Liopleurodon ferox Sauvage, 1873

Liopleurodon ferox Sauvage, 1873

Figures 2–8 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7

Type material. BHN 3 R 197 , a single tooth now kept in the Musée d’histoire naturelle de Lille ( Lille , France) .

Referred material. See Noè, 2001; ComCom Thouarsais_Geol.0121 (described herein).

Type locality and horizon. Le Wast, near Boulogne-sur-Mer (northern France), Quenstedtoceras lamberti Zone, Oxford Clay Formation , Upper Callovian (Sauvage, 1873; Noè, 2001) .

Other localities. Bathonian-Callovian boundary to Lower Oxfordian of North and Central France, England, Switzerland and Germany. Possibly from the Lower Bathonian of France, Callovian of Poland, Callovian and Oxfordian of Russia, and Lower Tithonian of Germany (see Noè (2001) for details; Zverkov et al., 2024).

Diagnosis. Andrews, 1913; Tarlo, 1960 (in part); for the cranial diagnostic characters see Noè (2001). Pliosaurid possessing short cervical centra (length less than half width or height); cervical centra with a ventral lip and no ventral keel; two cervical rib facets.

Description

Preservation. The Thouarsais specimen measures around 320 cm in length and consists of a sub-complete post-cranial skeleton of a pliosaurid plesiosaur ( Figure 2 View FIGURE 2 ), including 45 vertebrae (10 cervicals, 4 pectorals, 22 dorsals, 4 sacrals, and 4 caudals), remains of ribs and gastralia, partial pectoral and pelvic girdles, as well as partial front- and hind-limbs. For convenience, the vertebrae will be described by using a letter (i.e., C: cervical, P: pectoral, D: dorsal, S: sacral and CA: caudal) followed by an Arabic number (e.g., C1 for the first cervical vertebra). This system does not correspond to the anatomical positions of the vertebrae.

Cervical vertebrae. Ten cervical centra represent middle to posterior elements of the neck ( Figure 3 View FIGURE 3 ). They exhibit gently concave articular surfaces with a central depressed notochordal pit, as in Peloneustes and NHMUK R. 3536, a large specimen of Liopleurodon (Andrews, 1913; Tarlo, 1960; Ketchum, 2007). In anterior view, the centra are subcircular in outline, slightly heart-shaped with a slight ventral lip in the cervicals C1 to C8 ( Figure 3A, C, H View FIGURE 3 ). The presence of a ventral lip is reported in Marmornectes , Liopleurodon , Peloneustes , ‘Pliosaurus’ andrewsi, Eardasaurus , and Simolestes (Andrews, 1913; Tarlo, 1960; Ketchum and Benson, 2011, 2022) and may be a synapomorphy of a derived clade within Pliosauridae (Ketchum and Benson, 2011) . The cervical centra are wider than they are high and long (W>H>L), as in other pliosaurids (e.g., Andrews, 1913; Tarlo, 1960; O’Keefe, 2001; Ketchum and Benson, 2011). Centrum length represents approximately 1/2 to 3/5 of both the width and height of the vertebrae; similar proportions (length being less than half the vertical diameter of the articular faces) are given by Andrews (1913) for Liopleurodon ferox . The ventral surfaces of the centra are pierced by two small subcentral foramina ( Figure 3I View FIGURE 3 ). Similar to the pliosaurids Marmornectes , Pliosaurus westburyensis , Pliosaurus carpenter , and Liopleurodon ferox , these foramina are not separated from one another by a ridge and differ from the condition reported in Peloneustes , Pliosaurus brachydeirus , and the pliosaurid OUMNH J.02247 (Andrews, 1913; Tarlo, 1960; Ketchum and Benson, 2011; Benson et al., 2013b). The cervical rib facets are large and located on the ventral to ventrolateral surfaces of the centra ( Figure 3 View FIGURE 3 B-E). The centra of cervicals C1 to C8 bear two rib facets (diapophysis and parapophysis) subequal in size ( Figure 3B, E, F View FIGURE 3 ). The diapophysis is triangular with the apex pointing dorsally whereas the parapophysis exhibits a bowl shape with a rounded ventral margin. The two facets, taken together, are pear-shaped like those described by Andrews (1913) for Liopleurodon ferox . The cervical centra C9 and C10 bear a single oval, dorsoventrally elongated rib facet ( Figure 3G View FIGURE 3 ), similar to Liopleurodon ferox (Andrews, 1913) . Four cervical neural arches are sub-complete and still attached to their centra showing a V-shaped neurocentral suture (C3-C6; Figure 3 View FIGURE 3 D-E). In anterior view, the neural canals exhibit an ovoid keyhole-shape due to a bony bulge located at midheight on the medial surface of the neural arches, as in Marmornectes (Ketchum and Benson, 2011) and Liopleurodon ferox (Andrews, 1913) . The neural canals represent approximately 50% of the centra height ( Figure 3C View FIGURE 3 ). The preserved neural spines are straight and sub-rectangular. The neural spines are longer antero-posteriorly than they are wide, as in Liopleurodon but unlike the condition

VINCENT ET AL.: JURASSIC Liopleurodon OF FRANCE observed in Marmornectes (Andrews, 1913; Ketchum and Benson, 2011), and measure approximately half of the centra length. The tips of the neural arches widen as in Liopleurodon ferox (Andrews, 1913) , showing a teardrop to oval outline in dorsal view and appearing sub-rectangular in lateral view. The zygapophyses are narrower than the centra. They are angled at approximately 45 degrees from the horizontal and are separated from one another, the right and left pre- and postzygapophyses being not fused medially. The sutures between cervical centra and associated neural arches, as well as the sutures between cervical centra and the corresponding ribs, are visible on the surface of the bones.

Pectoral vertebrae. Four pectoral vertebrae were identified based on rib facets that are present on both the centrum and the neural arch ( Figure 4 View FIGURE 4 A- D). The pectoral centra are higher than long, but longer than the cervical centra. On the two first pectorals (P1 and P2), the rib facets show an open neurocentral suture ( Figure 4B, D View FIGURE 4 ) and appear elongated dorsoventrally ( Figure 4B, D View FIGURE 4 ). This differs from the last pectoral (P4), in which the rib facet is more rounded ( Figure 4E View FIGURE 4 ). The subcentral foramen on the right side of the P4 centrum is situated more laterally than in other pectorals, and a larger supplementary foramen is present ventrally.

Dorsal vertebrae. Centra of the 22 dorsal vertebrae are as high as wide, shorter than high, and bear a central notochordal pit ( Figure 4 View FIGURE 4 F-H). Both the lateral and ventral surfaces of the centra are concave. The articular surfaces of the centra are sub-oval and weakly concave. The centrum width is approximately half of the transverse process width. The rib facets are situated on short and robust transverse processes. Only one complete neural arch and neural spine is preserved in association with the centrum ( Figure 4G, H View FIGURE 4 ). The zygapophyses are relatively short and proportionally smaller than those on the cervical neural arches. The neural spines extend antero-posteriorly, almost as long as the centrum length (around 70%), and higher than the associated centrum. The thin neural spines are inclined postero-dorsally and present a slightly curved anterior margin in lateral view ( Figure 4H View FIGURE 4 ). The apices of the neural spines are convex in their centra and rectangular in cross-section. The posterior margin of the neural spine is vertically grooved, and the postzygapophyses are thus separated by deep medial fossae (observable on D6 and D19; Figure 4F View FIGURE 4 ).

Sacral vertebrae. The centra of the four sacral vertebrae ( Figure 5 View FIGURE 5 A-B) show an open neurocentral suture and bear a dorso-ventrally elongated rib facet, which is formed by both the neural arch and the centrum ( Figure 5B View FIGURE 5 ). The neural spines are inclined postero-dorsally, the apices being strongly expanded and oval in dorsal view.

Caudal vertebrae. Five caudal vertebral centra are preserved. The articular surfaces are nearly circular in outline and are slightly concave with a pronounced median pit ( Figure 5 View FIGURE 5 C-D). The caudal rib facets are simple. The chevron facets are located ventrally on the posterior edges of the centra. No neural arches have been preserved.

Ribs. The double-headed cervical ribs are short with a convex dorsal surface and separated by a wide aperture ( Figure 3E View FIGURE 3 ). Proximally, the width of the aperture ( Figure 3C View FIGURE 3 ) is much greater than that reported for Marmornectes (Ketchum and Benson, 2011) , but similar to that reported for CAMSM J.46912 (holotype of Liopleurodon pachydeirus Seeley, 1869 , considered con-specific with Liopleurodon ferox by Noè, 2001). The distal end of the complete ribs preserved on cervical vertebrae C3 and C6 ( Figure 3E View FIGURE 3 ) shows an undeveloped anterior process, the short posterior process being rounded and flattened as in most pliosauroids (e.g., Peloneustes , Liopleurodon , Simolestes ; Ketchum and Benson, 2011). Numerous fragments of elongated and curved dorsal ribs are preserved ( Figure 5E View FIGURE 5 ). The ribs have slightly concave articulatory surfaces. Differentiating gastralia from dorsal ribs among the various broken isolated elements has proved difficult. One element is double-forked ( Figure 5F View FIGURE 5 ) and is identified as a gastralium, a morphology already observed for gastralia in the Triassic sauropterygians Corosaurus (Storrs, 1991) and Nothosaurus (Koken, 1893) , the plesiosauroids Plesiosaurus and Brancasaurus (Storrs, 1997; Sachs et al., 2016), both juvenile and adult specimens of the elasmosaurids Mauisaurus , Terminonatator , and Fluvionectes (Hector, 1874; Martin et al., 2007; Campbell et al., 2021), and in a single specimen of Peloneustes philarchus, BMNH R 3318 (Ketchum, 2007). This character was not observed in the mounted specimen of Liopleurodon ferox GPIT-PV-30093 displayed in Tübingen (syn. PV 17998, GPIT/RE/3184, GPIT 1754/2; see Goller (2021) and Krahl et al. (2022)). This character is variously interpreted as an anomalous ossification (Sato, 2003) or a congenital deformity (Storrs, 1991), and considered rare (Martin et al., 2007), but it may be more widespread than initially thought.

Two sacral ribs are preserved ( Figure 5 View FIGURE 5 G-I). Similar to other pliosaurids (e.g., Liopleurodon and Marmornectes ), the distal end of the rib is dorsoventrally flattened and flared (Andrews, 1913; Ketchum and Benson, 2011). The proximal end has an elongated oval outline ( Figure 5I View FIGURE 5 ). The rib shaft is twisted as in Marmornectes (Ketchum and Benson, 2011) , so that the long axis of the proximal rib facet and the long axis of the distal ilium facet are strongly rotated relative to each other (~90°). A ridge extends dorsoventrally from the rib head to the flared distal end as in Peloneustes (Ketchum, 2007) .

Pectoral girdle. The pectoral girdle is partially preserved, with only remains of the scapulae and coracoids present, whereas the clavicular arch is missing. Other small and fragmentary elements, probably belonging to the pectoral girdle are preserved; however, they could not be identified and are omitted in this description. Only the posterior ramus of the right scapula is preserved, whereas the almost complete left scapula is triradiate, with the ventral, dorsal, and posterior rami visible ( Figure 6 View FIGURE 6 A-C). The medial surface of the scapula between the posterior and ventral rami, at the level of the pectoral fenestra, is concave. The ventral ramus is incomplete and directed antero-medially, whereas the complete dorsal process extends postero-laterally. The latter shows an approximately subequal antero-posterior width along its entire dorso-ventral height except for the apex that is slightly enlarged. The dorsal process presents a convex and acute anterior edge, as well as a roundly concave posterior margin. The anterior edge is slightly sinusoidal, as in Marmornectes and Peloneustes (Ketchum and Benson, 2011) and Liopleurodon ferox GPIT-PV-30093 mounted specimen (Linder, 1913; pers. obs.). The apex of the dorsal ramus is a rugose concave, elongated, teardrop-shaped facet, indicating that it was probably covered by cartilage in the living individual, and undulates, as in Marmornectes (Ketchum and Benson, 2011) . The posterior ramus is robust and bears facets for articulation with the coracoid and the humerus. A ridge extends from the tip of the glenoid facet of the scapula along its lateral surface, enhancing the angle between the ventral and lateral surfaces of the scapula.

A crushed portion of the right coracoid is preserved ( Figure 6F View FIGURE 6 ). It presents two adjacent facets: the anterior one, triangular, corresponds to the scapular facet of the coracoid, while the posterior facet is concave and oval, corresponding to the glenoid facet of the coracoid. An element preserved separately ( Figure 6 View FIGURE 6 D-E), showing an antero-posterior curvature and a thickening that culminates in the flattened medial margin, is identified as the anterior-most part of the robust right bar of the coracoid. The part of the symphysis where the coracoids meet is curved dorsally. The anterior process of the coracoid is very short with a sub-rectangular outline in dorsal view as in Liopleurodon ferox (Andrews, 1913) .

Forelimb. Both humeri are preserved, the left being almost complete ( Figure 7A View FIGURE 7 ) whereas the right is severely damaged and lacks most of the proximal and distal parts ( Figure 7B View FIGURE 7 ). The proximoventral part of the left humerus shows the tuberosity but is severely abraded, and the capitulum is not preserved ( Figure 7C View FIGURE 7 ). The humeral shaft curves dorso-distally as in other pliosaurids ( Figure 7D View FIGURE 7 ; Ketchum and Benson, 2011). Distally, the pre- and postaxial margins of the shaft are asymmetrical, similar to Liopleurodon ferox , Pliosaurus almanzaensis , and Peloneustes philarchus (Linder, 1913; Ketchum, 2007; O’Gorman et al., 2018). A prominent postaxial expansion is visible at the distal end and is much wider than in Pliosaurus funkei , Stenorhynchosaurus , and Marmornectes (Andrews, 1913; Ketchum and Benson, 2011; Knutsen et al., 2012; Páramo-Fonseca et al., 2016). The distal surface of the humerus is suboval in outline ( Figure 7E View FIGURE 7 ). The radial and ulnar facets are distinct as in Liopleurodon ferox (Linder, 1913) , contrasting with Marmornectes (although the propodials are abraded distally in the type and sole specimen of this taxon; Ketchum and Benson, 2011), and Stenorhynchosaurus , which both lack prominent articular surfaces for the epipodial elements (Páramo-Fonseca et al., 2016). Posterior to the ulna facet, a third facet is presumed to articulate with a postaxial accessory ossicle. This contrasts with Stenorhynchosaurus and Peloneustes in which the lack of postaxial ossicles in the epipodial row of the forelimb has been reported (Ketchum and Benson, 2011; Páramo-Fonseca et al., 2016).

Both left and right radii ( Figure 7 View FIGURE 7 A-B) form a compact and robust bone, longer (left radius: 92 mm) than wide (left radius: 87 mm), with concave postaxial borders and a slightly convex preaxial margin. The postaxial margin forms the preaxial margin of a large epipodial foramen that extends for 2/3 of the length of the epipodials. The humerus facet is extensive, being antero-posteriorly long and dorso-ventrally thick. Distally, the radius bears three facets. The middle one, being the largest, corresponds to the radial facet. Postaxially, a small but well-formed oblique facet contacts with the intermedium and appears relatively shorter than the one observed in Peloneustes philarchus (Ketchum, 2007) . A third, very reduced and oblique facet, located preaxially to the radial facet, may have contacted a preaxial ossicle.

Only the right ulna is preserved ( Figure 7B View FIGURE 7 ). It forms a pentagonal bone slightly wider (preaxially to postaxially) than long (proximally to distally), similar to the condition reported in other pliosauroids (Ketchum and Benson, 2011). The ulna (77 mm long; 95 mm wide) is shorter and wider than the radius, as in Simolestes vorax (Andrews, 1913) . The ulna bears a straight proximal facet for the humerus, as observed in Simolestes vorax and Liopleurodon ferox (Andrews, 1913; Linder, 1913), but not for a postaxial accessory ossicle. The two distal facets articulate with the ulnare and the intermedium. The preaxial margin of the ulna forms the postaxial border of the epipodial foramen and is concave as in Liopleurodon ferox (Linder, 1913) , but at a higher degree than in Marmornectes and Peloneustes (Andrews, 1913; Ketchum and Benson, 2011). The postaxial margin is almost straight and differs from the condition reported in Simolestes and Peloneustes (Andrews, 1913) , but is similar to that observed in Liopleurodon ferox (Linder, 1913) .

The forelimb intermedium ( Figure 7 View FIGURE 7 A-B) is a large element (right intermedium: 67 mm wide; 47 mm long) and exhibits distinct facets for the radius, ulna, radiale, ulnare, distal carpals 2 and 3, and distal carpal 4. The intermedium participates in the epipodial foramen, but its involvement is minimal.

The left radiale is a dorsoventrally thick, sub-hexagonal element 62 mm wide and 46 mm long. It bears a long and thick proximal facet for union with the radius, and an anterior facet for the preaxial accessory ossicle (not preserved). Distally, the radiale contacts the distal carpal (dc) 1 through a long facet, and both the intermedium and dc2 and dc3 along its postaxial margin.

The right ulnare is the smallest of the three proximal carpals. It is rounded and contacts the ulna proximally as well as the intermedium medially. Its thickness reduces towards its anterior part to form a very thin edge marked by a profound notch on its extremity, as in Liopleurodon and Simolestes (Andrews, 1913; Linder, 1913).

The distal carpal (dc) 1 is rounded and articulates with the radiale and the dc2 and 3. The dc2 and 3 is sub-hexagonal and larger than dc1. Additional elements partially preserved may be identified as metacarpal and phalange remains. Only one of them is complete and appears proximodistally elongated and hourglass shaped.

Pelvic girdle. Both pubes are fragmentary ( Figure 6G View FIGURE 6 ). The left pubis presents a gently concave acetabular facet that is longer than the ischial facet. Its posterior border, medial to the ischial facet, is concave, and forms the anterior border of the thyroid fenestra.

A large portion of the left ischium is preserved and forms a blade-like element, antero-posteriorly longer than wide (estimated length 560 mm), as in other pliosauroids (Tarlo, 1960; Ketchum and Benson, 2011) and polycotylids (Williston, 1903; O’Keefe and Carrano, 2005; Albright et al., 2007a), and which become thinner posteriorly. The dorsal surface of the bone is concave, and its anterior margin forms a broad and concave curve. On the anteromedial side, a forward projection comes to a broad point and forms the anterior end of the curved medial border of the element. Posteriorly, the shaft of the left ischium indicates that the two posterior parts of the ischia would probably diverge from one another, as in Simolestes but more pronounced than the divergence reported in Liopleurodon and Peloneustes (Andrews, 1913) . The posterior-most part of the ischium is partially preserved. The thyroid fenestra seems to be more rounded than that of Peloneustes (Andrews, 1913; Ketchum, 2007).

Both ilia are partially preserved; the dorsal end of the left one as well as the ventral end of the

E

D Dorsal Anterior F cap

f fi t fib int tib dt mt

t

E right one are abraded. The ilia are elongate, left, and right measuring 258 mm and 256 mm, respectively. They are expanded at both extremities, the dorsal one (sacral end) more than the ventral (acetabular) one. The mid-shaft of the ilium is sub-oval in outline. The ventral end is approximately rotated about 90 degrees relative to the iliac blade and bears two facets, which are sub-equal in size: a posteromedial ischium facet and a more anterolateral facet contributing in the acetabulum margin. The ilium does not bear any ridge contra that reported for Peloneustes (Andrews, 1913; Ketchum, 2007).

fi fib mt f

Hindlimb. Both femora are preserved ( Figure 8) and appear relatively elongate with concave preaxial and postaxial margins. They are slightly longer (right femur: 480 mm long; left femur: 470 mm long) than the humeri, similar to Pliosaurus rossicus (Halstead, 1971) and Liopleurodon ferox (Linder, 1913) , and as robust as the femora observed in other Callovian pliosauroids except Marmornectes (Ketchum and Benson, 2011) . Both femora are fractured and abraded ventrally ( Figure 8 D-E). The proximal ends are poorly preserved, preventing the observation, if present, of the postaxial process and the rugosities. The posterodistal end of the right femur is only partially preserved ( Figure 8 A-B), whereas the distal portion of the left femur is well-preserved ( Figure 8C). The latter is expanded and dorsoventrally flattened ( Figure 8F) and bears two well-developed, but poorly distinctive, facets for contact with the epipodials. The facet for the tibia is slightly longer than the one for the fibula.

The tibia and fibula are morphologically similar to the radius and ulna, respectively, but longer (left tibia: 120 mm long; right tibia: 117 mm long; left fibula: 93 mm long; right fibula: 96 mm long) as is typical in pliosaurids (e.g., Andrews, 1913; Tarlo, 1960). The overall morphology of the tibia is similar to the one reported in Liopleurodon , Peloneustes , and Marmornectes (Andrews, 1913; Tarlo, 1960; Ketchum and Benson, 2011). The tibia ( Figure 8 B- C) is longer than wide, and its proximal facet for the femur is straight as in Peloneustes philarchus and Liopleurodon ferox (Andrews, 1913) . The preaxial border is slightly convex as in Peloneustes and Liopleurodon ferox (Andrews, 1913; Ketchum, 2007), and the postaxial border is concave, forming the anterior margin of the epipodial foramen. The epipodial foramen is closed distally by contact between the tibia and fibula and is less extensive than that observed in the forelimb. The tibia shows a proximal facet for contact with the fibula, and a short and well-developed distal facet for the intermedium as in Peloneustes , Liopleurodon , and other pliosauroids (Andrews, 1913; Tarlo, 1960), but unlike Marmornectes (Ketchum and Benson, 2011) . A long and concave distal facet for the tibiale is also visible on the tibia, as well as a short and oblique facet, located preaxially, for a preaxial accessory ossicle which is not preserved.

The fibula ( Figure 8C, F) has a lunate outline with a concave preaxial margin that forms the posterior border of the epipodial foramen as in Liopleurodon ferox (Adrews, 1913; Linder, 1913). A posteriorly oblique proximal facet contacts a postaxial accessory ossicle preserved on the left paddle. The tibiale, intermedium, and ulnare are similar in shape to the elements of the forelimb. Only one distal tarsal (dt) is preserved and identified as the dt2 and 3. One metatarsal (mt) shows a triangular-shaped proximal end and is thus interpreted as the mt2. Only few phalanges are preserved preventing the determination of the phalangeal formula.

Cladistic Analysis

The parsimony analysis of the data matrix yielded 2000 most parsimonious trees of 1937 steps. We do not detail the results within the Rhomaleosauridae , nor the results within the Plesiosauroidea. The topology of the strict consensus ( Figure 9 View FIGURE 9 ) positions the Thouarsais specimen as the sister taxon of Liopleurodon ferox , and this clade is supported by four synapomorphies: the presence of transversely concave/convex cervical zygapophyseal facets (character 169, state 1), cervical centrum mediolateral width subequal to height or less (character 171, sate 0), caudal ribs facet located at midheight of centrum or lower in proximal–middle caudal vertebrae (character 188, state 2), and the chevron facets of caudal vertebrae that are located mainly on the posterior edge of the centra (character 192, sate 1). Thalassiodracon hawkinsii is sister taxon of the clade grouping Rhomaleosauridae and Pliosauridae . The clade that includes the three species of Hauffiosaurus is the sister taxon to all other members of Pliosauridae . The overall relationships among taxa within this latter clade remain largely unresolved, similar to the relationships identified by Sachs et al. (2023) in their analysis using unweighted parsimony.