28, jugal; 29, left prootic; 30, left exoccipital-opisthotic; 31, right partial pterygoid. (B) Close-up image of dentition showing apicobasally
extending ridges along the lingual face of the teeth but absent on the labial. Full-size
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The ventral surface of the supraoccipital is exposed (Fig. 5D). Ventromedially, the
surface of this element is smooth. Posteromedially, a short and sharply tapering process is
visible pointing posteriorly. The anterior semicircular canal, and crus communis are most
visible in the left-half of this element (Fig. 5D). Facets for the prootic and
exoccipital-opisthotic occur anteriorly and posteriorly, respectively.
Figure 3 Mandible and skull elements of MH 7. (A) Ventral view of the mandible and associated skull
elements. (B) Mandible and skull elements labeled. (C) Left lateral view of the mandible. Abbreviations:
an, angular; art, articular; d, dentary; j, jugal; R. mx, right maxilla; L. mx, left maxilla; p, palatine; R. pm,
right premaxilla; sp, splenial; s.r., symphyseal ridge; sur, surangular.
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The left exoccipital-opisthotic is preserved; however, only the posterior surface of this
complex is visible (Fig. 5E). The paraoccipital process is slightly compressed dorsoventrally
and extends ventrolaterally to a blunt end. The left prootic is also preserved and evinces a
short contact with the basisphenoid, with a smoothly curved posterior margin that
contributed to the fenestra ovalis. Dorsally, this element would have partly articulated with
the supraoccipital and the exoccipital-opisthotic (Fig. 5F). The medial face of the prootic in
Figure 4 Isolated skull elements of MH 7. (A) Close-up image with exposed dental alveoli and
replacement alveoli of the left maxilla. Plausible frontals are also in view along with the parabasisphenoid
in ventral view, part of the left squamosal, and an unidentified element. Also in view is the left temporal
bar with a facet for the postorbital located anteriorly. (B) Right suspensorium, right quadrate, right
quadrate flange of the pterygoid, and parietals. (C) Part of the left squamosal in lateral view along with an
articulated left quadrate. Abbreviations: R./L. bpt. pr, Right/Left basipterygoid process; d, dental; fr,
frontal; mx, maxilla; par, parietal; pbs, parabasisphenoid; po, postorbital; q, quadrate; q. pt., quadrate
process of the pterygoid; sq, squamosal. Full-size
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MH 7 is obscured by sediment, thus the vestibule and foramina for cranial nerves cannot
be described.
A well-preserved right hyoid is visible between the rami of the mandible, close to its life
position (Fig. 3B). This element is slightly curved in its morphology.
Mandible. The ventral surface of the dentary has a rough texture with numerous foramina
piercing the surface (Figs. 3A,3B). The symphysis is elaborated by a keel that becomes
shallow anteriorly (Fig. 3B). An exact count is not possible, however, the maximum
number appears to be close to thirty, as ~26 teeth are visible within the right dentary or
close to their original position in this element, and ~31 teeth are visible within and or near
the left dentary. The angular contacts the dentary and articular. The mandibular rami are
straight and not bowed laterally. The retroarticular processes of MH 7 are oriented slightly
posteromedially with a gentle posterodorsal inclination.
The splenial is an elongated element that extends for most of the entire length of the
mandible, but its contribution to the symphysis cannot be assessed. The surangular forms a
tall apex that contributes significantly to the coronoid process. The coronoid is not
Figure 5 Palate and braincase elements of MH 7. (A) Dorsal view of partial right pterygoid. The gray region highlighted on the bone denotes the
groove on the dorsal surface. (B) Ectopterygoid in view with a ventral boss. (C) Basioccipital in dorsal view. (D) Supraoccipital in ventral view. (E)
Left exoccipital-opisthotic in posterior view. (F) Lateral view of left prootic. Abbreviations: exo-op, exoccipital-opisthotic; f.o., fenestra ovalis; oc,
occipital condyle; pbs, parabasisphenoid; pr, prootic; pt, pterygoid; sc.c., semi-circular canal; so, supraoccipital.
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observable and matrix obscures any fenestrae on the medial side of the mandible. In lateral
view, the mandible has a smooth surface near the mandibular glenoid with a clear suture
between the dentary and angular (Fig. 3C).
Dentition. The tooth crowns are all recurved. They are elongated and oval in cross-section
with labio-lingual compression (Fig. 2A). The crowns are ornamented by narrow ridges
extending apicobasally along the lingual face but also on the mesial and distal surfaces,
whereas the labial face is smooth. The largest teeth occur mesially with subsequent size
reduction distally.
Axial skeleton
Cervical vertebrae. Thirty-five cervical vertebrae remain in articulation. The atlas-axis
complex and one additional cervical vertebra are dislocated from the anterior end of the
cervical column, giving a total cervical vertebra count of 38. The axis centrum and what
appears to be the atlas intercentrum are clearly distinguishable by a vertical suture running
along the lateral surface (Fig. 6). The neural arch of the atlas is short dorsoventrally and
contacts the axial neural arch posteriorly. The neural spine of the axis is low but elongated
and becomes transversally wider posterodorsally along its apex. The axial neural spine
becomes more compressed before merging ventrally with the postzygapophyses. The
lateral surface of the axis is smoothly concave before meeting the facet for the axial rib. The
posterior articular facet of the axis is platycoelous. Damage along the ventral surface of the
atlas-axis complex prevents interpretation of a hypophyseal ridge.
Most of the cervical vertebrae are slightly longer than high (Table S1). The neural arches
of the cervical vertebrae are completely fused to their adjoining centra with a U-shaped
neurocentral suture visible in lateral view (Figs. 7A–7C). A rugose bone texture is
observable on the lateral and ventral surfaces of the cervical centra, close to the articular
Figure 6 Atlas-axis complex of MH 7 in left lateral view. Full-size
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rims (Fig. 7A). The articular facets of the centra are obscured in almost all of the cervicals,
aside from the first two postaxial vertebrae, which exhibit amphicoelous facets. All the
cervical ribs are unfused. The pre- and postzygapophyses are oriented horizontally along
the entire cervical series. The rib facets are all double-faceted and co-joined in most of the
cervical vertebrae. The interface between the facets is delineated by a narrow ridge in the
anterior vertebrae that eventually inverts to form a groove in the posterior cervical
vertebrae. The rib facets in MH 7 are ellipsoidal in shape for almost all of the cervical
vertebrae, except for the posterior-most in which the diapophyses and parapophyses are
more circular in profile (Fig. 7C). The diapophyses have a slightly greater surface area than
the parapophyses in the cervical vertebrae. Rib facets for the last four cervical vertebrae in
the series are oriented posterolaterally.
Nearly all the cervical vertebrae exhibit ribs that are hatchet-shaped with a more
pronounced posterior process on the distal end (Figs. 7A,7B). The posterodistal process of
the cervical rib becomes increasingly more elongated moving posteriorly along the neck.
The posterodistal process forms a cylinder in the posterior-most cervical vertebrae
Figure 7 Axial skeleton of MH 7. (A) Anterior cervical vertebrae. (B) Middle cervical vertebrae;
(C) Posterior cervical vertebrae and a single exposed pectoral vertebra. (D) Anterior caudal vertebrae.
(E) Middle caudal vertebrae. (F) Posterior caudal vertebrae. Abbreviations: ca. r., caudal rib; ca. r. f.,
caudal rib facet; ca. v. 13, caudal vertebra 13; cr, cervical rib; c. r. f., cervical rib facet; ch, chevron; ch. f.,
chevron facet; fs, foramina subcentralia; ncs, neural central suture; ns, neural spine; prez, pre-
zygapophyses; poz, postzygapophyses; pv, pectoral vertebra; pv r. f., pectoral vertebra rib facet.
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(Fig. 7C). A pair of foramina on the ventral surface of the cervical centra is separated by a
subtle ventral ridge in the posterior cervical vertebrae (Fig. 7C). The middle and anterior
cervical vertebrae lack the mid-ventral ridge separating the foramina. A lateral longitudinal
ridge is absent in all the cervical vertebrae except for perhaps postaxial cervical vertebrae
four and five where it is possible an incipient ridge is present; however, this possible ridge
may be an artefact created by the rugose texture of bone on the lateral surface. Thus, it is
not certain that this trait is present in MH 7.
The shape of the neural spine varies along the cervical series. The anterior cervical
vertebrae have neural spines that are curved posteriorly and an anteroposteriorly elongated
base that tapers distally, forming a shark fin-shaped neural spine. The neural spines
become more rectangular in the posterior-half of the neck and are tall relative to the centra
with decreased spacing between adjacent neural spines (Fig. 7C).
Pectoral and dorsal vertebrae. A pectoral vertebra displaced from the vertebral series is
visible on the right side of the interclavicle in MH 7 (Fig. 7C). The rib facet is
dorsoventrally expanded and oriented ventrolaterally. The articular facet of the centrum is
platycoelous. The subsequent pectoral vertebra is mostly obscured by matrix. The dorsal
vertebrae are completely obscured by matrix, aside from the ventral surface of four dorsal
centra poking through the gastral basket. In what is exposed, these centra exhibit an
hourglass shape in ventral view with platycoelous articular facets.
Caudal vertebrae. At least 35 to 36 caudal vertebrae are visible in near perfect articulation,
mostly in right lateral view (Figs. 7D–7F). An exact count is difficult, as the last several
vertebrae are slightly disarticulated and partially covered by matrix. The total number of
caudal vertebrae is unlikely to have been much more than 36 because the anterior-most
caudal in the series exhibits a dorsoventrally expansive rib facet, similar to a sacral vertebra.
In lateral view, the neural arch contributes to the caudal rib facet (Fig. 7D). In ventral view,
foramina subcentralia are apparent (Fig. 7E). The neural spines of the anterior 10 caudal
vertebrae in the series exhibit tall neural spines that are approximately 1.5 times greater in
height than the centra with rounded distal ends (Fig. 7D). The neural spines of the
posterior-most caudal vertebrae significantly shorten (Figs. 7E,7F) and the last caudal
vertebrae are difficult to discern as their associated neural arches, ribs, and chevrons are
disarticulated and intermingled with the centra.
None of the caudal ribs and chevrons are fused to the centra. The first four caudal
vertebrae in the series exhibit facets for the chevrons that do not protrude far from the
centra relative to the sequential caudal vertebrae where these facets project further from
the ventral surface of the centrum. The articular facets of caudal centra 18–21 are
hexagonal in shape. The caudal ribs are dorsoventrally compressed and expand distally.
The hemapophyses of MH 7 are elongated in the anterior portion of the caudal series and
become shorter posteriorly with expanded proximal and distal ends. The 11th caudal
vertebra in the series of MH 7 exhibits a more posteriorly inflected neural spine as opposed
to the preceding vertebrae (Fig. 7E). From the 13th caudal vertebra to the 16th, the neural
spines are inflected posteriorly at a low angle (Figs. 7E,8).
Ribs and gastralia. The bodies of the anterior dorsal ribs are exposed in MH 7. These are
all circular in cross-section and oriented posteriorly (Fig. 9); in life they would therefore
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have had at least a slight posterior inclination when articulated. The gastral basket is a
tightly interlocking mesh of ~60 spindle-shaped gastralia arranged in at least three rows
(Fig. 9). The gastralia with the greatest thickness lay along the midline. Smaller and
narrower gastralia line the ventrolateral margins of the trunk (Fig. 9). Approximately 26
gastralia lie in the right row, while ~10 are aligned in the middle, and ~22 are present in the
left row (Fig. 9).
Appendicular skeleton
Pectoral girdle. The interclavicle is a broad star-shaped bone that forms the anteromedial
margin of the pectoral girdle ventral to the clavicles (Fig. 10A). The anterior end of the
interclavicle has a deep and broad U-shaped margin. Triangular anterolateral and lateral
processes extend from the interclavicle. A short posterior extension appears to be present
but is damaged.
Both clavicles have a triradiate outline in ventral view, with the posteromedial section
contacting the anterior margin of the coracoids; this prevents the scapulae meeting along
the midline, and from contacting the anteromedial processes of the coracoids. This
interpretation is supported by the left scapula which remains in articulation with the
clavicle and the coracoid (Fig. 10A).
The dorsal blade of the scapula curves posteriorly, terminating in a blunt apex that is
approximately as wide as the body of the blade (Fig. 10A). The anterior margin of the
dorsal blade is smoothly convex in outline, while the posterior margin of the blade is
concave. On the lateral surface, the blade is flat with no process or elaborations. A distinct
shelf on the ventrolateral margin is set at ~90from the dorsal blade. The ventral surface of
the scapula is smooth with a concave medial border. The facets for the coracoid and the
humerus are approximately equal in width.
The coracoid forms a blunt and short anterior process (Fig. 10A). Laterally, the element
widens to contact the scapula and receive the head of the humerus. A ventrally protruding
Figure 8 Caudal vertebrae of MH 7 in left lateral view; vertebrae positions are labeled above the
neural spines. Note the posterior inflection of the neural spines as the series shifts to the poster-
ior-half of the tail. Full-size
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Figure 9 Gastral basket of MH 7; some dorsal ribs remain near life position in the anterior portion of
the trunk. A minimum of three rows for the gastralia are apparent; two laterally and one medially.
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Figure 10 Pectoral and pelvic girdles of MH 7. (A) Pectoral girdle. (B) Pelvic girdle. Abbreviations: cl,
clavicle; cor., coracoid; icl, interclavicle; il, ilium; is, ischium; p, pubis; sc, scapula; v. pro., ventral process.
Note: the scale bar in panel B is also accurate for (A). Full-size
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eminence is noticeable along the center midline but flattens toward the glenoid and
anterior process. The glenoid surface on the coracoid is a flat surface that forms a 90angle
with smoothly concave lateral margins posterior to the glenoid. The posterior processes of
the coracoids extend as prominent cornua. See Table S2 for measurement data from the
pectoral girdle.
Pelvic girdle. The pubis forms a smoothly convex anterior margin (Fig. 10B). The presence
of an anterolateral cornu can be inferred in MH 7 as the lateral margin of the left pubis
transitions from a concave surface to a convex one anteriorly that extends lateral to the
acetabulum of the pelvic girdle; although the anterolateral corner of the left pubis is partly
reconstructed, and the anterior margin of the right one is partly obscured. The medial
surface is straight with the greatest thickness at mid-length. The surface of the acetabulum
on the pubis is relatively long compared to that of the ischium. A posteromedial projection
of the pubis completes a pelvic bar. The pelvic fenestra is well-defined and circular.
Along the anterior margin, the ischium forms a smoothly concave surface to contribute
to the pelvic fenestra (Fig. 10B). The anteromedial process of the ischium is blunt where it
contacts the pubis to form a pelvic bar. The medial surface of the ischium is nearly straight.
The posterior process of the ischium is smoothly rounded. As in the pubis, the deepest
region of the ischium is at approximately mid-length of the medial symphysis.
The proximal end of the ilium is double faceted to articulate with the ischium and
contribute to the acetabulum. The ilium is nearly straight with only slight anterior
curvature. There is no rotation of the distal end relative to the proximal end. A tubercule is
located on the posterior surface. The exposed lateral, anterior, and posterior surfaces are
otherwise smooth. The anterior face of the distal articular surface is more developed than
the posterior articular end.
Forelimb. The humerus has a flat proximal head (Fig. 11A), which would have been
covered by cartilage in life (Krahl, 2021). Posterior to the humeral head there is a
protuberance that faces posteriorly. The posterior margin of the humerus is smoothly
concave, and the anterior margin is almost straight. The distal end of the humerus has two
distinct articular facets. A gentle posterodistal expansion can be observed while the
anteriorodistal end of the humerus is only slightly expanded. The radius is columnar in
overall shape with a flat proximal articular facet for the humerus and two distal facets, the
larger being for the radiale and the smaller posterodistal facet for the intermedium. The
anterior margin of the radius is shallowly concave, and the posterior margin is more deeply
concave where it encloses the elongate epipodial foramen. The ulna is lunate in shape with
one facet proximally and two distally to articulate with the centrale and the ulnare. Two
rows of mesopodials are present in the forelimbs and hindlimbs of MH 7. There is no
evidence of any pisiforms or accessory elements. The carpal bones are discoidal in shape,
with distal carpal I being proximodistally compressed. The fifth metacarpal would have
been situated posterior to distal carpal IV in the second row. The phalanges of the forelimb
are spool-shaped and robust. The digit formula for the forelimb is 4-9-11-11-4. At the
distal end of the second digit, a small round ossicle is present beyond the eighth digit. The
distal end of both the forelimbs and the hindlimbs are strongly curved posteriorly.
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Hind limb. The proximal end of the femur has a cratered surface and appears slightly
rounder than the proximal head of the humerus (Fig. 11B). The pre- and postaxial margins
of the femur are straight with a nearly symmetric distal expansion, as opposed to the
humerus, which exhibits a greater posterodistal expansion. The distal width of the femur is
approximately the same as in the humerus; however, the femur is shorter proximodistally
(Table S3). The tibia is more robust than the radius in being proximodistally shorter with
less curvature on the posterior margin, thus producing a squared profile. Likewise, the
fibula is shorter and more robust in shape than the ulna.
No supernumerary elements are present in the fore or hind limbs. The tarsals are less
disc-like than the carpals. The centrale is rectangular with a broad proximal facet for the
tibia and the calcaneum is reniform with a slightly concave posterior margin. The
astragalus has two distinct facets for articulation with the tibia and fibula. Distal tarsal I is
rectangular in outline, and distal tarsal II + distal tarsal III is ellipsoidal. Distal tarsal IV
presents a flat proximal surface for articulation with the astragalus.
The phalanges are all spool-shaped and robust with proximal widths equal to a little
more than half the proximodistal length. In the more complete left hindlimb, four
phalanges remain articulated in the first digit, eight in the second digit, eight in the third
Figure 11 Limbs of MH 7. (A) Right forelimb. (B) Left hindlimb. Abbreviations: as, astragalus; cal,
calcaneum; ce, centrale; dcI, distal carpal I; dcII+III, distal carpal II and III; dcIV, distal carpal IV; dtI,
distal tarsal I; dtII+III, distal tarsal II+III; dtIV, distal tarsal IV; f, femur; fi,fibula; h, humerus; mcI-IV,
metacarpals I through IV; mcV, metacarpal V; mtI-IV, metatarsal I-IV; mtV, metatarsal V; r, radius; ra,
radiale; ti, tibia; u, ulna; ul, ulnare. Full-size
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digit, six in the fourth digit, and four in the fifth digit. The exact digit formula of the
hindlimb cannot be determined due to disarticulation of the phalanges at the distal end.
However, based on the eight disarticulated phalanges distal to the left hind limb, the digit
formula is likely 4-8-11-11-4, similar to that of the forelimb.
Comparisons
The skull of MH 7 is disarticulated, with many of the cranial elements being obscured by
matrix; however, some comparisons can be made with other Lower Jurassic plesiosaurians.
For example, the ornamentation of the teeth in MH 7 is comparable to that of Seeleyosaurus
guilelmiimperatoris (Maisch & Rücklin, 2000;Sachs et al., 2025), Microcleidus melusinae
(Vincent et al., 2017b) and the holotype of Plesiopterys wildi, but different from that in
Microcleidus brachyptergius, where small ridges are present along both the labial and lingual
surface (Maisch & Rücklin, 2000). There are 16 dental alveoli in the maxilla of MH 7; that is,
thesamenumberasinM.brachypterygius (Maisch & Rücklin, 2000), but more than
in S.guilelmiimperatoris (14 maxillary alveoli) (Maisch & Rücklin, 2000)andMicrocleidus
tournemirensis (12 maxillary alveoli) (Bardet, Godefroit & Sciau, 1999)andlessthanin
Plesiosaurus dolichodeirus Conybeare, 1824, which has 18 maxillary teeth (Maisch & Rücklin,
2000).
The tall dorsal ramus of the maxilla at the anterior end of the orbit in MH 7 is distinct
from the short and broader dorsal ramus of the maxilla in M.brachypterygius (Maisch &
Rücklin, 2000), M.melusinae (Vincent et al., 2017b), M.tournemirensis (Bardet, Godefroit
& Sciau, 1999), and P.dolichodeirus (Storrs, 1997). The holotype of P.wildi (SMNS 16812),
however, also exhibits a narrow ramus at the anterior end of the maxilla where it forms the
anterior margin of the orbit (Groβmann, 2007;fig. 4).
The short contribution to the ventral side of the temporal bar by the squamosal with a
vertical, interdigitating suture where the jugal would have articulated in MH 7 (Fig. 4C)is
distinct from that of P.dolichodeirus (Storrs, 1997), M.tournemirensis,M.brachypterygius
(Maisch & Rücklin, 2000;Smith, Araújo & Mateus, 2011), and Lusonectes sauvagei Smith,
Araújo & Mateus, 2011 which exhibit a longer contribution to the ventral side of the
temporal bar by the squamosal. How the squamosal in MH 7 would have contributed to
the dorsal margin of the temporal bar is, however, unknown as this portion of the skull is
missing.
The shape of the mandible in MH 7 is similar to that of M.melusinae (Vincent et al.,
2017b) and P.dolichodeirus (Storrs, 1997) with no apparent bowing. A ventral elaboration
along the symphysis is also shared with M.melusinae, although the holotype of P.wildi
(SMNS 16812) appears to lack this character, at least as reconstructed by Groβmann (2007,
fig. 5) contra O’Keefe (2004). In our first-hand observation, we also find that P.wildi lacks
this keel (Figs. 12A,12B).
With respect to the pterygoid in MH 7, this element is nearly identical to that of SMNS
16812 in several aspects. An elongated groove along the dorsal surface of the quadrate
flange was regarded by O’Keefe (2004) as unique to P.wildi, however, close inspection
reveals that the ‘groove’in SMNS 16812 is sediment fill between the quadrate ramus of the
pterygoid and the parabasisphenoid (Fig. 13). In the more mature MH 7, this groove on the
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pterygoid is apparent and has been previously regarded as a plesiomorphic condition (as it
is present also in nothosaurids; Rieppel, 1994;Rieppel & Werneburg, 1998;O’Keefe, 2006).
The quadrate flange of the pterygoid in MH 7 is narrow and straight (Fig. 12C), as in
SMNS 16812, but this character state is present also in M.tournemirensis (Bardet,
Godefroit & Sciau, 1999) and M.brachypterygius (Maisch & Rücklin, 2000). However, in
these latter two taxa, the pterygoid forms a sheet-like extension below the basicrania, which
does not occur in SMNS 16812 or MH 7. The quadrate flange in MH 7 and SMNS 16812
does not develop a vertical flange, like in M.melusinae (Vincent et al., 2017b) and M.
tournemirensis (Bardet, Godefroit & Sciau, 1999). O’Keefe (2004) interpreted the quadrate
flange in SMNS 16812 as terminating in a boss; however, Groβmann (2007) reported this
process as being broken. It is further worth noting that O’Keefe (2004) included the
anterior interpterygoid vacuity with a rounded posterior margin and sharp anterior border
Figure 12 Comparisons of the holotype of Plesiopterys wildi (SMNS 16812) with MH 7. (A, B) Palate of P. wildi holotype (SMNS 16812) with
right (R. pt) and left (L. pt) pterygoids labeled. (C) Right pterygoid of MH 7 in dorsal view (R. pt) with a symmetry of this element (L. pt) showing the
partially preserved quadrate flanges of the pterygoids and medial flanges. (D1) Basioccipital of SMNS 16812 in dorsal view. (D2) Sketch of SMNS
16812 basioccipital. (D3) Basioccipital of MH 7 in dorsal view. (D4) Sketch of MH 7 basioccipital. (E1) Supraoccipital of SMNS 16812 in ventral
view. (E2) Sketch of SMNS 16812 supraoccipital. (F1) Left lateral view of the left prootic from SMNS 16812. (F2) Sketch of prootic in SMNS 16812.
(F3) Left lateral view of left prootic from MH 7. (G1) Anterior cervical series of SMNS 16812. (G2) Middle caudal vertebrae of SMNS 16812 with
posteriorly reclining neural arches. Abbreviations: a. int. vac., anterior interpterygoid vacuity; a. sc.c, anterior semicircular canal; bs, basisphenoid; cc,
crus communis; oc, occipital condyle; ps, parasphenoid; pt, pterygoid. Full-size
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as an autapomorphy for P.wildi. However, this character is also present in Trinacromerum
bentonianum Cragin, 1888 (Williston, 1908;fig. 1) and possibly also Tricleidus seeleyi
Andrews, 1910;(fig. 74). Furthermore, the autapomorphy defined by O’Keefe (2004, p. 973)
for P.wildi:“exposure of the cultriform process of the parasphenoid almost to margin of
anterior pterygoid vacuity”was reassessed and rather, the parasphenoid does extend to the
margin of the anterior interpterygoid vacuity (Figs. 12A,12B).
Anterior to the quadrate flange, a short and rounded medial flange from the pterygoid is
apparent in MH 7, where the posterior interpterygoid vacuity would have terminated
anteriorly (Fig. 12C). This is similar but more rectangular in outline in SMNS 16812
(Figs. 12A,12B). In MH 7, the anterior portion of the pterygoid narrows rapidly following
this medial flange. Thus, the anterior portion of the parasphenoid would likely have been
close to the posterior opening of the anterior interpterygoid vacuity or contributed to its
posterior border. The morphology of the pterygoid is different between MH 7 and SMNS
16812 with respect to the lateral margin, where the pterygoid of MH 7 is slightly concave as
opposed to straight (Figs. 12B,12C). These shared states of the palate are distinguishable
from those of M.brachypterygius,M.tournemirensis,P.dolichodeirus,M.melusinae,
Microcleidus homalospondylus (NHMUK 36184), and L.sauvagei which all lack the medial
flange of the pterygoid separating the elongated interpterygoid vacuities (Smith, Araújo &
Mateus, 2011;Vincent et al., 2017b). O’Keefe (2004) reported the medial extension of the
pterygoid of SMNS 16812 as being dorsal to the plane of the palate. However, because the
skull of SMNS 16812 is compressed, we consider this feature to be a taphonomic artefact.
In MH 7, the parabasisphenoid has a concave ventral surface (Fig. 4A), and thus is not
flat as in M.brachypterygius (Maisch & Rücklin, 2000). No foramina are evident in ventral
view, as opposed to the condition M.tournemirensis (Bardet, Godefroit & Sciau, 1999). The
parasphenoid differs from that of SMNS 16812 in the lack of any processes or extensions
Figure 13 Dorsal view of the basicranium of SMNS 16812 (holotype of Plesiopterys wildi). (A) Dorsal
view of the basicranial without labels. (B) Dorsal view of the basicranial with the pterygoids, parabasi-
sphenoid, and prootics labeled. Abbreviations: pbs, parabasisphenoid; pr, prootic; pt, pterygoid.
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on the ventral surface where O’Keefe (2004, p. 981) reports a “ventro-posterior process of
the basisphenoid”similar to Thalassiodracon hawkinsii (Owen, 1838) and OUM J.28585,
with the parasphenoid extending beyond this process to articulate with the quadrate
flanges of the pterygoid and the basioccipital tuber (O’Keefe, 2004,fig. 3). We have
reinterpreted the basicranium from that of O’Keefe (2004) (Figs. 12A,12B,13). However,
the parabasisphenoid in T.hawkinsii apparently lacks this structure (Benson et al., 2011b).
O’Keefe (2004) considered the extension of the parasphenoid beyond this process of the
basispheniod to be unique to P.wildi. However, Groβmann (2007) did not interpret any
process of the basisphenoid in P.wildi.
In view of the basioccipital, an eminence with a triangular outline tapers posteriorly
along the dorsal surface. This same eminence can be found in SMNS 16812 (Figs. 12D1,
D2); however, it is incipient and not as developed as in MH 7 (Figs. 12D3,12D4). The
basioccipital tubers are round and small processes, like those of SMNS 16812, but differing
from those of Stratesaurus taylori Benson, Evans & Taylor, 2015 which are
anteroposteriorly long. Similar to M.tournemirensis, there is no contribution to the
occipital condyle by the exoccipital-opisthotics in MH 7.
O’Keefe (2004,fig. 5) interpreted the anterior semicircular canal as piercing the
supraoccipital in SMNS 16812, similar to the condition in T.hawkinsii (Benson et al.,
2011b), rather than an impression on the bone as in MH 7 and S.taylori (Benson, Evans &
Taylor, 2015). We could not identify a foramen for the anterior semi-circular canal on the
supraoccipital of SMNS 16812 as described by O’Keefe (2004). Rather, impressions in the
bone for the anterior semicircular canal and the crus communis were apparent in the
left-half of the supraoccipital (Figs. 12E1,12E2). A smaller impression is apparent in the
right-half and may represent an asymmetry in the contribution of the supraoccipital to the
semicircular canals and crus communis is SMNS 16812 (Fig. 12E2). The endosseous
labyrinth in MH 7 thus appears nearly identical to that of SMNS 16812.
The prootic is not often documented in Lower Jurassic plesiosaurians (Bardet, Godefroit
& Sciau, 1999;Maisch & Rücklin, 2000;Smith & Benson, 2014;Vincent et al., 2017b). Yet,
the prootic of MH 7 is very similar to that of S.taylori (Benson, Evans & Taylor, 2015,fig.
6D, E) but differs from that of M.homalospondylus (Brown, Vincent & Bardet, 2013,figs.
3.2, 3.6), which has an anteroventral process that Brown, Vincent & Bardet (2013)
considered to be plesiomorphic among Plesiosauria. The prootic of SMNS 16812 also
differs from MH 7 in possessing an elongate and flat dorsal margin without any obvious
facets for the supraoccipital or exoccipital-opisthotic (Figs. 12F1–12F3). The ‘prootic’
bones described by Groβmann (2007) are isolated skeletal fragments located at the
distal-most portion of the quadrate ramii of the pterygoids, and appear to belong to these
elements, where they would have articulated with the quadrate. The medial face of the
prootic in MH 7 is obscured by sediment; thus, the vestibule and foramina for cranial
nerves cannot be described or compared (Fig. 12F3).
The hyoid of MH 7 exhibits an anterior end that is slightly greater in diameter than the
posterior end, similar to the condition in Meyerasaurus victor and S.taylori (see Smith &
Vincent, 2010;Benson, Evans & Taylor, 2015).
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The estimated total cervical count for MH 7 (38) is higher than that of M. melusinae
(32) (Vincent et al., 2017b), S.guilelmiimperatoris (34) (Dames, 1895;Sachs et al., 2025),
but lower than that of M. tournemirensis (41) (Bardet, Godefroit & Sciau, 1999) and close
to SMNS 16812 (39) (O’Keefe, 2004). The diapophyses and parapophyses on the
posterior-most cervical vertebrae of MH 7 are not separated, unlike the condition in
Microcleidus spp. (Benson, Evans & Druckenmiller, 2012). Nearly all the cervical vertebrae
exhibit ribs that are hatchet-shaped with a more pronounced posterior process on the
distal end and a reduced process anteriorly (Figs. 7A,7B). This contrasts with the
condition in S.guilelmiimperatoris, where significantly more pronounced anterior and
posterior processes are present on the anterior cervical ribs (Dames, 1895); however, it is
similar to Franconiasaurus brevispinus Sachs, Eggmaier & Madzia, 2024 and Plesiopterys
wildi (Fig. 12G1). A keel on the lateral surface of the cervical vertebrae may be present in
two anterior cervical vertebrae. However, unlike the condition in M.homalospondylus
(Owen, 1865) and M.tournemirensis (Bardet, Godefroit & Sciau, 1999), this character is
absent from the rest of the cervical column. Nevertheless, the presence of this character in
MH 7 may be ontogenetically influenced. A lateral keel is not apparent in SMNS 16812
(Fig. 12G1). The shark fin-shaped anterior neural spines in MH 7 are similar to those of
SMNS 16812 (Fig. 12G1) and F.brevispinus (Sachs, Eggmaier & Madzia, 2024) but unlike
those of S.guilelmiimperatoris (Dames, 1895), which exhibits rectangular neural spines
oriented straight dorsally. The presence of shark fin-shaped cervical neural spines in the
osteologically immature holotype of Brancasaurus brancai Wegner, 1914 might suggest
that this character is indicative of immature individuals (Sachs, Hornung & Kear, 2016). M.
melusinae exhibits cervical neural spines inclined slightly anteriorly (Vincent et al., 2017b).
The anterior cervical vertebrae of M. tournemirensis have a straight posterior margin that
is slightly inclined posteriorly and a smoothly convex anterior margin (Bardet, Godefroit &
Sciau, 1999). With respect to the caudal series, a minimum of 35 or 36 caudal vertebrae in
MH 7 is greater than that of M.brachypterygius (28 caudal vertebrae in SMNS 51143, and
MH 8), but less than SMNS 16812 (41) (O’Keefe, 2004). The posterior inflection of the
caudal neural spines in the posterior half of the caudal series in MH 7 is identical to SMNS
16812 (Fig. 12G2).
The broad star-shaped interclavicle in MH 7 is similar to that of SMNS 16812 (Figs.
14A,14B). M.tournemirensis (Bardet, Godefroit & Sciau, 1999), and Westphaliasaurus
simonsensii Schwermann & Sander, 2011 exhibit interclavicles that are more funnel-shaped
in outline. The scapulae in SMNS 16812 are similarly distanced from the anteromedial
extensions of the coracoid. The clavicles of SMNS 16812 appear distinct from those of MH
7 in being crescent-shaped when viewed together ventrally, with a convex anterior margin
and a more concave one posteriorly (Figs. 14A,14B). The scapular blade of MH 7 is
identical in morphology to that of F.brevispinus (Sachs, Eggmaier & Madzia, 2024)
with a convex surface along its anterior margin, unlike that of SMNS 16812 which is
more concave (Figs. 14C1,14C2). The cornua in MH 7 do not extend laterally beyond
the pectoral glenoid, similar to the condition in M.tournemirensis (Bardet, Godefroit &
Sciau, 1999).
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The anterior margin of the pubis in MH 7 is convex unlike that of M. tournemirensis
(Bardet, Godefroit & Sciau, 1999)orPlesiopharus moelensis Puértolas-Pascual et al., 2021,
which has a shallow excavation on the anterior border. The medial surface of the ischium is
nearly straight, similar to that of P.moelensis (Puértolas-Pascual et al., 2021) but not
M. tournemirensis, where the medial margins are smoothly curved (Bardet, Godefroit &
Sciau, 1999).
With respect to the limbs, the radii of MH 7 lack the flange present on the proximal part
of the radius in Microcleidus spp. and S.guilelmiimperatorus (Dames, 1895;Benson, Evans &
Druckenmiller, 2012;Sachs et al., 2025). Similarly, SMNS 16812 lacks this flange (Figs. 14D1,
14D2). Additionally, MH 7 does not have supernumerary elements in either the fore—or
hind limbs, as otherwise seen in M.brachypterygius (von Huene, 1923)andS.
guilelmiimperatoris (Dames, 1895) but again, similar to SMNS 16812 (Figs. 14D1,14D2).
Phylogenetic analysis
We used similar analysis parameters to those of Sachs, Eggmaier & Madzia (2024) with an
unweighted ‘New Technology’search and two additional searches using implied
weighting. These analyses were done with the Plesiopterys wildi holotype (SMNS 16812)
and MH 7 included in the matrix, and a second round of analyses with the holotype
excluded, for a total of six analyses. MH 7 was scored for 135 characters out of 270 used by
Sachs, Eggmaier & Madzia (2024). Space for 200,000 trees was allocated to RAM
(command: hold 200000;). Then, a ‘New Technology’search with 1,000 addition sequences
and default settings for tree fusing, ratchet, drift, and sectorial searches was used, which
retained 41 trees (CI: 0.190; RI: 0.684; RCI: 0.130). A TBR search was then run on trees
saved to RAM. This returned a best score of 2,098 with a consensus of Plesiosauroidea,
including MH 7 in a more derived position than SMNS 16812, but basal to
Figure 14 Girdle elements of SMNS 16812 (Plesiopterys wildi holotype) for comparison. (A) Pectoral girdle of SMNS 16812 in ventral view. (B)
Sketch of the pectoral girdle of SMNS 16812. Cross-hatched areas are reconstructed areas/mount support. (C1) Left lateral view of the scapula from
SMNS 16812. (C2) Lateral view of the right scapula from MH 7 (flipped asymmetrically to compare with SMNS 16812). (D1) Left front limb of
SMNS 16812 in ventral view. (D2) Right front limb of MH 7 in ventral view. Abbreviations: cl, clavicle; cor, coracoid; h, humerus; icl, interclavicle; r,
radius; sc, scapula; u, ulna. Full-size

DOI: 10.7717/peerj.18960/fig-14
Marx et al. (2025), PeerJ, DOI 10.7717/peerj.18960 23/33

Franconiasaurus brevispinus. Together, SMNS 16812, MH 7 and F. brevispinus are
reconstructed successively closer to cryptoclidian plesiosaurians; the placement of SMNS
16812 basal to MH 7 can be attributed to its juvenile ontogenetic stage. Bremer supports
were calculated using the implemented script in TNT (“Trees”>“Bremer supports”>
“Calculate Bremer supports”>“TBR from existing trees”>“Retain trees suboptimal by
three steps”)(Fig. 15A). The second ‘New Technology’search with SMNS 16812 removed
retained 20 trees (CI: 0.191, RI: 0.683, RCI: 0.131; best score: 2,085). Subsequent TBR
returned MH 7 again in an intermediate position between Microcleididae and F.
brevispinus (Fig. 15B). This analysis recovered the following combination of character
states diagnosing MH 7: morphology of ventral surface of the parasphenoid within the
interpterygoid vacuity concave (ch.83: 2 -> 0), axial neural spine transversely broad
(ch.150: 0 -> 1), chevron facets of middle and distal caudal vertebrae project significantly
ventrally (ch.193: 0 -> 1), ratio of coracoid to scapula length is greater than or equal to 1.9
(ch.196: 1 -> 0), scapular dorsal process length relative to posterior ramus of the scapula is
Figure 15 Phylogenetic analyses incorporating MH 7. (A) Phylogenetic placement of MH 7 from the ‘New Technology’search with subsequent
TBR. Bremer support indicated below nodes. (B) Phylogenetic placement of MH 7 excluding SMNS 16812 from the ‘New Technology’search with
subsequent TBR. Bremer support indicated below nodes. (C) Implied weighting analyses (k = 6/9) including SMNS 16812; symmetric resampling
values above 50 below the nodes. (D) Implied weighting analyses (k = 6/9) excluding SMNS 16812; symmetric resampling values above 50 below the
nodes. Full-size

DOI: 10.7717/peerj.18960/fig-15
Marx et al. (2025), PeerJ, DOI 10.7717/peerj.18960 24/33

subequal or shorter (ch.204: 0 -> 1), ventral projection of the coracoid extends from the
intercoracoid symphysis (ch.215: 0 -> 1).
Two implied weighting analyses were performed using two different concavity
constants (k = 6 and 9). Space for 200,000 trees was again allocated and the same ‘New
Technology’search parameters were input for the implied weighting analyses. After the
‘New Technology’search, the k = 6 weighted analysis retained 28 trees (CI: 0.188; RI: 0.680;
RCI: 0.128), while the k = 9 analysis retained 13 trees (CI: 0.189; RI: 0.682; RCI: 0.129).
After TBR swapping, the k = 6 analysis returned 32,319 trees, and the k = 9 analysis
returned 161,595 trees. In both cases, MH 7 was returned as an early-diverging
plesiosauroid and sister to Franconiasaurus + Cryptoclidia, with SMNS 16812 branching
basally (Fig. 15C). Implied weighting (k = 6) without the holotype of P.wildi retained 15
trees (CI: 0.190, RI: 0.680, RCI: 0.129) with subsequent TBR swapping retaining 32,319
trees with MH 7 being intermediate again to F.brevispinus and more basal Lower Jurassic
plesiosaurs, including Microcleididae. Implied weighting (k = 9) without the holotype
retained 10 trees (CI: 0.190, RI: 0.681, RCI: 0.130) with subsequent TBR swapping resulting
in 32,319 trees found with MH 7 being recovered in the same position as the k = 6 implied
weighting analysis (Fig. 15D).
DISCUSSION
MH 7 shares two autapomorphic character states with the holotype of Plesiopterys wildi
(SMNS 16812): medial flanges of the pterygoid separating the posterior interpterygoid
vacuity from the anterior vacuity, and a star-shaped interclavicle with a deep anterior
margin. Additional character states shared by SMNS 16812 and MH 7 include: cultriform
process of the parasphenoid at the posterior margin of the anterior interpterygoid vacuity;
possession of a narrow and straight quadrate ramus of the pterygoid; tall dorsal ramus of
the maxilla; lack of supernumerary elements in the limb; lack of a proximal flange on the
radius; and caudal neural spines of posterior caudal vertebrae inflected posteriorly. Given
these similarities, we consider MH 7 to be osteologically more mature than SMNS 16812,
and attribute all morphological differences to intraspecific and/or ontogenetic variation.
The slight stratigraphic age difference between these two specimens is also minor, with
MH 7 deriving from εII
6
and SMNS 16812 from εII
4
.
Ontogenetically mature osteological character states apparent in MH 7 include:
complete fusion of the neural arches to the centra; well-developed cornua on the coracoids;
and well-defined facets on the propodial and epipodial elements (Brown, 1981;Araújo
et al., 2015). However, MH 7 lacks fusion of the cervical and caudal ribs to the centra (see
Brown, 1981). Additionally, the shark-fin shaped neural spines in MH 7, like those found
in the osteologically immature holotype (GPMM A3.B4) of Brancasaurus brancai (Sachs,
Hornung & Kear, 2016) and SMNS 16812 may be another juvenile character. This mixture
of ‘adult’and ‘juvenile’character states implies a sub-adult ontogenetic stage for MH 7.
Groβmann’s(2007) assignment of SMNS 16812 as a juvenile Seeleyosaurus
guilelmiimperatoris was based on shared character states of the dentition and craniofacial
skeleton (Groβmann, 2007), in conjunction with a reinterpretation of the autapomorphic
characters identified by O’Keefe (2004) for the pterygoid and palate. Groβmann (2007)
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reconstructed the medial extensions of the pterygoids anterior to the posterior
interpterygoid vacuity as being the ventral surface of the parasphenoid (Groβmann, 2007,
fig. 5). However, as we show, Groβmann (2007) reinterpretation of the pterygoid is
incorrect. Although Groβmann (2007) was correct in dismissing the grooves O’Keefe
(2004) reported on the quadrate process of the pterygoid in SMNS 16812. Yet, MH 7 does
have elongated grooves on the dorsal surface of the pterygoids (Fig. 5A).
Character states from the dentition used to assign SMNS 16812 to S.guilelmiimperatoris
are unreliable because the teeth cannot be accurately counted. Furthermore, the presence
of striae (ridges) on the lingual side of the dentition in SMNS 16812 and lack of striae on
the labial face is a common condition in plesiosaurians (Brown, 1981;Sachs et al., 2025)
and therefore of poor taxonomic utility.
Additional ambiguous characters used by Groβmann (2007) to assign SMNS 16812 to S.
guilelmiimperatoris include: (1) horizontally oriented and elongated rectangular jugal that
forms part of the ventral orbit; (2) a maxillary-jugal suture with a right angle between its
nearly vertical dorsal part and and horizontal ventral part; (3) squamosal with an
anterodorsal process that contacts the postorbital and excludes the jugal from the temporal
opening. For states (1) and (3), we could not substantiate these states in our assessment nor
does Groβmann (2007;fig. 4, 5). Additionally, character (2) can be seen in other Early
Jurassic plesiosaurs, such as M. brachypterygius (Maisch & Rücklin, 2000) and M.
homalospondylus (Brown, Vincent & Bardet, 2013). The autapomorphies in SMNS 16812
shared with MH 7 thus establish P.wildi as a valid taxon. Furthermore, we recommend
using MH 7 for Plesiopterys wildi in future phylogenetic analyses to avoid scoring of
ontogenetically influenced states.
Our phylogenetic analyses recovered MH 7 as a derived Early Jurassic plesiosauroid
more derived than Microcleididae. These results, along with the lack of widely separated
rib facets of the posterior cervical vertebrae and a prominent flange extending anteriorly
from the proximal portion of the radius (Benson, Evans & Druckenmiller, 2012), suggest a
non-microcleidid affinity for MH 7 (Plesiopterys wildi). Derived cryptoclidian
plesiosaurians appeared abruptly after the Toarcian (Fischer, Weis & Thuy, 2021). Our
results in conjunction with those from Sachs, Eggmaier & Madzia (2024), reveal a stepwise
evolution toward this derived clade and thus fills the gap for their appearance in the Early
to Middle Jurassic.
Moreover, these results retain a high plesiosauroid diversity and possible endemism for
the Toarcian Posidonienschiefer Formation of the Southwestern German Basin. It is of
course possible that P.wildi could have dispersed into other basins in Central Europe (Sachs
et al., 2016). However, the diverse plesiosaur fauna (at least five distinct genera and species)
of the Southwestern German Basin (Maisch & Rücklin, 2000;O’Keefe, 2001;Smith &
Vincent, 2010;Sachs et al., 2025) are distinct from that of the nearby Yorkshire Basin. With
that being said, it is worth pointing out that the intraspecific variability among plesiosaurs
within these basins remains understudied; some taxonomically segregated species of
plesiosaurs may in fact be different ontogenetic stages of the same species or reflect sexual
dimorphism. Nonetheless, the endemism of these Toarcian plesiosaurs is at odds with
Marx et al. (2025), PeerJ, DOI 10.7717/peerj.18960 26/33

coeval ichthyosaurs (Fischer, Guiomar & Godefroit, 2011) and actinopterygians (Wretman,
Blom & Kear, 2016), which were more dispersed in the European Epicontinental Sea.
Perhaps plesiosaurs were more ecologically specialized than previously recognized and thus
restricted to local areas. The Yorkshire Basin for example includes large-bodied macro-
predatory rhomaleosaurids (Taylor, 1992), such as Rhomaleosaurus thorntoni,
Rhomaleosaurus cramptoni,andRhomaleosaurus zetlandicus, which reached sizes greater
than five meters (Smith & Dyke, 2008). By comparison, the rhomaleosaurid Meyerasaurus
victor, in the Southwestern German Basin, only reached 3.35 m in length (Smith & Vincent,
2010;fig. 1). P.wildi,S.guilelmiimperatoris,Microcleidus brachypterygius,and
Hauffiosaurus zanoni similarly reached lengths of between 3 and 3.5 m (Groβmann, 2007;
Vincent, 2011;Sachs et al., 2025). Perhaps niche separation can explain this disparity, with
rhomaleosaurids in the more northern Yorkshire Basin possibly feeding on larger fish and
cephalopods (Taylor, 1992;Sachs et al., 2023), while plesiosaurs in the Southwestern
German Basin preyed on smaller-sized food items.
CONCLUSIONS
MH 7 is a new plesiosaur from Holzmaden, represented by an articulated skeleton. MH 7
represent a sub-adult individual of Plesiopterys wildi. Shared autapomorphies between MH
7 and the holotype (SMNS 16812) demonstrate that P.wildi is a valid taxon and not
synonymous with Seeleyosaurus or Microcleidus. This reinforces the high diversity of
plesiosaurs in the Southwestern German Basin and its unique assemblage from coeval
localities in the Yorkshire Basin of England. Our results thus infer possible endemism and
regionalization of plesiosaur taxa within the Central European Basin during the Early
Jurassic. Lastly, MH 7 is a derived non-microcleidid plesiosaur that together with
Franconiasaurus brevispinus, form a stepwise evolution toward derived cryptoclidids. The
Early Jurassic was therefore a critical interval for not only the early radiation of plesiosaurs
but also the establishment of precursors to more derived forms that dominated the Late
Jurassic.
INSTITUTIONAL ABBREVIATIONS
GPIH Institut für Geologie, Universität Hamburg, Hamburg, Germany
GPMM Geomuseum der Universität Münster, Münster in Westfalen, Germany
MH Urwelt-Museum Hauff, Holzmaden, Germany
NHMUK Natural History Museum, London, United Kingdom
OUM Oxford University Museum of Natural History, Oxford, United Kingdom
SMNS Staatliches Museum für Naturkunde Stuttgart, Stuttgart, Germany
ACKNOWLEDGEMENTS
We thank Franziska Hauff, Rolf Hauff, Bernard Hauff, and Andi Fitchner for assistance
during collection visits to Urwelt-Museum Hauff. We are especially grateful to Franziska
Hauff for translating the German excavation notes of MH 7 into English. We also thank
Erin E. Maxwell for assistance during a collection visit to Stuttgart Museum of Natural
Marx et al. (2025), PeerJ, DOI 10.7717/peerj.18960 27/33

History, Ingmar Werneburg, and Anna Krahl for organizing and assisting with the visit to
the paleontological collection of the University of Tübingen. We thank Oliver Hampe and
Sacha Thiel for assistance during a collection visit to the Berlin Museum of Natural History
and Günter Schweigert for insight on correct terminology of the Posidonia Shale
ammonite zones. We would also like to thank Daniel Madzia for the assistance and advice
with the phylogenetic analyses, and the Willi Hennig Society for sponsoring TNT and its
availability.
ADDITIONAL INFORMATION AND DECLARATIONS
Funding
The Swedish Research Council (project grant #2020-03542 to Johan Lindgren, #2019-
03516 to Mats E. Eriksson, and #2020-03423 to Benjamin P. Kear) and the Royal
Physiographic Society of Lund funded this project (application #42011 to Miguel Marx).
The funders had no role in study design, data collection and analysis, decision to publish,
or preparation of the manuscript.
Grant Disclosures
The following grant information was disclosed by the authors:
The Swedish Research Council: #2020-03542, #2019-03516, #2020-03423.
Royal Physiographic Society of Lund: #42011.
Competing Interests
The authors declare that they have no competing interests.
Author Contributions
.Miguel Marx conceived and designed the experiments, performed the experiments,
analyzed the data, prepared figures and/or tables, authored or reviewed drafts of the
article, and approved the final draft.
.Sven Sachs analyzed the data, authored or reviewed drafts of the article, and approved the
final draft.
.Benjamin P. Kear analyzed the data, authored or reviewed drafts of the article, and
approved the final draft.
.Mats E. Eriksson analyzed the data, authored or reviewed drafts of the article, and
approved the final draft.
.Klaus Nilkens analyzed the data, prepared figures and/or tables, authored or reviewed
drafts of the article, and approved the final draft.
.Johan Lindgren conceived and designed the experiments, analyzed the data, authored or
reviewed drafts of the article, and approved the final draft.
Data Availability
The following information was supplied regarding data availability:
Marx et al. (2025), PeerJ, DOI 10.7717/peerj.18960 28/33

The matrix with taxon names and morphological character scores, along with our trees
from one of our consensus analyses analyzing all 131 OTUs, are available in the
Supplemental Files.
Supplemental Information
Supplemental information for this article can be found online at http://dx.doi.org/10.7717/
peerj.18960#supplemental-information.
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