Eurhinosaurus mistelgauensis, Spicher & Miedema & Heijne & Klein, 2025

Spicher, Gaël E., Miedema, Feiko, Heijne, Jelle & Klein, Nicole, 2025, A new Eurhinosaurus (Ichthyosauria) species from the Lower Jurassic (Toarcian) of Mistelgau (Bavaria, Southern Germany), Fossil Record 28 (2), pp. 249-291 : 249-291

Eurhinosaurus mistelgauensis sp. nov.

Etymology.

The specific epithet is derived from Mistelgau, the locality in Bavaria where the specimens were discovered.

Holotype.

UMO BT 011 221.00 , an almost complete skeleton (Figs 3 , 6 ).

Paratype.

UMO BT 011 235.00 , an almost complete skeleton (Figs 4 , 7 ).

Referred specimens.

UMO BT 011 240.00 , a partial snout (both premaxillae) with preserved articulated teeth and rib fragments (Fig. 5 ).

Locality.

Mistelgau, Bavaria, Germany (Fig. 1 ).

Stratigraphic horizon.

Jurensismergel Formation (Upper Toarcian), Haugia variabilis Zone, Vitiosa Subzone ( UMO BT 011 235.00 ), and Jurensismergel Formation (Upper Toarcian), Grammoceras thouarsense Zone and Subzone ( UMO BT 011 221.00 and UMO BT 011 240.00 ) (Fig. 2 ).

Diagnosis.

Eurhinosaurus species with the combination of the following autopomorphic characters: basioccipital with the ventral extracondylar area ( ECA) greatly extending anteroposteriorly and absence of ventral protrusion so that the extracondylar area is not visible in posterior view (no dorsoventral orientation of the extracondylar area); ventral extracondylar area slightly concave; absence of basioccipital peg; prominent basioccipital condyle forming the entire posterior surface of the basioccipital; markedly thickened and robust ribs that are round in cross-section, lacking a longitudinal groove along the shaft, particularly in the dorsal region.

Description.

Skull

Orbit and sclerotic ring. The internal orbit is partially preserved in UMO BT 011 235.00 (Figs 4 , 7 ). The orbit is approximately 14 centimeters in diameter (Suppl. material 1: table S 1). The orbit is extremely large compared to the size of the skull (Figs 4 , 7 ). The orbit is surrounded by the jugal ventrally, the lacrimal anteriorly, the prefrontal anterodorsally, the postfrontal dorsally, and the postorbital posteriorly (Fig. 7 ). Some bony material visible in the orbital region can tentatively be attributed to fragments of sclerotic plates (Fig. 7 ).

Premaxilla. The premaxilla is a paired bone preserved in all three specimens (Figs 3 , 4 , 5 , 6 , 7 ; Suppl. material 3: fig. S 2). In UMO BT 011 235.00 and UMO BT 011 221.00 , it is heavily damaged and fragmentary (Figs 3 , 4 , 6 , 7 ). In UMO BT 011 235.00 , solely the posterior portions of both premaxillae are preserved. The right premaxilla is articulated and visible in medial view. A small portion of the left premaxilla partially covering the right premaxilla is visible in lateral view and is slightly shifted upwards. In UMO BT 011 221.00 , a large portion of the right premaxilla is preserved in medial view. However, the anterior and posterior ends are missing. In the same specimen, a small portion of the left premaxilla is preserved in lateral view. The premaxillae are disarticulated in UMO BT 011 221.00 . In UMO BT 011 240.00 , the premaxillae, exposed in both lateral (left element) and medial (right element) views, are well-preserved and articulated (Fig. 5 ). The most visible element, the left premaxilla, measures approximately 63 centimeters as preserved (the posterior portion is broken). (Suppl. material 1: table S 1).

The premaxilla is an extremely elongated, straight, and slender bone forming most of the snout (anterior to the narial opening). The posterior portion of the premaxilla is dorsoventrally wide but narrows gradually towards its anterior tip (Figs 3 , 5 , 6 ). In UMO BT 011 221.00 , the right premaxilla preserved in medial view, shows a medial flange parallel to the lateral wall (corresponding to the ventral margin of the premaxilla) along the entire length of the premaxilla (Fig. 6 ; Suppl. material 3: fig. S 2). The medial shelf and lateral wall of the premaxilla form a shallow alveolar groove, in which the teeth insert when in articulation. No individual alveoli are observable in the premaxilla.

Laterally, the premaxilla carries a longitudinal groove extending over its entire length: the fossa premaxillaris (Fig. 5 ), like already observed in known Eurhinosaurus specimens (e. g., von Huene 1922, 1928; Maisch 2022). The fossa premaxillaris is discontinuous anteriorly and alternates between external exposure with small open pits and intraosseous pathways (Fig. 5 ).

As mentioned above, premaxillae articulated to the rest of the skull are uniquely preserved in UMO BT 011 235.00 , but the posterior portion of the bone is too damaged to identify the exact contact with the maxilla and nasal (Fig. 7 ). However, the right premaxilla, visible in medial view in specimen UMO BT 011 240.00 , preserves a clearly defined facet for the nasal, located on the posterior mediodorsal surface of the bone (Fig. 5 ). In addition to this posterior mediodorsal contact with the nasal, the premaxilla also articulates posteroventrally with the maxilla (Fig. 7 ). Both premaxillae contact each other medially along most of their length, as also observed in UMO BT 011 240.00 (Fig. 5 ). In this specimen, the medial suture accounts for approximately 64 % of the preserved premaxillary length, while the nasal facet comprises the remaining ~ 36 %. However, the posterior-most region of the premaxilla is absent, precluding a complete assessment of the nasal facet’s full extent. Since the anterior margin of the rounded nasal opening is not preserved and the overall nasal region is heavily damaged, the contribution of the premaxilla’s posterior end to the anterior margin of the narial opening is not visible in UMO BT 011 235.00 (Fig. 7 ).

Maxilla. The maxillae are disarticulated in UMO BT 011 221.00 and articulated in UMO BT 011 235.00 (Figs 3 , 4 , 6 , 7 ; Suppl. material 3: fig. S 2). In UMO BT 011 221.00 , the left maxilla is exposed in lateral view, and in UMO BT 011 235.00 , the right maxilla is visible in medial view. In UMO BT 011 235.00 specifically, despite being articulated, the maxilla is strongly damaged. In lateral view, the maxilla has an anteroposteriorly elongated outline (Fig. 6 ). Both posterior and anterior portions of the maxilla narrow towards the respective ends. In UMO BT 011 221.00 , on the anterior projection of the maxilla, there is an elongated and deep groove laterally that faces dorsally, into the direction of the suture with the premaxilla (Suppl. material 3: fig. S 2). The limits of the anterior end and dorsal suture of the maxilla are not visible, and the posterior suture is uncertain in UMO BT 011 235.00 . Anteriorly and anterodorsally, the maxilla contacts the premaxilla (Fig. 7 ). In medial view, the posterior extension of the maxilla runs under the lacrimal so that it contacts the latter posterodorsally. Also in medial view, the posterior end of the maxilla contacts the jugal along a short contact (Fig. 7 ). In UMO BT 011 235.00 , due to the heavily damaged region of the narial opening, it is uncertain whether the dorsal portion of the maxilla contributes to the ventral margin of the narial opening in medial view. In the maxilla of UMO BT 011 221.00 , the anteroposterior extent of the facets for the premaxilla and the jugal indicates that these do not contact one another. This suggests that the dorsal surface of the maxilla at its midpoint, between the premaxilla and jugal facets, contributed to the ventral margin of the narial opening in lateral view. The alveolar groove of the maxilla is not visible in either specimen and since no teeth are found articulated in the maxilla, it remains unknown how many teeth the maxilla bears.

Jugal. The left jugal is completely preserved in medial view in UMO BT 011 235.00 but disarticulated (Figs 4 , 7 ), while the right jugal is preserved in medial view and partially articulated to the skull (Fig. 7 ). The right jugal is heavily damaged in UMO BT 011 235.00 and its posterior portion is missing. In UMO BT 011 221.00 , two fragments of bones are tentatively identified as anteriorly incomplete jugals (Fig. 6 ; Suppl. material 3: fig. S 3). However, based on its general morphology, the bone fragment identified by us as left jugal may also represent the left postorbital (Fig. 6 ; Suppl. material 3: fig. S 3). Nevertheless, the fragment cannot be assigned to either element with certainty. The fragment identified as a right jugal is also incomplete posteriorly in UMO BT 011 221.00 . In the same specimen, both jugals are preserved in lateral view. The jugal is a long and curved bone that forms the ventral mid-orbital margin when in articulation, extending posterodorsally beyond the orbit (Fig. 7 ). The anterior ramus of the jugal is thin and tapers towards its anterior process (Fig. 7 ). The anterior end of the jugal contacts the posterodorsal end of the maxilla anteroventrally and the lacrimal anterodorsally (Fig. 7 ). Based on the jugal facet of the maxilla of UMO BT 011 221.00 , the contact between the jugal and the maxilla was shorter than the one between the maxilla and the premaxilla (Suppl. material 3: fig. S 2). The posterodorsal ramus ends in a broad dorsal facet that articulates with the descending process of the postorbital (Fig. 7 ). The postorbital facet of the jugal presents numerous thick ridges and grooves on its lateral surface (Fig. 7 ). In UMO BT 011 221.00 , the jugals are more angular than in UMO BT 011 235.00 (Suppl. material 3: fig. S 4). The postorbital facet is also smaller in lateral view, as preserved in UMO BT 011 221.00 , than in medial view, as preserved in UMO BT 011 235.00 (Fig. 6 ; Suppl. material 3: fig. S 3). The overall shape and posterodorsal ridges are very reminiscent of Argovisaurus ( Miedema et al. 2024).

Lacrimal. Lacrimals are solely preserved in UMO BT 011 235.00 (Figs 4 , 7 ; Suppl. material 3: fig. S 3). The left lacrimal is disarticulated and preserved in lateral view (Fig. 4 ). The right lacrimal is part of the skull bones that are preserved in articulation and is exposed in medial view (Fig. 7 ). The lacrimal is thin and has a triangular shape (Suppl. material 3: fig. S 3). The lacrimal forms the anteroventral margin of the orbit (Fig. 7 ). Its dorsal process contacts the nasal and prefrontal and forms the posteroventral margin of the narial opening (Fig. 7 ). Despite the poor preservation of this region in UMO BT 011 235.00 , it appears that, in medial view, the ventral lacrimal mostly contacts the anterodorsal jugal, as in lateral view, based on the jugal facet morphology of the maxilla of UMO BT 011 221.00 . (Fig. 7 , Suppl. material 3: fig. S 2). The posteroventral lacrimal contacts the anterior process of the jugal (Fig. 7 ).

Nasal. The nasal is solely preserved in UMO BT 011 235.00 . The right nasal is found articulated in medial view (Figs 4 , 7 ) and the left nasal is missing. As mentioned above, the skull has been laterally compressed, and damages obscure the exact shape of the nasal. Therefore, the suture between the nasal and the premaxilla is not clearly visible. In medial view, the nasal contacts the premaxilla anteriorly and anteroventrally. Posteriorly, the nasal has a long vertical contact with the prefrontal (Figs 4 , 7 ). The nasal further contacts the dorsal process of the lacrimal posteroventrally (Figs 4 , 7 ) and forms the dorsal margin of the narial opening (Figs 4 , 7 ).

Prefrontal. The prefrontal is preserved in UMO BT 011 235.00 and UMO BT 011 221.00 (Figs 3 , 4 , 6 , 7 ; Suppl. material 3: fig. S 3). In UMO BT 011 221.00 , the right prefrontal is disarticulated and partially covered posteriorly by the dentary and prearticular (Figs 3 , 6 ; Suppl. material 3: fig. S 3). The bone is still mostly embedded in the matrix and is partially broken so that only the posterior part of the prefrontal is visible in dorsal view. In UMO BT 011 235.00 , the right prefrontal is partially in articulation with other elements and visible in medial view (Fig. 7 ). However, the right prefrontal is damaged; a portion of the posterior part, visible in ventral view, is broken off and located now within the orbit (Fig. 7 ). The posterior portion of the left prefrontal is exposed in ventral view and disarticulated from the rest of the skull, but still in contact with fragments of the left postfrontal and parietal (Figs 4 , 7 ; Suppl. material 3: fig. S 3). Nevertheless, in the left prefrontal of UMO BT 011 235.00 , the contacts with the postfrontal and parietal do not appear entirely genuine, as these bones seem to have overlapped with thin, fragile sheets of bone that appear to have partially broken, likely altering the genuine shape of the suture in some areas. The prefrontal is curved in medial view and slightly crescentic. It comprises a descending process that contributes to the anterior rounded orbit, and a flattened dorsal portion that contributes to the skull roof and borders the anterodorsal orbit margin (Fig. 7 ). The posterior prefrontal portion contributes to the skull roof and has a broad ridge ventrally, running longitudinally, so that the prefrontal appears thick in this portion. This ridge is not visible on the dorsal surface, so the prefrontal appears flat in dorsal view (Fig. 6 ). In UMO BT 011 221.00 , the dorsal surface of the prefrontal is slightly concave and almost entirely covered with striations, suggesting that it was largely overlapped by a bony sheet, most likely the parietal and postfrontal (Suppl. material 3: fig. S 3). Dorsolaterally, the prefrontal presents a thin flange that contacts the postfrontal. In medial view, along its anteroventral surface, the prefrontal has a contact with the dorsal process of the lacrimal (Fig. 7 ). The prefrontal extensively contacts the nasal anteriorly along a curved suture (Fig. 7 ). This contact excludes the prefrontal from contributing to the narial opening in medial view. The prefrontal contacts the parietal posteromedially. The prefrontal likely contacted the frontal medially, but no frontal is preserved in the Mistelgau specimens. Consequently, the morphology of the frontal remains unknown in Eurhinosaurus , as it has never been described for other specimens of the genus. The prefrontal forms the major portion of the anterodorsal margin of the internal orbit (Fig. 7 ).

Postfrontal. The anterior part of the left postfrontal is preserved in ventral view in UMO BT 011 235.00 (Figs 4 , 7 ). In the articulated part of the skull in UMO BT 011 235.00 , bone fragments are potentially identified as the partial right postfrontal in medial view, due to their location in the skull between the prefrontal and the postorbital (Fig. 7 ). However, in this specimen, in addition to the damage, the postfrontal fragments are partially covered by the dislocated right angular, which obscures the exact shape of the postfrontal. Nevertheless, despite the damage, it is observable that the postfrontal contributes to the dorsal margin of the orbit (Fig. 7 ). In UMO BT 011 235.00 , the left postfrontal is thin and flat anteriorly. Anteromedially, the postfrontal contacts the posterolateral part of the prefrontal (Fig. 7 ). Posterior to the contact with prefrontal, a short contact between the medial margin of the postfrontal and the lateral margin of the parietal is visible in ventromedial view (Fig. 7 ).

Squamosal. An unambiguous squamosal is only preserved with UMO BT 221.00 (Fig. 6 ; Suppl. material 3: fig. S 3). In UMO BT 011 235.00 , a small fragment of bone in contact with the dorsal process of the postorbital is visible and can be identified as the right squamosal in medial view, based on its location (Fig. 7 ). The bone is hidden behind the postorbital and only a small strip is visible, preventing a detailed description. In UMO BT 221.00, the squamosal is the left element, disarticulated and preserved in dorsolateral view. The squamosal is largely complete with partially broken anterodorsal and ventral edges. The element is triangular in outline with the largest portion directed anteriorly (Suppl. material 3: fig. S 3). The squamosal is convex in dorsal view, forming part of the natural curve of the posterior cheek and skull roof (Suppl. material 3: fig. S 3). Ventrolaterally a distinct, slightly offset round process is visible (Suppl. material 3: fig. S 3). The process is concave medially, which would have held part of the dorsal lamella of the quadrate. Just dorsomedial to the quadrate process the posterior margin of the squamosal is slightly concave (Suppl. material 3: fig. S 3).

Postorbital. The right postorbital is preserved in medial view in UMO BT 011 235.00 (Figs 4 , 7 ). The postorbital is still in correct anatomical position and oriented vertically (Fig. 7 ), resulting in a semi-lunate shape with well-developed dorsal squamosal and ventral jugal facets. The anterior margin of the postorbital forms most of the posterior margin of the orbit (Fig. 7 ). The postorbital appears broader dorsally and slightly narrows gradually towards its ventral tip. The squamosal facet of the postorbital is broad and extends anterolaterally (Fig. 7 ). A small anterodorsal contact of the postorbital with the postfrontal is visible. The jugal facet of the postorbital is slightly roughened and oriented ventrally (Fig. 7 ). The jugal facet presents a shallow groove that contacts the postorbital facet of the jugal when articulated. The jugal facet is slightly longer and less broad than the squamosal facet. A contact with the quadratojugal is not preserved, and the quadratojugal facet is not clearly identifiable. A short ridge located dorsal to the jugal facet could correspond to a posteriorly oriented quadratojugal facet, but this is highly uncertain due to the preservation and may also be an artefact of taphonomy.

Quadratojugal. The right quadratojugal is preserved in UMO BT 011 221.00 and visible in lateral view (Figs 3 , 6 ; Suppl. material 3: fig. S 3). The bone has a dorsally slightly convex sub-rectangular plate-like shape (Fig. 6 ; Suppl. material 3: fig. S 3). Almost the entire dorsal surface of the bone is covered by long and thin striations, suggesting that the bone was partially covered by other bones, most probably the squamosal and postorbital (Suppl. material 3: fig. S 3). The posterior rim of the bone is slightly concave, and the anterior rim is roughly serrated (Fig. 6 ). The quadrate facet is directed ventrally like in Stenopterygius and Temnodontosaurus ( McGowan 1973; Maisch and Hungerbühler 2001; Miedema and Maxwell 2022) and the offset process of the quadrate facet is not visible and most probably still embedded in the matrix.

Parietal. Only a small portion of the left parietal is preserved in ventral view in UMO BT 011 235.00 (Figs 4 , 7 ). The fragment corresponds to the anterior-most parietal. The parietal contacts the posteromedial aspect of the prefrontal in medial view (Fig. 7 ). Posterior to this contact with the prefrontal, the parietal has a minute contact with the postfrontal laterally (Fig. 7 ).

Pterygoid. In UMO BT 011 235.00 , an isolated elongated bone can tentatively be identified as a highly fragmented left pterygoid in dorsal view (Figs 4 , 7 ). However, the bone is heavily damaged, preventing a detailed description. The anterior portion of the pterygoid is broken off and missing. The posterior portion of the pterygoid is round and forms a wide bulge (Fig. 7 ). Anterior to the bulge, the pterygoid is constricted before expanding mediolaterally on the anterior portion of the bone (Fig. 7 ).

Epipterygoid. The epipterygoids are well-preserved in lateral view in UMO BT 011 221.00 but completely disarticulated (Figs 3 , 6 , 8 A, B ; Suppl. material 3: fig. S 3). They are well-ossified and triradiate with two diverging dorsal and posterior projections, and a short but broader ventral foot, giving the element a forked shape (Figs 6 , 8 A, B ; Suppl. material 3: fig. S 3). The dorsal projection is referred to here as the epipterygoid dorsal ramus and the posterior projection as the posterior process of the epipterygoid (Fig. 8 A, B ). The dorsal ramus and posterior process that probably contact the parietal dorsally and posterodorsally when in articulation, diverge from the pedicel-like ventral foot that likely contacts the pterygoid when articulated. Both epipterygoids of UMO BT 011 221.00 differ in shape: the left epipterygoid has a broad bony bridge that connects the dorsal and posterior projections, making them less apparent and forming a small, rounded epipterygoid foramen at the center of the bone (Figs 6 , 8 A, B ). The right epipterygoid lacks such a bony bridge, and the lack of damage to the bone suggests a primary absence of the bridge (Fig. 8 B ), which could indicate a difference in ossification in that specific bone. The dorsal ramus is long and slender (Fig. 8 A, B ). The posterior process is much shorter (more than twice as short as the other divergent dorsal ramus) and wider (Fig. 8 A, B ). This is less apparent in the left epipterygoid due to the presence of the afore-mentioned bony bridge. The long epipterygoid ramus narrows towards its dorsal end (Figs 6 , 8 A, B ). The short dorsal epipterygoid process slightly widens towards its dorsal end (Figs 6 , 8 A, B ). The ventral foot is the broadest portion of the epipterygoid and expands ventrally (Figs 6 , 8 A, B ). The epipterygoids were unknown in Eurhinosaurus and the Mistelgau specimens are the first record of such bones in this genus. Preserved epipterygoids are rare in the ichthyosaurian fossil record and have been found ossified in the Triassic Chaohusaurus ( Yin et al. 2021) and Besanosaurus ( Bindellini et al. 2021) , as well as in the Jurassic Ichthyosaurus ( McGowan 1973) , Stenopterygius ( Miedema and Maxwell 2022) , and Argovisaurus ( Miedema et al. 2024). However, the morphology of the epipterygoids in Eurhinosaurus from Mistelgau, by showing the presence of a diverging dorsal portion and a foramen (in the left element) dorsal to the foot of the epipterygoid, differs greatly from the other known epipterygoids preserved in the afore-mentioned taxa. Nevertheless, based on its location on the epipterygoid of UMO BT 011 221.00 , the shorter dorsal process may be analogous to the medial bulge and potential prootic facet present in the epipterygoids of Stenopterygius ( Miedema and Maxwell 2022, supplementary figure 11). However, the comparison is limited due to different preservational states (3 D preserved in this study but flattened in Miedema and Maxwell 2022) and the scarce knowledge of ichthyosaurian epipterygoids.

Quadrate. The right quadrate is well-preserved and exposed in anterior view in UMO BT 011 221.00 although partially covered by the quadratojugal (Figs 3 , 6 ; Suppl. material 3: fig. S 3). The left quadrate is broken, and only a small portion of the dorsal part is preserved (Fig. 6 ). The quadrate is one of the most massive skull elements and has a general crescentic outline (Fig. 6 ; Suppl. material 3: fig. S 3). The dorsal and ventral portions of the quadrate in UMO BT 011 221.00 are composed of a laterally directed dorsal supratemporal ramus and a ventral pterygoid lamella. The pterygoid lamella is not accessible for description as it is entirely covered by the quadratojugal. The supratemporal ramus and articular area are very similar in outline. Both supratemporal ramus and articular area are mediolaterally wide and show a convex dorsal and ventral margin. The surface of the entire anterior quadrate is roughened (Suppl. material 3: fig. S 3). The quadrate is concave laterally (Fig. 6 ).

Parabasisphenoid. The parabasisphenoid is preserved in UMO BT 011 221.00 in dorsal view (Figs 3 , 6 , 8 C, D ). However, it was prepared from the other side of the slab as well so that the ventral surface of the parabasisphenoid is exposed (Fig. 8 C, D ). The parabasisphenoid has a quadrangular shape. A long, robust rod-like bone fragment is preserved near the parabasisphenoid and is interpreted as the potential cultriform process that has broken off (Fig. 6 ). The partial cultriform process is broad posteriorly and narrows gradually towards its anterior end in ventral view. The internal carotid foramen passes through the parabasisphenoid and is located on the anteroposterior midpoint on the ventral side and anteriorly on the dorsal side (Figs 6 , 8 C, D ). The internal carotid foramen is oval and unpaired (Figs 6 , 8 C, D ), as already described in Eurhinosaurus ( Maisch and Matzke 2000) . Note that, despite the unpaired condition of the internal carotid foramen, the lateral indentations for the carotid arteries are visible (Fig. 8 D ). In UMO BT 011 221.00 , in ventral view, the basipterygoid processes are slightly bulged and anterolaterally oriented. The basipterygoid processes form laterally directed pterygoid facets (Figs 6 , 8 C, D ). The shape is similar to Ichthyosaurus although the basipterygoid processes do not protrude anteriorly as strongly ( McGowan 1973). A small, rounded to subtriangular depression oriented laterally is visible on the dorsal surface of the basipterygoid process. The dorsal surface of the parabasisphenoid comprises a small dorsum sellae located posterior to the internal carotid foramen (Fig. 8 C ). On the dorsal surface of the parabasisphenoid, the sella turcica is extremely small and located just anteriorly to the carotid foramen (Fig. 8 C, D ). The basioccipital facet of the parabasisphenoid is projecting posterodorsally and forms the entire posterior side of the parabasisphenoid. The basioccipital facet has a strongly pitted texture. The basioccipital facet also shows a narrow and shallow groove that runs dorsoventrally along the entire posterior side of the parabasisphenoid.

Prootic. The paired prootics are preserved in UMO BT 011 221.00 (Figs 3 , 6 ; Suppl. material 3: figs S 2, S 3). Both elements are still mostly embedded in the matrix, and only visible in anterior view. The prootic has a rounded to oval shape and its anterior surface is slightly domed (Fig. 6 ; Suppl. material 3: figs S 2, S 3). It shows an overall smooth surface except for a few rare and shallow pits that are heterogeneously distributed (Suppl. material 3: figs S 2, S 3).

Opisthotic. Solely the right opisthotic is preserved in medial view in UMO BT 011 221.00 (Figs 3 , 6 ). The opisthotic is partially covered by a neural arch, obscuring most of its shape (Fig. 6 ), and therefore only the impression of the membranous labyrinth is accessible for description (Fig. 8 E ). The impression is complete and well-preserved. The texture visible within the impression is smooth. The impression of the membranous labyrinth is deep and V-shaped (Figs 6 , 8 E ). The V-shaped impression shows the horizontal and posterior semicircular canals that meet ventrally while contacting the impression of the sacculus (Fig. 8 E ). The posterior semicircular canal is slightly broader than the horizontal semicircular canal. Both semicircular canals are slightly curved. The horizontal semicircular canal curves anteriorly and the posterior semicircular canal posteriorly. Neither semicircular canal contacts the medial surface of the opisthotic (Figs 6 , 8 E ). The sacculus is broad and has a rounded shape (Figs 6 , 8 E ).

Exoccipital. Both exoccipitals are preserved in UMO BT 011 221.00 (Fig. 6 ; Suppl. material 3: figs S 3, S 4). The left exoccipital is visible in dorsolateral view and the right exoccipital in ventral view. The left element is slightly broken laterally and is slightly compressed vertically. The exoccipitals are small and shaped like a short column with a ventral articulation facet for the basioccipital and dorsal facet for the supraoccipital (Suppl. material 3: figs S 3, S 4). Both the basioccipital and supraoccipital facets show a roughened texture, but the supraoccipital surface is more heavily pitted than the basioccipital surface. Both facets have a rounded outline. The supraoccipital facet is slightly convex, while the basioccipital facet is more flattened, but still slightly bulged. The shaft of the exoccipital is short and constricted so that the exoccipital is the broadest at its dorsal and ventral ends (Suppl. material 3: fig. S 3). The shaft of the exoccipital slightly extends anteroventrally. In ventral view, this small extension is subtriangular in shape and is clearly distinguishable from the basioccipital facet by its slightly roughened texture (Suppl. material 3: figs S 3, S 4). Therefore, the shaft of the exoccipital is broader ventrally than dorsally. The medial surface of the shaft forms the wall of the foramen magnum. On the lateral surface of the exoccipital, two foramina for the hypoglossal nerve (XII) are present.

Basioccipital. The basioccipital is exposed in dorsal view in UMO BT 011 221.00 (Figs 3 , 6 , 9 A, D ; Suppl. material 3: fig. S 4). In UMO BT 011 235.00 , most of the ventral surface of the basioccipital is exposed (Figs 4 , 7 , 9 B, E ). In this specimen, the occipital condyle is clearly visible (Figs 7 , 8 B, E ). The basioccipital is one of the largest and most massive bones among the posterior bones of the skull. In dorsal view, the basioccipital is flattened and has an overall rounded outline with a slightly convex anterior surface (Figs 6 , 9 A, D ). On the dorsal surface, the basioccipital presents two large circular exoccipital facets that have a pitted texture (Figs 6 , 9 A, D ). The exoccipital facets are separated by the floor of the foramen magnum that runs anteroposteriorly (Figs 6 , 9 A, D ). The exposed portion of the floor of the foramen magnum is approximately as long as the exoccipital facets. The anterior half of the dorsal surface of the basioccipital (anterior to the exoccipital facets) has rugose and pitted texture and presents a small anterior notochord pit (Fig. 9 A, D ). The facet for the parabasisphenoid is formed by the entire anterior surface of the basioccipital. On the anterior face of the basioccipital, a basioccipital peg is absent, which contrasts with the condition described for other species of Eurhinosaurus in which a basioccipital peg is present ( McGowan and Motani 2003). Anterolaterally, the basioccipital shows large rounded articulation facets for the stapes (Fig. 9 B, E ). The posterior face of the basioccipital is entirely formed by the occipital condyle. The occipital condyle is prominent and has a rounded and strongly convex shape. The occipital condyle in the Mistelgau Eurhinosaurus is larger and more massive than in other Eurhinosaurus specimens, especially in posterior view (Fig. 9 A – F ) (e. g., SMNS 18648 , SMNS 56632 ). A posterior notochord pit is visible dorsally on the occipital condyle (Fig. 9 A, D ). On the ventral surface of the basioccipital, the extracondylar area is extensive (Figs 6 , 9 B, E ). It extends greatly posteriorly and is reduced laterally. It is also slightly concave medially (Figs 6 , 9 B, E ). However, the condition of the ventral extracondylar area in the Mistelgau specimens differs greatly from the condition described in Eurhinosaurus (Fig. 9 B, C, E, F ) ( McGowan and Motani 2003). As mentioned, the ventral extracondylar area extends greatly anteroposteriorly but does not protrude ventrally so that it is not visible in posterior view (Figs 6 , 9 B, E ), while in Eurhinosaurus the basioccipital is described to present an extensive extracondylar area that greatly extends ventrally so that in posterior view, the extracondylar area is well-visible ventral to the condyle (Fig. 9 C, F ) ( von Huene 1949, 1952; McGowan 1979; McGowan and Motani 2003), like in closely related ichthyosaurs such as Excalibosaurus ( McGowan 2003) , Leptonectes tenuirostris ( Lomax and Massare 2012) , and Wahlisaurus ( Lomax 2016) . The peculiar condition of the basioccipital in the Mistelgau specimens is similar to the condition observed in Hauffiopteryx ( Maxwell and Cortés 2020) and, to some extent, Temnodontosaurus zetlandicus ( Laboury et al. 2022) and Temnodontosaurus trigonodon ( Fraas 1913; von Huene 1931 b).

Stapes. Stapes are preserved in UMO BT 011 221.00 and in UMO BT 011 235.00 (Figs 3 , 4 , 6 , 7 ; Suppl. material 3: fig. S 4). In UMO BT 011 221.00 , the right stapes is visible in posterior view, and the left stapes in anterior view. In UMO BT 011 235.00 , it is uncertain whether the stapes is the left or right element. Stapes are small bones presenting a lateral and medial articular facet which are separated by a slightly constricted shaft (Figs 6 , 7 ; Suppl. material 3: fig. S 4). The medial head of the stapes, contacting the basioccipital when articulated, is massive and broad. (Figs 6 , 7 ; Suppl. material 3: fig. S 4). It is convex in anterior and posterior view and shows a roughened texture. The shaft is slightly constricted at the midshaft (Figs 6 , 7 ; Suppl. material 3: fig. S 4). The lateral end of the stapes is the articulation facet for the quadrate (Suppl. material 3: fig. S 4). The quadrate facet extends dorsoventrally and has a sub-rectangular shape that is slightly convex. This morphology is reminiscent of other Early Jurassic ichthyosaurs such as Stenopterygius and Hauffiopteryx ( Miedema and Maxwell 2019; Marek et al. 2015).

Dentary. A single partial left dentary is preserved in lateral view in UMO BT 011 221.00 (Figs 3 , 6 ; Suppl. material 3: fig. S 2). The preserved bone is crushed, missing the anterior and posterior parts. The dentary is an elongated bone that forms most of the anterior part of the mandible. The bone narrows towards its posterior and anterior ends and is the broadest at its midpoint (Fig. 6 ). The lateral surface of the dentary is convex. The shallow dentary fossa is present on the entire length of the lateral side of the bone (Fig. 6 ; Suppl. material 3: fig. S 2). The fossa is shallower posteriorly than anteriorly. Anteriorly, the fossa is discontinuous, showing an alternation of external furrows and foramina with passage inside the bone (Suppl. material 3: fig. S 2), as also observed in the premaxilla. The dentary normally bears teeth, but teeth are completely disarticulated in UMO BT 011 221.00 , and the alveolar grooves are obscured by matrix.

Splenial. The splenial is preserved in both UMO BT 011 221.00 and UMO BT 011 235.00 (Figs 3 , 4 , 6 , 7 ). The splenials are well-preserved in medial view in UMO BT 011 221.00 but their posterior end is obscured by sediment. In UMO BT 011 235.00 , both splenials are damaged and fragmented. In UMO BT 011 235.00 , the right splenial is visible in medial view and the left splenial is too damaged to distinguish which view of the bone is visible. The splenial is a long bone that forms most of the medial aspect of the mandible. The splenial is deep but narrows dorsoventrally towards its anterior end. The anterior end of the bone is forked, showing the two symphyseal processes (Fig. 6 ). The symphyseal processes run from the dorsal and ventral edges of the splenial (Fig. 6 ). The ventral process is more than twice the length of the dorsal process, which is also less robust than the ventral process (Fig. 6 ). This is standard morphology for parvipelvian ichthyosaurs, except for Ophthalmosaurus icenicus in which the processes are closer to equal length (Moon and Kirton 2016). A short sheet of bone is present between the two major symphyseal processes (Fig. 6 ), which to our knowledge could be an autapomorphy of Eurhinosaurus , although the morphology is unclear in many taxa e. g., Excalibosaurus ( McGowan 1989 a) . The processes are rounded in cross-section.

Surangular. The surangulars are preserved in medial view in UMO BT 011 235.00 (Figs 4 , 7 ). In UMO BT 011 221.00 , the left surangular is preserved in medial view, and the right surangular in lateral view (Figs 3 , 6 ; Suppl. material 3: fig. S 2). The surangular is a massive and elongated bone that forms the major part of the posterior lower jaw. The right surangular in UMO BT 011 221.00 is still articulated with the right angular. The posterior and ventral margins of the surangular contact the dorsal margin of the angular. The suture between the two bones is well-visible laterally (Fig. 6 ). Anteriorly, the surangular narrows towards its anterior end, becoming thin and reducing greatly in height. On the medial surface of the surangular, the Meckelian fossa and canal extend longitudinally along the entire length of the bone. The Meckelian fossa becomes shallower towards the posterior end of the bone and disappears at the posterior-most aspect of the surangular (Fig. 6 ). On the posterior portion of the surangular, the dorsal surface gives place to a rounded preglenoid process (sensu Bindellini et al. 2021) that extends dorsomedially (Figs 6 , 7 ; Suppl. material 3: fig. S 2). Anterior to the preglenoid process, the dorsal margin forms a broad paracoronoid process (Suppl. material 3: fig. S 2). On the posterior-most portion of the surangular, the dorsal margin develops a thin and convex articular facet (Figs 6 , 7 ; Suppl. material 3: fig. S 2). The articular facet is covered on its medial surface by minute and thin ridges for articulation with the articular. The preglenoid process and articular facet are distinctly separated by a small glenoid fossa on the dorsal margin (Suppl. material 3: fig. S 2). The depth of the fossa varies between the specimens, being slightly less apparent in UMO BT 011 235.00 than in UMO BT 011 221.00 (Figs 6 , 7 ). A large foramen is located anterior to the preglenoid process on the medial side of the surangular (Figs 6 , 7 ; Suppl. material 3: fig. S 2).

Angular. The right angular is preserved in lateral view in UMO BT 011 221.00 and the left in medial view (Figs 3 , 6 ). In UMO BT 011 235.00 , the right angular is preserved in medial view (Figs 4 , 7 ). The right angular in UMO BT 011 221.00 is complete and still preserved in articulation with the surangular. The suture between the two bones is well-identifiable on the lateral side of the mandible. The angular is an elongated bone that curves dorsally in its posterior portion, where it reaches its greatest dorsoventral height (Figs 6 , 7 ). The angular tapers anteriorly (Figs 6 , 7 , and its medial surface, anterior to the curvature, is slightly concave in UMO BT 011 221.00 .

Articular. The left articular is preserved in medial view in specimen UMO BT 011 235.00 (Suppl. material 3: fig. S 2). It is relatively small and robust and exhibits a slightly quadrangular outline. The medial surface is strongly concave anteroposteriorly, resulting in maximal width at the anterior and posterior ends and giving the element a saddle-like appearance in medial view (Suppl. material 3: fig. S 2). The ventral articulation facet for the prearticular is slightly concave and elongate anteroposteriorly (Suppl. material 3: fig. S 2). The anterior surface of the articular is entirely formed by the glenoid, which is dorsally slightly convex and displays a roughened, heavily pitted texture (Suppl. material 3: fig. S 2). The posterior surface is well defined, convex, and contributes to the posterior margin of the mandible. Like the glenoid, it is roughened, although it lacks pitting.

Prearticular. In UMO BT 011 221.00 , the right prearticular is preserved in medial view (Figs 3 , 6 ; Suppl. material 3: fig. S 2). In the same specimen, the left prearticular is potentially preserved in lateral view (Figs 3 , 6 ; Suppl. material 3: fig. S 2). In UMO BT 011 235.00 , one bone fragment can tentatively be identified as a right prearticular preserved in medial view (Figs 4 , 7 ). However, in this specimen, the bone is too fragmentary to be able to state this with certainty. In both specimens, most of the anterior and posterior portions are missing. The prearticular is thin, but the ventral margin is thicker than the dorsal margin (Fig. 6 ). The prearticular narrows towards its anterior end and its posterior portion is slightly angled (Fig. 6 ).

Dentition. Teeth are mainly well-preserved in UMO BT 011 240.00 and UMO BT 011 221.00 . In UMO BT 011 240.00 , numerous teeth are preserved in articulation with the two premaxillae (Fig. 5 ), but teeth are completely disarticulated and dispersed in UMO BT 011 221.00 . In UMO BT 011 240.00 , the articulated teeth are placed vertically in the premaxilla. Very few dispersed teeth are also preserved with UMO BT 011 235.00 . Due to the disarticulation of the teeth and the preservational state of the maxilla and the dentary in UMO BT 011 235.00 and UMO BT 011 221.00 , the condition of the dentition and teeth alveoli in the maxilla and lower jaw (dentary) is unknown for the Mistelgau specimens. In the Mistelgau specimens, the premaxilla shows a shallow alveolar groove (see section on the premaxilla).

The exact number of teeth per ramus is unknown in the Mistelgau specimens. In UMO BT 011 240.00 , the best-preserved specimens regarding the teeth, 44 teeth are preserved in the left premaxilla (none are identified in the right premaxilla) (Fig. 5 ). In this same specimen ( UMO BT 011 240.00 ), the roots of most of the in-place teeth are visible (Fig. 5 ). The teeth are mainly straight (Fig. 8 F ), but some are slightly curved (Fig. 5 ). The exact distribution of straight and curved teeth along the jaw remains uncertain, primarily due to the high degree of disarticulation. However, some teeth of UMO BT 011 221.00 , found in situ, suggest that the anterior portion of the upper jaw was primarily composed of straight teeth, with the exception of those at the tip, which appear slightly curved. The teeth show no carina and have a narrow conical shape (Fig. 8 F ). The surface of the crown is completely smooth, unlike the root, which is formed by plicidentine on its entire length (Fig. 8 F ). The shape of the teeth can typically be associated with the pierce 1 guild, which corresponds to a diet of soft prey such as cephalopods or small fish ( Massare 1987). No graduation in tooth size is observable and the replacement teeth are randomly distributed along the rami. The functional tooth size ranges from less than 15 millimeters to 20 millimeters in height for the largest (root of the tooth included) (Suppl. material 1: table S 1). The largest crowns reach 11 millimeters in height, and the smallest are approximately 8 millimeters in height (Suppl. material 1: table S 1). The higher the tooth, the greater the diameter. The largest diameter reaches 6 millimeters for the root and 5 millimeters for the diameter at the base of the crown (Suppl. material 1: table S 1).

Hyoid apparatus. Parts of the hyoid apparatus are preserved in ventral view in UMO BT 011 221.00 (Figs 3 , 6 ; Suppl. material 3: fig. S 4). The hyoids consist of two slightly curved rod-like elements identified as ceratobranchial I (Fig. 6 ; Suppl. material 3: fig. S 4). At approximately their midpoint, the cross-section is round, becoming flatter towards both ends. (Suppl. material 3: fig. S 4). The ventral surface of the ceratobranchial is smooth. Another isolated element in UMO BT 011 221.00 close to the ceratobranchials, is interpreted as the hyoid corpus (Figs 6 , 8 G ). It is a small element that has a rounded subtriangular shape with a roughened, domed, and convex base (Fig. 8 G ). The other sides of the bone show coarser roughening and slightly convex surfaces. The general morphology of the hyoid corpus of the Mistelgau specimens corresponds to the rare hyoid corpus findings in other ichthyosaurs ( Hauffiopteryx typicus : Motani et al. 2013; Maxwell and Cortés 2020; Stenopterygius quadriscissus : Miedema and Maxwell 2022; Ichthyosaurus sp. : Delsett et al. 2023; Mixosaurus cornalianus : Miedema et al. 2023).

It is important to note that the general morphology and size of the element we interpret as the hyoid corpus is also similar to that of an atlantal intercentrum. However, the element lacks a dorsal mediolateral concavity that would typically accommodate the rounded shape of the centra, which supports our current interpretation as a hyoid corpus. Still, we recognize that both the hyoid corpus and the atlantal intercentrum are rarely preserved and seldom described in detail, which makes distinguishing between them particularly challenging. Due to this limited comparative material, we do not rule out the possibility that this element could represent an atlantal intercentrum. Future discoveries and more comprehensive data may allow for a clearer distinction, but based on our current observations, we provisionally maintain the identification as a hyoid corpus.

Unidentified cranial bones. Several preserved bony remains are too fragmentary or damaged to be correctly identified (mainly in UMO BT 011 235.00 ). These bone fragments surely correspond to some of the missing, undescribed skull bones. These missing bones are the following: supraoccipital, supratemporal, frontal, palatine, and vomer.

Axial skeleton

Atlas-Axis complex. The atlas-axis complex is well-identifiable in UMO BT 011 221.00 (Figs 3 , 6 ; Suppl. material 3: figs S 4, S 6). The atlas-axis complex lacks cortical bone and thus exhibits a roughened texture that differs from the other preserved centra. In UMO BT 011 221.00 , the centra of the atlas and axis are still mostly embedded in sediment and so they are mostly visible from dorsolateral view. In UMO BT 011 235.00 , the atlas-axis complex is heavily damaged and still mostly embedded in the matrix, as in UMO BT 011 221.00 , thus exposing only the axis in posterior view. Due to extensive damage, identifying the element interpreted as the atlas-axis complex in specimen UMO BT 011 235.00 remains challenging. The identification is supported by morphological congruence with the atlas-axis complex of UMO BT 011 221.00 , as well as the lack of cortical bone on the element. The atlas has a polygonal outline. In lateral view, the centra of the atlas and axis are fused; however, the two elements remain differentiated due to a shallow groove demarcating their junction (Fig. 6 ; Suppl. material 3: figs S 4, S 6). In UMO BT 011 221.00 , the anterior atlantal facet is concave for the articulation with the occipital condyle of the basioccipital. In UMO BT 011 235.00 , the visible posterior axial facet is slightly concave as well.

The atlas and axis are approximately the same length. The articulation facets of the corresponding cervical ribs are poorly defined and are strongly damaged. However, it is visible on both atlas and axis that only one articulation facet for the rib is present. These broad and rounded articulation facets are anterodorsally placed on both centra. Only one half of the atlantal neural arch is preserved in lateral view in UMO BT 011 221.00 (Fig. 6 , Suppl. material 3: fig. S 3). It is strongly damaged and has a slight triradiate outline (Fig. 6 ; Suppl. material 3: fig. S 3). The pedicel is short (Suppl. material 3: fig. S 3). The neural spine is about twice the height of the pedicel (Suppl. material 3: fig. S 3). The neural spine is oriented posteriorly and slightly narrows towards its dorsal tip (Fig. 6 ; Suppl. material 3: fig. S 3). Pre- and postzygapophyses are not visible. No neural arch of the axis is preserved in any of the specimens.

Postaxis vertebrae. The vertebral columns of UMO BT 011 221.00 and UMO BT 011 235.00 are well-preserved (Figs 3 , 4 ). They range from disarticulated to well-articulated, depending on the region and specimen (Figs 3 , 4 ; Suppl. material 3: figs S 5, S 8). The condition of the vertebral column does not permit an exact length measurement in any of the two specimens. Due to disarticulation, an exact count of vertebrae cannot be given in either of the specimens. UMO BT 011 235.00 has a total of 128 centra preserved, and UMO BT 011 221.00 has 107. The caudal region is very incomplete in UMO BT 011 221.00 but mostly preserved in UMO BT 011 235.00 . In UMO BT 011 235.00 , 47 preserved postflexural centra, among which 25 are articulated, form the ventrally descending tail fluke that is elongated (Fig. 4 ; Suppl. material 3: fig. S 9). However, no wedge-shape centra from the tail flexure are observable. In UMO BT 011 235.00 , 81 centra are preflexural vertebrae. In UMO BT 011 221.00 , among the 107 preserved centra, 89 centra are preflexural and 18 are interpreted to be postflexural, based on their small size (compared with postflexural centra in UMO BT 011 235.00 ) and absence of rib articulation facets. 15 of the 18 postflexural centra are preserved on two slabs found isolated from the rest of the specimen, one is completely isolated, and two are preserved on the main slabs composing the specimen (Suppl. material 3: fig. S 1).

Due to the exposure in articular view of numerous centra, the exact determination of the position of the centra is not always possible. Nevertheless, a general distinction between cervical, dorsal, and caudal vertebrae is still possible based on the general morphology, position of the rib facets, and position of the centra on the different slabs composing the specimens. The caudal region of the vertebral column might have started in UMO BT 011 221.00 around the 51 st preserved centrum (Fig. 3 ) since it is the first vertebra showing fused diapophysis and parapophysis (not visible in UMO BT 011 235.00 ). Thus, all posterior vertebrae are considered caudals.

All centra are amphicoelous, but the overall shape of the centra varies throughout the vertebral column. The cervical centra are rounded-pentagonal with a distinct keel ventrally, while the dorsal and caudal centra are rounded. The posterior postflexural centra have a more oval shape, with the dorsoventral height being slightly greater than the lateral width. Dorsally, the centrum presents two facets to which the neural arch attaches. Between the two facets, there is the floor for the neural canal. The cervical and anterior dorsal centra are of moderate height compared to more posterior dorsal vertebrae (Fig. 10 A ). The dorsoventral height of the centra increases throughout the vertebral column until the anterior caudal centra, which are the highest (Fig. 10 A ). Then the height of the centra starts to reduce gradually until the posterior portion of the tail where the centra reduce rapidly in height up to the tail bend region (Fig. 10 A ). The anterior-most postflexural centra are small, and their size reduces slowly until the end of the tail fin, where the centra are the smallest in height (Fig. 10 A ). The variation of the height of the centra along the body of the Mistelgau specimens (Fig. 10 A ) is similar to the condition observed in Eurhinosaurus ( Buchholtz 2001) . The width of the centra of Mistelgau specimens follows the same variation pattern as the height of the centra described above (Fig. 10 B ). The length of the centra is difficult to assess because the vast majority of them are either lying on their anterior or posterior side, making measurements of the length impossible. However, it is observable that the length of the centra appears to have a similar trend as the height and width of the centra. The length increases from the cervical centra to the posterior dorsal centra. The posterior dorsal and anterior caudal centra are the longest. After the sacral region, the length of the centra decreases rapidly up to the postflexural centra which are the shortest. The di- and parapophyses, located on the lateral sides of the centrum, also follow a pattern throughout the vertebral column. Anteriorly in the cervical centra, the diapophysis is contacting the dorsally placed facets for articulation with the neural arch. Throughout the vertebral column, the diapophysis shifts gradually more ventrally and becomes separated from the facet for the neural arch. All cervical and dorsal centra have a distinctly separated diapophysis and parapophysis. Anteriorly, the diapophysis is larger than the parapophysis, but both facets tend to be approximately the same size in the dorsal centra. As the position of the facets lowers throughout the vertebral column, the distance between the diapophysis and parapophysis narrows until the two facets fuse. While both facets fuse, it appears that the parapophysis reduces in size at the same time. The diapophysis and parapophysis of the anterior centra are connected to the anterior rim of the centra by a bony bridge. This bridge disappears in the posterior dorsal centra, and the facets remain isolated from the edges of the centra. The first centrum with a synapophysis is considered to be the first caudal vertebra (as mentioned above). All the caudal centra have a synapophysis placed lower on each lateral side of the centra. They continue to lower throughout the tail and gradually reduce in size until they disappear. The posterior-most preflexural centra and the postflexural centra have no articulation facets for the ribs. The apophyses of the cervical and dorsal centra have a rounded shape. Most of the caudal centra show rounded apophyses as well. However, the shape of the synapophyses of the anterior caudal centra is irregular, where the diapophysis and parapophyses fuse. In these specific centra, the shape is initially hourglass-like and becomes rounder throughout the fusion of the two apophyses.

The neural arches are well-preserved in UMO BT 011 221.00 , but much less in UMO BT 011 235.00 , where most of them are missing or damaged. UMO BT 011 221.00 has 71 neural arches preserved, and UMO BT 011 235.00 has 6. In both UMO BT 011 221.00 and UMO BT 011 235.00 , no neural arches are preserved in articulation with their respective centrum, and all are dispersed around the specimens (Suppl. material 3: fig. S 5). Ventrally, the neural arches have broad and slightly convex articulation facet for the centrum. The neural arches slightly narrow toward their dorsal portion where they bear the zygapophyses and the neural spine. The neural arches in the posterior caudal region show a porous surface, unlike the other neural arches which have a smoother surface (Suppl. material 3: fig. S 5). The same size variation throughout the vertebral column as in the centra is expected for the neural arches in the Mistelgau specimens. However, since numerous neural arches are missing, it is difficult to assess in detail their size variation. Nevertheless, based on the preserved neural arches in UMO BT 011 211.00 , the neural arches are higher in the cervical and dorsal region than in the caudal region where they rapidly reduce in height. The prezygapophyses and postzygapophyses are located dorsally on the neural arch pedicels (Suppl. material 3: fig. S 5). In the presacral vertebrae, the prezygapophyses project anterodorsally and the postzygapophyses project posterodorsally (Suppl. material 3: fig. S 5). The prezygapophyses and postzygapophyses have an oval to rounded shape (Fig. 8 E ; Suppl. material 3: fig. S 5). Both pre-and postzygapophyses are paired, one on each side of the neural arch in the presacral vertebrae. However, each pair tends to gradually converge medially along the vertebral column and be confluent in anterior caudal vertebrae up to the tip of the tail. The size variation of the pre- and postzygapophyses follows a similar pattern as the size of the neural arches. They are well-developed and large in the cervical and dorsal region, but rapidly reduce in size in the caudal region, to almost completely disappear in the posterior-most caudal neural arches, where they only appear almost as a notch in the neural arch (Suppl. material 3: fig. S 5). The neural spine that is projected posteriorly throughout the vertebral column extends dorsally to the arch (Fig. 8 E ; Suppl. material 3: fig. S 5). All neural spines have a sub-rectangular shape with a distal end that is often slightly convex (Suppl. material 3: fig. S 5). The sub-rectangular shape becomes less apparent in the posterior-most caudal neural spine due to the almost complete reduction of the spines (Suppl. material 3: fig. S 5). The cervical and anterior dorsal neural spines are mediolaterally broader than the more posterior spines which are thinner (Fig. 8 E ; Suppl. material 3: fig. S 5). The neural spines increase in height rapidly in the first anterior vertebrae. The neural spines are the highest in the dorsal region and they can almost reach twice the height of the neural canal in the largest vertebrae. The neural spines drastically reduce in height in the caudal region and are the smallest in the posterior-most caudal region, where they are extremely short and reduced (Suppl. material 3: fig. S 5). No neural arches of postflexural vertebrae are preserved, although neural arch facets are visible on postflexural centra of UMO BT 011 221.00 (Suppl. material 3: fig. S 1).

Ribs. Numerous ribs are preserved in UMO BT 011 235.00 and UMO BT 011 221.00 (Figs 3 , 4 ). Only one well-preserved rib is found in UMO BT 011 240.00 (Fig. 5 ), next to two smaller rib fragments. In UMO BT 011 235.00 and UMO BT 011 221.00 , the ribs are disarticulated and mainly completely preserved (Figs 3 , 4 ; Suppl. material 3: figs S 6 – S 8). However, the ribs of the anterior dorsal region are less dispersed than the cervical ribs, posterior dorsal ribs, and caudal ribs (Figs 3 , 4 ). Ribs of the atlas centrum are not preserved in either specimen. However, the two ribs of the axis centrum are preserved in UMO BT 011 221.00 (Fig. 6 ; Suppl. material 3: figs S 4, S 6). Additionally, one rib of the third cervical centrum is possibly preserved (Fig. 6 ; Suppl. material 3: fig. S 4). The two ribs of the axis are the smallest and shortest preserved anterior ribs. They are curved and narrow greatly towards their distal end (Fig. 6 ; Suppl. material 3: figs S 4, S 6). The single head of the ribs of the axis is well-developed and wide and has a shallow groove proximally that extends on the shaft of the rib (Suppl. material 3: figs S 4, S 6). The preserved third cervical rib is about twice as long as the axis rib (Suppl. material 3: fig. S 4). The third cervical rib is single-headed as the axis rib but contrasts with the latter by showing no groove proximally that extends towards its distal end. The third cervical rib has a round cross-section.

In UMO BT 011 221.00 , an element exhibiting an unusual and presumably pathological morphology is preserved in the region of the anterior cervical ribs (Fig. 9 H ). Although its general morphology is consistent with that of a rib, the element is highly elongate and needle-shaped, characterized by a rounded proximal extremity and a shaft that tapers progressively distally (Fig. 9 H ). In its proximal portion, the element displays a notably greater diameter than adjacent anterior cervical ribs, supporting the interpretation of an abnormal condition. Accordingly, this element is interpreted as a pathologically modified anterior cervical rib, most likely exhibiting pseudarthrosis.

Posterior cervical ribs and dorsal ribs show a similar morphology. Both show a form of bicapitate head with oval-shaped tuberculum and capitulum which are confluent proximally (Suppl. material 3: fig. S 6). This morphology is highly similar to the “ sheathed-bicapitate ” condition present in the ribs of Argovisaurus martafernandezi ( Miedema et al. 2024). The tuberculum and capitulum are of equal size and form an obtuse angle (Suppl. material 3: fig. S 6). This angle gradually reduces towards the posterior dorsal ribs, and finally gradually disappears in the caudal ribs. The posterior-most dorsal ribs show more proximally fused but still recognizable tuberculum and capitulum than the anterior or mid-dorsal ribs, showing a gradual transition to caudal ribs. In some dorsal ribs, a shallow groove restricted to the head of the rib is observable between the tuberculum and capitulum, making them appear more distinct, while this groove is absent in other dorsal ribs (Suppl. material 3: fig. S 6). However, no trend in the distribution of this proximal groove is observable. Cervical ribs and dorsal ribs show no groove along their shaft, and the shaft is completely round or oval in cross-section (Suppl. material 3: figs S 6 – S 8). The dorsal ribs are the longest and have the greatest diameter at midshaft (Suppl. material 1: table S 1).

The Mistelgau specimens show no preserved posterior caudal ribs. The most posterior preserved ribs are the anterior caudal ribs. The caudal ribs are single-headed and the tuberculum and capitulum are completely fused to form a synapophysis which is oval and flat. The caudal ribs are round to oval in cross-section and are greatly reduced in length compared to the dorsal ribs. All ribs of the Mistelgau specimens narrow towards their distal end, except for the very last ribs (posterior-most ribs in the anterior caudal region), which show a widened distal end (Suppl. material 3: fig. S 5). In these posterior-most ribs, the head of the rib is as wide as the distal end, and the midshaft is slightly constricted (Suppl. material 3: fig. S 5). As the cervical and dorsal ribs, the caudal ribs show no groove along their shaft and are round in cross-section. A variation in the length of the ribs is observable in the Mistelgau specimens. The length of the ribs rapidly increases in the cervical region to reach a maximum in the dorsal region. The last dorsal ribs slightly decrease in length compared to mid-dorsal ribs. The caudal ribs drastically decrease in length compared to dorsal ribs. Within the caudal ribs, their length gradually decreases, whereas the very last ribs are the shortest of the entire body. The same variation applies to the diameter of the ribs, where the thickest ribs are in the dorsal region and the thinnest ribs are in the caudal region (Suppl. material 1: table S 1). Additionally, the ribs are curved in the cervical and dorsal regions, but become straighter in the caudal region, as they reduce in length.

The ribs of the Mistelgau specimens appear to be especially thick and massive. Especially the dorsal ribs appear pachyostotic compared to the rest of the body of the Mistelgau specimens with a very thickened cortex and small medullary cavity (Fig. 9 G ; Suppl. material 3: figs S 6 – S 8). At midshaft, the cortex of the dorsal ribs is moderately thick, with radii ranging from 3 to 4.7 mm in UMO BT 011 235.00 and from 3.5 to 5.4 mm in UMO BT 011 221.00 . The radius of the medullary cavity varies between 1.8 and 2.3 mm in UMO BT 011 235.00 and between 1.5 and 3.1 mm in UMO BT 011 221. These measurements are based on the best-preserved rib cross-sections and are presented as indicative intervals (see Suppl. material 1: table S 1); poorly preserved specimens not included in the measurements may have affected the actual range. It is noteworthy that the absence of a groove along the shaft may also enhance the perception of the thickness of the ribs. Nevertheless, cross-sections of the ribs confirm that the bone is particularly thick (Fig. 9 G ; Suppl. material 3: fig. S 6). The morphology of the ribs in the Mistelgau Eurhinosaurus specimens, especially in the dorsal region, differs notably from that observed in the holotype of Eurhinosaurus longirostris from Whitby ( NHMUK PV OR 14566 ), which is stratigraphically older than the material from Mistelgau. The dorsal ribs of the holotype do not appear to have undergone significant compression. The holotype exhibits thin ribs with longitudinal grooves along the shaft, resulting in an hourglass-shaped cross-section (Suppl. material 3: fig. S 7). In contrast, the Mistelgau specimens show rib morphology more similar to that of specimens from Baden-Wuerttemberg which are also stratigraphically older. These southwestern specimens exhibit some variability, with ribs that may be grooved or ungrooved (typically oval in cross-section in the latter case) (e. g., von Huene 1922, 1926, 1928, 1951; Suppl. material 3: figs S 7, S 8), although they are generally thin (e. g., von Huene 1926, 1928, 1951). In this regard, the Mistelgau specimens more closely resemble these specimens from Baden-Wuerttemberg than the holotype material from England.

However, none of the previously known Eurhinosaurus specimens exhibit ribs as thick as those in the Mistelgau material. In the Mistelgau specimens, dorsal rib diameters range from 9 mm to 14 mm at midshaft, with the majority exceeding 10 mm. By contrast, in the holotype material from Whitby ( NHMUK PV OR 14566 ), dorsal ribs do not exceed 10 mm at midshaft and are often thinner. Even in the southwestern German material, where ribs are occasionally preserved ungrooved, they do not appear as robust as those from Mistelgau, although they are generally thicker when compared to those of the holotype.

Haemal arches. In the proximal portion of the tail in UMO BT 011 221.00 , a few small curved and rod-like elements are preserved near the vertebral centra (Fig. 3 ; Suppl. material 3: fig. S 5). Some of these elements are distinctly smaller and shorter than the rod-like caudal ribs. These elements are interpreted as haemal arches (Suppl. material 3: fig. S 5). The haemal arches show a slightly porous surface. Thus, the ossification level of these small elements tends to vary. Some elements resemble the description made by von Huene (1926), being short, straight to curved rod-like elements that, paired, would have formed the haemal arch. However, in the posterior-most portion of the tail, the ribs tend to vary greatly in shape, which makes it sometimes difficult to distinguish between haemal arches and posterior-most caudal ribs. Since the vertebral material is disarticulated, it is difficult to identify the exact true location of the caudal ribs to distinguish which element is a rib or potentially a partial haemal arch. However, very few elements distinctly differ from the rib morphology by being much more curved and presenting a distinct proximal articulation facet (Suppl. material 3: fig. S 5). Due to disarticulation and different preservational states, their exact number and position remain unknown. Based on the preserved material, there are at least two vertebrae with haemal arches in the proximal portion of the tail in UMO BT 011 211.00 .

Gastralia. Gastralia are thin, long, and sometimes slightly curved rod-like bones that are preserved in UMO BT 011 221.00 and UMO BT 011 235.00 (Figs 3 , 4 ). The gastralia are completely disarticulated and dispersed in both specimens. In both specimens, UMO BT 011 221.00 and UMO BT 011 235.00 , the gastralia have a rounded cross-section. The diameter is the largest at the midshaft and decreases towards both ends (Figs 3 , 4 ). In UMO BT 011 221.00 , the gastralia are all isolated, but in UMO BT 011 235.00 , some gastralia are exceptionally preserved and are showing articulation; some pairs of gastralia are fused to each other medially, forming a single boomerang-shaped element that slightly expands anteroposteriorly, as described in the genus Hauffiopteryx ( Maxwell and Cortés 2020) .

Appendicular skeleton

Pectoral girdle. Specimen UMO BT 011 221.00 preserves nearly all elements of the pectoral girdle, which are disarticulated; however, the interclavicle is missing. In specimen UMO BT 011 235.00 , the pectoral girdle elements are disarticulated and highly dispersed, with only the left clavicle and the interclavicle missing.

Clavicle. The clavicles are preserved in ventral view in UMO BT 011 221.00 (Fig. 11 ). In UMO BT 011 235.00 , the right clavicle is preserved in dorsal view (Fig. 11 ). The clavicle is an elongated, slender bone with a tapering lateral end bearing striations that indicate the attachment point for the scapula (Figs 3 , 4 ). The medial part is flared (Fig. 11 ) and shows striations. The clavicle curves laterally after the flatter medial part (Fig. 11 ). The medial and distal portions of the clavicle are somewhat equally thick. When articulated, the clavicle possibly overlapped the interclavicle medially and contacts the proximal portion of the scapula posterolaterally along a long contact. The medial and lateral flanges of the clavicle are approximately equal in thickness. This is common in early diverging parvipelvians ( Temnodontosaurus , Excalibosaurus and Suevoleviathan, but not in Stenopterygius and Hauffiopteryx ( McGowan 1989 a; Maisch 1998 a; Maisch 2008; Maxwell and Cortés 2020).

Coracoid. The coracoids are preserved in ventral view in UMO BT 011 221.00 (Figs 3 , 11 ), and in dorsal view in UMO BT 011 235.00 (Fig. 4 ). The coracoid is plate-like and slightly rounded (Figs 3 , 4 , 11 ). It has a small semi-circular notch anterolaterally (Figs 3 , 4 , 11 ), as in most parvipelvians ( McGowan and Motani 2003). However, the coracoid in Wahlisaurus greatly differs in shape from Eurhinosaurus ( Lomax 2016; Lomax et al. 2019), since in addition to an anterior notch, it has a posterior notch and an ovoid foramen, which are absent in the specimens described in this study. The coracoid presents three long articulation facets (Fig. 11 ). The first one, the intercoracoid facet, is placed medially and the other two laterally (Fig. 11 ). The articulation facets are especially deep and thus make the coracoid dorsoventrally thicker at its medial and lateral portions. The intercoracoid facet slightly extends on the dorsal surface, forming a small rim on the medial margin of the coracoid (Fig. 11 ). The anterior-most lateral facet is the scapular facet (Fig. 11 ). Posterior to the scapular facet is a longer, slightly concave articulation facet, the glenoid facet. Both articulation facets show a rough texture of numerous large pits. The anterior notch is covered with cortical bone.

Scapula. The scapulae are visible in external view in UMO BT 011 221.00 (Figs 3 , 11 ) and in internal view in UMO BT 011 235.00 (Fig. 4 ). The scapula is an anteroposteriorly elongated bone with a fan-like, flared proximal and slightly flared distal end (Fig. 11 ). At midshaft, the scapula is rounder than at its ends, showing here an oval cross-section. The proximal flared end of the scapula is much wider and convex proximally than the distal end (Fig. 11 ). The posterior shaft of the scapula is strongly curved, unlike the anterior shaft, which is almost straight. The wide proximal portion of the scapula takes part in the glenoid articulation. The entire proximal margin of the scapula is not strongly differentiated and thus no well-defined acromion process is visible (Fig. 11 ). Consequently, the acromion process of the scapula of the Mistelgau specimen is not prominent like in other leptonectid ichthyosaurs (e. g., Leptonectes moorei : McGowan and Milner 1999; Excalibosaurus : McGowan 2003, and Wahlisaurus : Lomax 2016). The texture of the articular surface of the scapula is roughened. The proximal blade of the scapula articulates with the coracoid anteromedially. When in articulation, the scapula also contacts the clavicle dorsally from its proximal end towards the mid-shaft.

Pelvic girdle. The pelvic girdle is almost completely preserved in UMO BT 011 221.00 (Figs 3 , 12 ) including the right ilium, both pubes, and both ischia (Fig. 12 ). UMO BT 011 235.00 shows a left ilium, the right ischium, and both pubes (Fig. 4 ). The pubis and ischium show no fusion and are separate elements, as in most non-baracromians ( McGowan and Motani 2003).

Ilium. The right ilium is preserved in medial view in UMO BT 011 221.00 (Figs 3 , 12 ), and the left ilium in lateral view in UMO BT 011 235.00 (Fig. 4 ). The ilium is an elongated and slender rod-like bone with a rounded cross-section. It is thinnest dorsomedially but becomes wider towards its ventrolateral acetabular end that contacts the pubis and ischium when in articulation, forming the acetabulum. The bone is slightly curved, showing a slight angle in the middle of its length (Fig. 12 ). The ilium is slightly longer than the ischium and pubis (Fig. 12 ).

Ischium. Both ischia are preserved in external view in UMO BT 011 221.00 (Figs 3 , 12 ), and the right ischium in UMO BT 011 235.00 is visible in internal view (Fig. 4 ). The ischium is a flattened, plate-like bone, and the largest bone of the pelvic girdle (Figs 3 , 4 , 12 ). The bone is mediolaterally wider than anteroposteriorly long (Fig. 12 ). The ischium is narrowest at midshaft, showing a widening towards both ends. The widening of the medial end of the ischium is more developed than the one of the lateral acetabular end. (Fig. 12 ). The medial margin is strongly curved, unlike the lateral margin, which shows an almost straight but oblique margin (Fig. 12 ).

Pubis. The pubes are preserved in lateroventral view in UMO BT 011 221.00 (Figs 3 , 12 ), and in mediodorsal view in UMO BT 011 235.00 (Fig. 4 ). The pubis is an elongated flat bone with a constricted shaft and widened ends (Fig. 12 ). The lateral acetabular end is slightly wider than the medial end forming the pubic symphysis when in articulation. The pubis is the narrowest at the midshaft (Fig. 12 ). The lateral margin is slightly curved, and the medial margin is straight (Fig. 12 ).

Humerus. In both specimens, UMO BT 011 221.00 and UMO BT 011 235.00 , the humeri are robust and elongated elements. In both specimens, the diaphysis of the humerus is constricted, giving the margins a concave outline (Fig. 13 ; Suppl. material 3: fig. S 10). Proximally, the humerus is slightly expanded and exposes a large surface for articulation with the glenoid (Figs 13 , 14 A, B, C ). This proximal expansion is more developed in UMO BT 011 235.00 and almost non-existent in UMO BT 011 221.00 (Figs 13 , 14 B ). In UMO BT 011 235.00 , the proximal articulation of the humerus is slightly convex (Fig. 14 A ). In UMO BT 011 221.00 , the proximal surface is flattened and extremely pitted (Figs 13 , 14 B ), which is considered abnormal and interpreted as being potentially pathological (see section on pathologies in the discussion). Note that the proximal end of the left humerus in UMO BT 011 235.00 shows a zone of subsidence above the dorsal process (Fig. 14 C , Suppl. material 3: fig. S 10), which is interpreted as an indicator of avascular necrosis (see section on pathologies in the discussion). The dorsal process is only visible in UMO BT 011 235.00 (Fig. 14 C ; Suppl. material 3: fig. S 10). The dorsal process is more developed than the deltopectoral crest and runs further distally along the diaphysis, while the deltopectoral crest is short and does not extend along the diaphysis. Proximally, the dorsal process forms a descending sub-triangular surface that extends the glenoid facet (Suppl. material 3: fig. S 10). The texture of the dorsal process is roughened proximally. The deltopectoral crest differs between both specimens. While in UMO BT 011 221.00 , it appears as a rectangular structure that is confluent with the edge of the proximal humeral head (Figs 13 , 14 B ). The deltopectoral crest, especially in UMO BT 011 235.00 , is broad and less differentiated from the humeral shaft than the dorsal process (Fig. 14 A ). The texture of the deltopectoral crest of UMO BT 011 235.00 is slightly roughened (Fig. 14 A ). In UMO BT 011 221.00 , the texture is heavily pitted as visible on the proximal head of the humerus (Figs 13 , 14 B ). In both specimens ( UMO BT 011 221.00 and UMO BT 011 235.00 ), the deltopectoral crest exhibits limited distal extension along the diaphysis and is confined to the proximal half of the humerus.

Distally, the humerus is greatly expanded and bears two large articulation facets for articulation with the radius and ulna. These facets form an obtuse angle (Figs 3 , 4 , 13 ). The radial facet is slightly longer than the ulnar facet (Fig. 13 ). In distal view, both articulation facets are sub-rectangular to sub-pentagonal in shape and are concave. The texture of the articulation facets is roughened and pitted, but not to the extent observed in the proximal articulation facet in UMO BT 011 221.00 . At midpoint of the humeral anteroposterior width, where the radial and ulnar facets meet, the texture is smoother.

Epipodials of the forelimb. In UMO BT 011 221.00 , the radius is larger than the ulna and has a sub-rectangular shape (Fig. 13 ). It is thick and anteroposteriorly wider than proximodistally long (Fig. 13 ). The radius contacts the radiale distally, the intermedium posterodistally and the ulna posteriorly. The anterior margin of the radiale is notched (Figs 3 , 12 ). The posterior margin of the radius is strongly convex in ventral view (Fig. 13 ). The ulna is polygonal and is as thick as the radius. However, both elements are thicker at their edges than at the center of their surface, giving their ventral surface a concave shape. This concavity of the surface is visible on all limb elements distal to the humerus. This is likely a compression artifact caused by trabecular collapse in the least dense part of the bone. The ulna contacts the ulnare distally and the intermedium anterodistally (Fig. 13 ). The ulna lacks a notch, and the posterior margin is convex (Fig. 13 ). The texture of the articular surfaces of the radius and the ulna is slightly pitted.

Carpals. UMO BT 011 221.00 preserves three proximal carpals (radiale, intermedium, and ulnare) and three distal carpals (Figs 3 , 13 ). The intermedium has a deformed pentagonal shape and is slightly smaller than the ulna (Fig. 13 ). It is anteroposteriorly wider than proximodistally long (Fig. 13 ). The intermedium contacts the radius anteroproximally and the ulna posteroproximally. Anterodistally, the intermedium contacts the radiale, and posterodistally it likely contacts the ulnare when in articulation (Fig. 13 ). Distally, it contacts the distal carpal III (Fig. 13 ). The radial and ulnar facets are shorter than the facet for distal carpal III (Fig. 13 ). The facets for the radiale and ulnare are the shortest (Fig. 13 ).

The radiale is a sub-rectangular element about the same size as the intermedium and similar in shape to the radius (Fig. 13 ). As in the radius, the radiale is notched anteriorly (Fig. 13 ). The elements distal to the radiale lack notches (Fig. 13 ). The facet for the intermedium of the radiale is posteriorly slightly convex (Fig. 13 ). The anterior margin is longer than the posterior margin (Fig. 13 ). The texture of the articular facets of the radiale is slightly pitted. This condition is also visible in the ulnare. The radiale contacts the radius proximally, the distal carpal II distally, the intermedium posteroproximally, and the distal carpal III posterodistally (Fig. 13 ). The ulnare is oval and is the smallest proximal carpal (Fig. 13 ). It contacts the ulna proximally, the intermedium anteriorly, and distal carpal IV distally (Fig. 13 ). Since the ulnare is not preserved in perfect articulation, a contact with the distal carpal V is not visible but cannot be ruled out.

The distal carpals are slightly smaller than the proximal carpals (Fig. 13 ). The distal carpal II is the largest, and the distal carpal IV is the smallest (Fig. 13 ). The distal carpals, by their oval shape, are rounder than the proximal carpals. This follows the general trend in the shape of the paddle elements which become increasingly rounded distally (Fig. 13 ). The distal carpals are slightly thinner dorsoventrally than the more proximal elements, and the texture of their articular surfaces is slightly pitted.

Metacarpals and phalanges. Metacarpals II-IV are oval-to-round elements smaller than the proximal limb elements (Fig. 13 ). Their shape is similar to the shape of phalanges, making their identification in complete disarticulation of the limbs difficult. The articular surfaces of the metacarpals have a roughened texture and, in some places, are pitted. These elements are much thinner than the more proximal elements. However, all limb elements distal to the humerus are thicker at their edges than at their center, giving them a dished surface (Fig. 13 ). The proximal phalanges are oval, and the more distal the phalanges, the smaller, dorsoventrally thinner, and rounder they become (Fig. 13 ). The more distal preserved phalanges are completely discoidal in ventral view (Fig. 13 ). However, these phalanges are shaped more spool-like when observed in articular view. This shape is caused by a slight constriction of the articular surface compared to the ventral and dorsal margins of the phalanx.

Femur. Both femora are preserved in ventral view in UMO BT 011 221.00 , and the left femur is preserved in dorsal view in UMO BT 011 235.00 (Figs 3 , 4 , 15 ). The shape of the femur resembles that of the humerus; however, the femur is smaller than the humerus (Figs 3 , 4 ; Suppl. material 1: table S 1). The femur is wider distally than proximally, with a proximally flat acetabular articulation facet. At the proximal end, it bears a ventral and a dorsal process. Both are positioned near the proximal margin, with the ventral process located closer to the anterior edge and the dorsal process extending across the entire anteroposterior width of the proximal femur. The ventral process is relatively small and proximodistally short (Fig. 15 ), while the dorsal process is more prominent and extends farther distally. The ventral process is primarily recognizable by the angle it forms where it contacts the acetabular facet (Fig. 15 ). Due to its offset from this facet, the ventral process appears triangular in lateral view (Fig. 15 ). Distally, the femur exhibits two articulation facets for the tibia and fibula, with the tibial facet being slightly larger than the fibular one.

Epipodials of the hindlimb. The tibia and fibula are well-preserved in semi-articulation only in the left hindlimb of UMO BT 011 221.00 (Fig. 15 ). Both tibia and fibula are preserved in ventral view. The tibia has an oval shape (Fig. 15 ). However, it has a straight distal margin, while the other margins are convex. The tibia is smaller than the fibula (Fig. 15 ). The articular surfaces of the tibia and fibula are roughened and slightly pitted. No notching is visible in the tibia.

The tibia contacts the femur proximally (Fig. 15 ). The tibia further contacts the tibiale distally (Fig. 15 ). A contact with the astragalus posterodistally is not preserved. The fibula has a rounded shape and is the largest epipodial of the hindlimb (Fig. 15 ). The fibula contacts the humerus proximally (Fig. 15 ).

Distal elements of the hindlimb. The limb elements distal to the epipodials (tarsals, metatarsals, and digits) are poorly preserved in both UMO BT 011 221.00 and UMO BT 011 235.00 (Fig. 15 ). Numerous elements are missing in both specimens, and the preserved elements are completely disarticulated and dispersed. Therefore, the exact number of digits and elements per digit in the hindlimbs is unknown. UMO BT 011 221.00 shows the best-preserved distal elements of the hindlimb. The tibiale of the left hindlimb of UMO BT 011 221.00 is preserved (Fig. 15 ). The tibiale presents four slight angles, giving it a sub-rectangular to oval shape (Fig. 15 ). It is anteroposteriorly wider than proximodistally long. All other elements present a rounded or oval outline (Fig. 15 ). This similarity in shape makes the precise identification of the disarticulated and dispersed elements difficult. Semi-lunate, notched anterior phalanges are preserved (Fig. 15 ). However, the tibia and tibiale are unnotched (Fig. 15 ). Distal phalanges are rounder and thinner than the more proximal elements. As in the forefin, the phalanges gradually decrease in size distally (Fig. 15 ).

Figures

Figure 1. Locality of the Eurhinosaurus specimens from Mistelgau. A. Map showing the location of Bavaria in Germany; B. Close-up of Bavaria showing the location of the Mistelgau locality.

Figure 2. Stratigraphy of the Mistelgau locality. A. Lithographic section of the Mistelgau locality (modified after Arp et al. 2021). B. Stratigraphic chart depicting the ammonite zones of northwestern Europe and the corresponding ammonite subzones of southwestern Germany, which are applicable to the Mistelgau locality in southeastern Germany, along with the associated geological formations (modified after Arp et al. 2021). The two subzones in which the Eurhinosaurus of Mistelgau were unearthed are highlighted in yellow. C. Close-up of the lithographic section of the Mistelgau locality showing the exact stratigraphic position of the Eurhinosaurus specimens (modified after Arp et al. 2021). The red line corresponds to the maximum regression surface and the dashed blue line to the maximum transgression surface (after Arp et al. 2021).

Figure 3. Holotype specimen of Eurhinosaurus mistelgauensis sp. nov. (UMO BT 011 221.00). A. Photograph; B. Interpretative line drawing. Abbreviations: fc, first caudal centrum (number 51, first centrum with fused rib facets).

Figure 4. Specimen UMO BT 011 235.00 from Mistelgau. A. Photograph; B. Interpretative line drawing. Note that UMO BT 011 235.00 lays on a belemnite battlefield. Abbreviations: at / ax, atlas / axis complex; cl, clavicle; co, coracoid; f, femur; h, humerus; il, ilium; is, ischium; la, lacrimal; pb, pubis; sc, scapula.

Figure 5. Specimen UMO BT 011 240.00 from Mistelgau. A. Photograph; B. Interpretative line drawing. The area highlighted by the dashed line visible on the posterior portion of the right premaxilla in medial view indicates the nasal facet. Abbreviations: nf, nasal facet; pmx, premaxilla.

Figure 6. Close-up on the skull of UMO BT 011 221.00. A. Photograph; B. Interpretative line drawing. Abbreviations: ang, angular; ar, articular; at / ax, atlas / axis complex; atna, neural arch of the atlas; axrb, rib of the axis; boc, basioccipital; cb 1, ceratobranchial I; cp, cultriform process; cr, cervical rib; den, dentary; epi, epipterygoid; ex, exoccipital; hc, hyoid corpus; j, jugal; mx, maxilla; op, opisthotic; pbs, parabasisphenoid; pmx, premaxilla; pre, prearticular; prf, prefrontal; pro, prootic; q, quadrate; qj, quadratojugal; sp, splenial; sq, squamosal; sta, stapes; sur, surangular.

Figure 7. Close-up on the skull of UMO BT 011 235.00. A. Photograph; B. Interpretative line drawing of the cranial bones. The dashed lines show uncertain sutures. Note that the suture between the right premaxilla and right maxilla is not visible. Abbreviations: ang, angular; boc, basioccipital; j, jugal; la, lacrimal; mx, maxilla; na, nasal; no, narial opening; pa, parietal; pmx, premaxilla; po, postorbital; pof, postfrontal; pre, prearticular; prf, prefrontal; pt, pterygoid; scp, sclerotic plate; sp, splenial; sq, squamosal; sta, stapes; sur, surangular.

Figure 8. Selections of photographs of skull elements in UMO BT 011 221.00. A. Left epipterygoid in lateral view; B. Right epipterygoid in lateral view; C. Parabasisphenoid in dorsal view; D. Parabasisphenoid in ventral view; E. Opisthotic in medial view and cervical neural arch in anterior view; F. Isolated tooth in lingual view; G. Identified hyoid corpus in ventral view. Abbreviations: bp, basipterygoid process; ds, dorsum sellae; ebb, epipterygoid bony bridge; edr, epipterygoid dorsal ramus; ef, epipterygoid foramen; epp, epipterygoid posterior process; evf, epipterygoid ventral foot; hsc, horizontal semicircular canal; icf, internal carotid foramen; na, neural arch; przy, prezygapophyses; psc, posterior semicircular canal; sa, sacculus; st, sella turcica; tc, tooth crown; tr, tooth root.

Figure 9. Selection of morphological characteristics and pathologies of Eurhinosaurus mistelgauensis sp. nov. A. Photograph of the basioccipital in UMO BT 011 221.00 in dorsal view; B. Photograph of the basioccipital in UMO BT 011 235.00 in ventrolateral view; C. Photograph of the basioccipital of SMNS 18648 in posterior view, photograph by Erin Maxwell (Staatliches Museum für Naturkunde, Stuttgart); D. Interpretative line drawing of the basioccipital in UMO BT 011 221.00; E. Interpretative line drawing of the basioccipital in UMO BT 011 235.00; F. Interpretative line drawing of the basioccipital in SMNS 18648; G. Photograph of a dorsal rib of UMO BT 011 221.00 in cross-section at midshaft; H. Photograph of the anterior cervical rib with pseudarthrosis in UMO BT 011 221.00. Abbreviations: anp, anterior notochord pit; co, occipital condyle; eca, extracondylar area; exf, exoccipital facet; fmf, floor of the foramen magnum; pop, posterior notochord pit; stf, articulation for stapes.

Figure 10. Charts showing the size variation of the centra along the vertebral column in the Mistelgau Eurhinosaurus. A. Variation of the centra height; B. Variation of the centra width. Note that the graphs are based on the measurements provided in Suppl. material 1: table S 1. As centra are missing in both specimens (particularly in the caudal region, see description), the centra numbers do not necessarily exactly correspond to their anatomical positions, but to the preserved consecutive order (measured from anterior to posterior).

Figure 11. Close-up on the pectoral girdle of UMO BT 011 221.00. A. Photograph; B. Interpretative line drawing. Abbreviations: cl, clavicula; co, coracoid; sc, scapula.

Figure 12. Close-up on the pelvic girdle of UMO BT 011 221.00. A. Photograph; B. Interpretative line drawing. Abbreviations: il, ilium; is, ischium; pb, pubis.

Figure 13. Close-up on the right forelimb of UMO BT 011 221.00 in ventral view. A. Photograph; B. Interpretative line drawing. Abbreviations: 2–4, distal carpals; II – IV, metacarpals; dc, deltoptectoral crest; H, humerus; i, intermedium; R, radius; re, radiale; U, ulna; ue, ulnare.

Figure 14. Morphology and pathologies of the proximal surface of the humeri in the Mistelgau Eurhinosaurus. A. Right humerus in UMO BT 011 235.00 in ventral view; B. Right humerus in UMO BT 011 221.00 in proximoventral view, C. Left humerus in UMO BT 011 235.00 in dorsal view showing a subsidence area indicative of avascular necrosis, D. Left humerus in Ichthyosaurus larkini in dorsal view with subsidence area compatible with avascular necrosis (photograph modified after Lomax and Massare 2017), E. Right humerus in Ichthyosaurus sp. in dorsal view with avascular necrosis (Copyright Pardo-Pérez et al. 2017, CC BY 4.0). The dashed lines highlight the zones of subsidence congruent with avascular necrosis. Abbreviations: dc, deltopectoral crest; dp, dorsal process.

Figure 15. Close-up of the left hindlimb of UMO BT 011 221.00 in ventral view. A. Photograph; B. Interpretative line drawing. Abbreviations: 4, fourth distal carpal; cal, calcaneum; F, femur; Fi, fibula; sle, semi-lunate element; te, tibiale; Ti, tibia; vp, ventral process.