Cryonectes neustriacus, Vincent & Bardet & Mattioli, 2013

Vincent, Peggy, Bardet, Nathalie & Mattioli, Emanuela, 2013, A new pliosaurid from the Pliensbachian, Early Jurassic of Normandy, Northern France, Acta Palaeontologica Polonica 58 (3), pp. 471-485 : 473-482

Cryonectes neustriacus sp. nov.

Etymology: Derived from the Frank kingdom of Neustrie that covered northwest France, created after the death of Clovis in AD 511.

Holotype: MAE 2007.1.1 (J), an incomplete skull and articulated mandible, and 10 associated vertebrae.

Type locality: Roche−Blain quarry, Fresney−le−Puceux, near Laize−la−Ville, south of Caen, Calvados Department, Northern France.

Type horizon: “ Calcaire à Bélemnites” Formation, uppermost Pliensbachian ( Lower Jurassic ), Normandy ( Dugué et al. 1998) .

Material.— Holotype only.

Diagnosis.—Moderate−sized plesiosaurian (skull length: approximately 470 mm from the tip of the snout to the posterior end of the mandible) presenting the following unique combination of characters: contacts between premaxillae and maxillae marked by a very slight constriction; snout well elongated; very thin anterior interpterygoid vacuity with pointed anterior and posterior margins; two ventrolateraly orientated low flanges on the posterior rami of the pterygoids; mandible with a long symphysis bearing seven tooth position and retaining a ventral mandibular ridge; cervical centra with relatively platycoelous articular surfaces; ventral sides of cervical centra almost flat, without ventral keel nor depression around the nutritive foramina.

Description

Preservation: When discovered, the specimen was prepared first with acid, which unfortunately damaged some parts, especially the teeth, which are now broken and lack enamel. Mechanical preparation was subsequently performed. The premaxillae and maxillae are incompletely preserved ( Fig. 3 ). The most important part of the palate is preserved separate from the skull ( Fig. 4). It is damaged but some structures are recognizable. The complete mandible and the skull are in occlusion with teeth in situ ( Figs. 5–7 ). One separated tooth is almost complete but lacks enamel as well ( Fig. 7 ). Ten associated vertebrae are preserved in a non−natural sequence, nine being cervical, but the last one cannot be determined with confidence ( Figs. 8–10).

Ontogenetic stage: Distinct neurocentral sutures are visible on all vertebrae preserving neural arches, except vertebra 9, which presents a closed suture. Accordingly, Cryonectes could be regarded as being at a relatively early stage in ontogenetic development (sensu Brown 1981). The closed neurocentral suture of vertebra 9 suggests that it was probably a subadult.

Skull ( Figs. 3–6 ): The premaxillae are massive and the rostrum is long and narrow ( Fig. 3 ). The skull roof is absent, thus preventing estimation of any snout elongation value. Nevertheless, the snout appears much longer than that of the Liassic plesiosauromorph taxa Microcleidus (NHMUK 36184; Brown et al. 2013), Plesiosaurus ( Storrs 1997) , Occitanosaurus ( Bardet et al. 1999) , Hydrorion and Seeleyosaurus ( Grossmann 2007) , the basal plesiosaurian taxa Thalassiodracon hawkinsii ( Storrs and Taylor 1996) and Anningasaura lymense Vincent and Benson, 2012 , and Rhomaleosauridae (e.g., Meyerasaurus victor ; Smith and Vincent 2010 and Rhomaleosaurus zetlandicus ; Taylor 1992; Vincent and Smith 2009). The snout appears as elongated as those of Attenborosaurus conybeari ( Sollas, 1881) and Archaeonectrus rostratus (NHMUK 38525; Owen 1865), but larger than that of Hauffiosaurus zanoni O’Keefe, 2001 and Hauffiosaurus tomistomimus Benson, Ketchum, Noè, and Gómez−Pérez, 2011b ; and not as extended as that of Hauffiosaurus longirostris (MMUM LL 8004: O’Keefe 2001: fig. 9; Ketchum and Benson 2010: fig. A6; MCZ 1033: White 1940: fig. 4b). In dorsal view, the premaxillae unite in a well−developed medial suture along their whole preserved length. The long slender facial processes of the premaxillae, running back on the dorsal surfaces of the skull, are not preserved entire, but seem to constitute the anterior part of the low parasagittal ridge ( Fig. 6A ), as in

http://dx.doi.org/10.4202/app.2011.0113 premaxilla maxilla dentary section Fig.7D

coronoid surangular articular

H. longirostris (MCZ 1033; White 1940), Archaeonectrus (NHMUK 38525; Owen 1865), and Macroplata tenuiceps ( Ketchum and Smith 2010) . The premaxillae bear five pairs of teeth, whose relative sizes are not easily observable because of bad preservation. Weak ridges ornament the external surface of the premaxilla. The sockets for the large premaxillary teeth run posteriorly, medially and dorsally, deep into the bone. The contacts between premaxillae and maxillae are marked by a very slight constriction of the muzzle as in Marmornectes candrewi Ketchum and Benson, 2011a , Peloneustes philarchus (SMNS 10113; Ketchum and Benson 2011b), Pliosaurus brachyspondylus (BRSMG Cc332; Taylor and Cruickshank 1993), and H. zanoni O’Keefe, 2001 (see also Vincent 2011), but not as marked as in Rhomaleosauridae (e.g., Meyerapalatine anterior interpterygoid vacuity pterygoid parasphenoid quadrate ramus of the pterygoid saurus victor, Smith and Vincent 2010 ; Rhomaleosaurus zetlandicus, Taylor 1992 ). The premaxillae−maxillae sutures are straight on the right side and slightly interdigitating on the left. They run posteromedially from the margin of the tooth row. The maxillae are long and slender and not entirely preserved. The right maxilla bears at least nine alveoli.

posterior interpterygoid vacuity pterygoids suture 10 cm

The palatines are partially preserved, the left one being the most complete of the two ( Fig. 4). It is an elongate bone running from the anteriormost part of the snout towards the posterior part of the ectopterygoid, which is not preserved. Their medial margins contact the pterygoids. They are not preserved anteriorly but probably join the vomers. The specimen does not present lateral palatal fenestration bordered by palatine and pterygoid as in Plesiosaurus ( Storrs 1997) , Leptocleidus capensis ( Cruickshank 1997) , Peloneustes ( Ketchum and Benson 2011b) , and Liopleurodon ( Andrews 1910) .

The pterygoid is a large bone forming the main part of the palatal surface ( Fig. 4). It includes the anterior, lateral, posterior and quadrate ramus. The anterior ramus is very long, broad and flat, with a thickened front portion. Laterally it contacts the palatine in a nearly straight suture. In the median line, the anterior ramus parts to form a very narrow anterior interpterygoid vacuity as in Pliosaurus brachyspondylus ( Taylor and Cruickshank 1993) , small individuals of Peloneustes ( Ketchum and Benson 2011b) , and Hauffiosaurus tomistomimus ( Benson et al. 2011b) , but much more extended. The two pterygoids meet for a short distance anteriorly and posteriorly to the anterior interpterygoid vacuity ( Figs. 4, 6B, C ). Behind, they are separated by the anterior part of the parasphenoid. Posteriorly, the medial edges of the pterygoids curve away from the parasphenoid to enclose the posterior interpterygoid vacuities. They meet again posterior to the posterior interpterygoid vacuities ( Fig. 6B, D ). Behind the posterior interpterygoid vacuities, the posterior portion of each pterygoid bears a slight and short flange ventrolaterally orientated ( Figs. 4, 6D ). Only the quadrate ramus of the right pterygoid is preserved; it is poorly preserved. It is a long, lat−

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erally compressed process running posterolaterally from the basicranium to the non−preserved quadrate.

The anterior interpterygoid vacuity, very long and slender, is present on the middle of the palate ( Figs. 4, 6C ), distinguishing Cryonectes from Hauffiosaurus zanoni ( O’Keefe 2001; Vincent 2011), which does not possess an anterior interpterygoid vacuity. The anterior interpterygoid vacuity of Cryonectes has very pointed anterior and posterior margins. The posterior interpterygoid vacuities are bordered laterally by the pterygoids and are separated on the midline by the parasphenoid. These are deep, anteroposteriorly extended, mediolaterally orientated and kidney−shaped ( Figs. 4, 6B ).

The parasphenoid is a thin flat bone anteriorly, forming a short triangular cultriform process that extends onto the surface of the palate and tapers to a sharp anterior point ( Figs. 4, 6B ). The parasphenoid extends well anterior to the anterior border of the posterior interpterygoid vacuities, but less than the parasphenoid of Peloneustes ( Ketchum 2007) or P. brachyspondylus ( Taylor and Cruickshank 1993) , and divides the pterygoids along their midline. It contacts the pterygoids with strong interdigitating sutures. The parasphenoid divides the posterior interpterygoid vacuities into two openings and is ventrally keeled between them. The palate is here slightly concave from side to side. The posterior portion of the parasphenoid cannot be differentiated from the basisphenoid because of damages.

Mandible ( Figs. 5–7 ): The mandible is well preserved and complete. It is narrow and tapers anteriorly ( Fig. 5 ).

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From the anterior extremity to the posterior end of the retroarticular process, it is 470 mm long. The mandible is slightly bowed when viewed in ventral view, but less than that of Rhomaleosauridae (the condition in Archaeonectrus in unknown). A natural break provides a section ( Fig. 7D ) revealing the internal arrangement of the mandibular elements.

The dentary rami are joined anteriorly in an elongated symphysis ( Fig. 6E ), about 110 mm long, approximately one quarter of the length of the mandible; the ratio is approximately the same for Archaeonectrus (NHMUK 38525; Owen 1865). The snout tip is narrow and slightly transversally expanded, as in Marmornectes candrewi Ketchum and Benson, 2011a , being intermediate between the unexpanded snout tip of longirostrine polycotylids (e.g., Dolichorhynchops herschelensis, Sato 2005 ; Thililua longicollis, Bardet et al. 2003 ), Hauffiosaurus ( Vincent 2011; Benson et al. 2011b), and H. longirostris ( White 1940; Benson et al. 2011b), and the spatulated snout tip of longirostrine pliosauroids (e.g., Peloneustes, Ketchum 2007 ; Liopleurodon, Noè 2001 ) and Rhomaleosauridae (e.g., Meyerasaurus victor, Smith and Vincent 2010 ). The condition in Archaeonectrus in unknown as the mandible is preserved laterally compressed. In the symphyseal region, the two rami form a median crest where they join ( Fig. 6E ), here poorly preserved, as in H. longirostris ( White 1940) and distinguishing Cryonectes from Hauffiosaurus and Marmornectes which do not possess ventral mandibular ridge ( Vincent 2011; Benson et al. 2011b; Ketchum and Benson 2011a). The condition is unknown in Archaeonectrus , as the mandible is preserved laterally compressed. Posteriorly, the two dentary rami are separated by the splenials entering into the symphyseal region for a short distance on the ventral side. Posteriorly, the rami diverge by about 40° and increasingly become laterally compressed and more vertical. Posteriorly, the dentary contacts the surangular and overlaps internally the suture between the surangular and the coronoid. The lateral surface of the dentary is flat without a trough occupying the lateral surface, as is the case in H. tomistomimus ( Benson et al. 2011b) .

The splenial is a long, transversely compressed bone ( Fig. 7 ). It takes part in the symphysis for a short distance where it forms the internal and ventral border of the mandibular ramus. Throughout its length, it is very closely applied to the medial face of the coronoid extremity. The splenial is not preserved over the Meckelian canal because it is thin and liable to breakage, as is the case in many other plesiosaurs ( O’Keefe 2001).

The coronoid is a very thin and long bone extending from the coronoid eminence, to the anterior part of the mandible posterior to the symphysis so that it does not participate into the symphyseal region. It is not exposed in lateral view ( Fig. 7B ). Its ventral surface contacts the splenial and is overlapped by it ventrally. Posteriorly, it deepens over the Meckelian canal, forming its roof ( Fig. 7C ). It unites with the surangular in an almost vertical suture. Its dorsal edge is close to and nearly parallel with the alveolar border of the dentary bone.

The angular is prolonged forwards as a narrowing process ending a few centimetres from the symphysis. In this region, it joins the dentary on the lateral side of the mandible and the coronoid on its medial side ( Fig. 7B ). It forms the ventral surface of the coronoid eminence. In lateral view, the angular forms a well−marked suture with the surangular.

The surangular forms the dorsal part of the mandible between the coronoid eminence and the glenoid. Its dorsal surface is transversally expanded as in Marmornectes ( Ketchum and Benson 2011a) , broader than that of Thalassiodracon ( Storrs and Taylor 1996; Benson et al. 2011a), but narrower than that of Peloneustes ( Ketchum and Benson 2011b) . It does not bear any depression anterior to the glenoid dorsally nor a foramen on its medial surface. The suture between articular and surangular is clearly visible in this specimen ( Fig. 7B ), whereas it is generally closed, and thus not visible, in almost all plesiosaurs (e.g., Brown 1981; Ketchum and Benson 2011a; Sato 2003, 2005). The suture runs from a point situated at the level of the coronoid eminence on the ventral surface of the mandible, to a point situated dorsally, anterior to the anterior margin of the glenoid.

The articular is a robust bone, exposed mainly dorsally and forming the dorsomedial flange of the jaw ramus between the surangular, the glenoid fossa and the retroarticular process. The glenoid fossa is laterally expanded and has two concavities to fit the double condyles of the quadrate. The medial area is deeply concave from side to side and the small lateral area is only slightly concave. The anterior border of the glenoid cavity forms a prominent lip. The retroarticular process is long, narrow, almost vertical and medially recurved.

Dentition ( Fig. 7 ): About 34 teeth are represented. Each premaxilla bears 5 tooth sockets, distinguishing Cryonectes from Peloneustes (6 teeth in each premaxilla), H. zanoni (7), H. longirostris (10), and Marmornectes candrewi (6). The left maxilla bears 9 sockets and the right 6, but the maxillae are not entirely preserved. Fifteen sockets are exposed on the left dentary and 19 on the right. Since some sections of the dentaries are obscured or slightly damaged, it is most likely that there were originally a few more than 19 teeth on both sides, a number close to that observed for Archaeonectrus (NHMUK 38525; Owen 1865), which presents approximately twenty teeth on each dentary. It is likely that there were at least 66 functional teeth in the specimen. The mandibular symphysis occupies at least seven tooth positions, and is in this regard similar to that of Archaeonectrus (NHMUK 38525; Owen 1865) and Macroplata tenuiceps (NHMUK R5488; Ketchum and Smith 2010), which both present seven or eight dentary tooth pairs adjacent to the mandibular symphysis ( Ketchum and Smith 2010). This number is higher than in Attenborosaurus conybeari ( Sollas, 1881) and NHMUK 39514 (Vincent 2012), which both present a mandibular symphysis occupying less than seven tooth positions, and distinctly lower than in Peloneustes ( Ketchum 2007) , Marmornectes candrewi Ketchum and Benson, 2011a , and H. longirostris ( White 1940) , which all have a mandibular symphysis occupying at least 12 tooth positions. The mandibular teeth of Cryonectes are close to each other, distinguishing it from Archaeonectrus (NHMUK 38525; Owen 1865) in which the teeth are well separated; Owen (1865) noted that “the teeth are divided by intervals of rather more than their own basal breadth”.

The teeth vary in size and orientation along the jaw. The anteriormost teeth are tilted whereas the posterior ones are more vertical. The first pair of teeth is very small in comparison to adjacent teeth. The teeth are broadly similar in size and shape across the premaxilla−maxilla suture. They all show the typical plesiosaurian form with curved, conical crowns and

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thin roots. Each tooth is subcircular in cross section without carinae, distinguishing Cryonectes from Pliosaurus brachydeirus ( Owen, 1842) and P. brachyspondylus (BRSMG Cc332; Taylor and Cruickshank 1993) which exhibit trihedral teeth in cross−section with carinae. The crowns of the best−preserved teeth are needle−like, slightly curved, slender and sharp, and their tips are compressed ( Fig. 7E, F ) contrary to those of Attenborosaurus conybeari ( Bakker 1993) , Archaeonectrus (NHMUK 38525), H. zanoni ( Vincent 2011) , Peloneustes ( Ketchum 2007) , and Liopleurodon ( Noè 2001) which have more robust and large teeth without compression. Most of the tooth row in Cryonectes is slightly higher than the glenoid fossa.

Axial skeleton ( Figs. 8–10, Table 1): The centra of the cervical vertebrae are relatively short, with their width slightly exceeding their length and their height (W> L> H) ( Table 1). Broad and short cervical centra are also seen in Archaeonectrus ( Owen 1865) . The cervical proportion of Cryonectes (H <L; Table 1) differs from Attenborosaurus ( Sollas 1881) , P. brachyspondylus (BRSMG Cc332; Taylor and Cruickshank 1993) and H. tomistomimus ( Benson et al. 2011b) , whose cervical vertebrae are higher than long. Brown (1981) proposed short cervical centra as a character of Pliosauroidea and Andrews (1913) and Ketchum (2007) stated that the cervical centra of pliosaurids are approximately half as long as they are high or wide. Accordingly, the cervical centra of Cryonectes show pliosauroid, but not pliosaurid, proportions.

5 cm rib facet nutritive foramina

In Table 1 and in Figs. 8, 9 , and 10, the vertebral numbers refer to their position in the vertebral column as preserved, not the natural position. The articular surfaces are elliptical and moderately amphicoelous, and the margins form an abrupt angle with the centrum side. Each articular surface possesses a marked, central, horizontally elongated pit. The ventral sides are almost flat and lack a keel, distinguishing Cryonectes from H. zanoni ( Vincent 2011) , Archaeonectrus (NHMUK R.1337; Owen 1865), P. brachydeirus ( Tarlo 1960) , and Rhomaleosauridae (e.g., M. victor Smith and Vincent, 2010 ), which possess a ventral keel on cervical vertebrae. This character is probably not ontogenetic because juvenile specimens of Thalassiodracon (e.g., NHMUK 2018; PV personal observations)

neural arch rib facet already possess this feature. The ventral paired nutritive foramina are well developed and form ovate depressions. They are close to each other. The lateral surfaces of the centrum beneath the bases of the neural arch are moderately concave lengthwise. The margins of the lateral surfaces show marked rugosities. There are two distinct articular facets for the ribs. The first facet is oval whereas the second is sub−circular. The rib facets are located ventrally and slightly nearer to the posterior than the anterior edge of the centrum. Only partial neural arches are preserved for vertebrae 1, 2, 3, and 9 and a partial isolated one is also preserved. The neural spine appears to have been narrow. The zygapophyses are poorly preserved and few characters are observable. The posterior zygapophyses are small, with elongate and only slightly concave articular surfaces. No groove runs on the posterior surface of the neural spine dorsally to the postzygapophyses.

The undetermined vertebra is not well preserved. Its centrum is shorter than high and the articular facets appear more rounded ventrally than other accompanying vertebrae. 5 4 3 2 1

The two nutritive foramina are laterally placed. The ribs articulations are not preserved.

Remarks.—In general morphology Cryonectes does not resemble any known plesiosaurian taxon. Long−snouted plesiosaurs are present among pliosauroids and Leptocleidia ( Ketchum and Benson 2010). No leptocleidian has been described from the Lower Jurassic, and the absence of a troughlike ventral surface of the posterior ramus of the pterygoid lateral to the posterior interpterygoid vacuity excludes Cryonectes from Leptocleidia (diagnosis of Ketchum and Benson 2010). Cryonectes differs from all other Early Jurassic pliosauroids in its cranial and postcranial skeleton. Attenborosaurus conybeari differs from Cryonectes neustriacus in that the mandibular symphysis extends for fewer than seven teeth, the teeth are robust and large and the cervical vertebrae are higher than long ( Sollas 1881). Cryonectes differs from Hauffiosaurus , which possesses 7–10 premaxillary teeth, no anterior interpterygoid vacuity, no ventral mandibular ridge and a ventral keel on cervical vertebrae ( O’Keefe 2001; Vincent 2011; Benson et al. 2011b). Hauffiosaurus longirostris Benson, Ketchum, Noè, and Gómez−Pérez, 2011b (MCZ 1033; Macroplata in White 1940) presents a very long snout, much more extended than in Cryonectes neustriacus , including 10 teeth in each premaxilla and at least 13 tooth positions along its mandibular symphysis ( White 1940). Cryonectes and Archaeonectrus (NHMUK 38525; Owen 1865) share a long, narrow snout and a long mandibular symphysis bearing seven teeth, the ratio between the length of the mandibular symphysis and the length of the mandible is approximately the same. However, the premaxillae of Archaeonectrus (NHMUK 38525) are more laterally expanded than those of the specimen studied here; the teeth are not so curved and not so large in Cryonectes than in Archaeonectrus (NHMUK 38525); Archaeonectrus possesses cervical vertebrae with width always exceeding their length and their height and with two deep depressions around the ventral nutritive paired foramina, separated by a sharp keel (NHMUK R.1337; personal observations). Cryonectes also differs markedly from Rhomaleosauridae , which exhibit a marked constriction at the premaxillary−maxillary suture, a short and spatulate mandibular symphysis, a bowed mandible, a broad anterior interpterygoid vacuity with round extremities, robust teeth, a ventral keel on the ventral side of its cervical vertebrae and both large nutritive foramina sunk in deep depressions on cervical

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10 9 8 7 6 vertebrae ( Watson 1909; Taylor 1992; Cruickshank 1994, 1996; O’Keefe 2001; Ketchum and Benson 2010: character 86.0; Ketchum and Smith 2010; Smith and Dyke 2008: characters 60.1 and 64.1; Smith and Vincent 2010).

As noted earlier, the single diagnostic Pliensbachian plesiosaurian taxon is the plesiosaurid Westphaliasaurus simonsensii Schwermann and Sander, 2011 from the lower Pliensbachian ( Tragophylloceras ibex Ammonite Zone ), and so slightly older than Cryonectes . Since W. simonsensii is known from postcranial remains, the two specimens preserve only a few overlapping portions. The cervical vertebrae of Westphaliasaurus and Cryonectes present both ventral sides almost flat lacking a keel, gently amphicoelous articular facets, with a marked, central pit (the pit of Cryonectes being more elongated), and two distinct articular facets for the ribs. Nevertheless, the articular margins of the cervical vertebrae of Cryonectes form an abrupt angle with the centrum side, whereas they are more rounded in Westphaliasaurus .

Cryonectes presents characters that are consistent with the diagnosis of Pliosauridae proposed by Druckenmiller and Russell (2008), namely ventral pterygoid flanges lateral and posterior to the posterior interpterygoid vacuities and a flat ventral surface of the cervical centra. In addition, Cryonectes has a coronoid with a long lingual process and cervical centra with flat ventral surfaces, characters that are consistent with the diagnosis of Pliosauridae proposed by Ketchum and Benson (2010).

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig. 3. Pliosaurid plesiosaur Cryonectes neustriacus gen. et sp. nov., MAE 2007.1.1(J), holotype; Late Pliensbachian, Early Jurassic, Calvados, France. Skull in dorsal view (A) and explanatory drawing (B).

Fig. 5. Pliosaurid plesiosaur Cryonectes neustriacus gen. et sp. nov., MAE 2007.1.1(J), holotype; Late Pliensbachian, Early Jurassic, Calvados, France. Skull in ventral view (A) and explanatory drawing (B).

Fig. 6. Pliosaurid plesiosaur Cryonectes neustriacus gen. et sp. nov., MAE 2007.1.1(J), holotype; Late Pliensbachian, Early Jurassic, Calvados, France. A. Rostrum in dorsal view. B. Close−up photograph of the parasphenoid and posterior interpterygoid vacuities. Close−up photograph of the anterior interpterygoid vacuity (C) and the posterior part of the palate in oblique anterior view (D). E. Rostrum in ventral view.

Fig. 7. Pliosaurid plesiosaur Cryonectes neustriacus gen. et sp. nov., MAE 2007.1.1(J), holotype; Late Pliensbachian, Early Jurassic, Calvados, France. A. Skull in left lateral view. Posterior part of the right mandibular ramus in lateral (B) and lingual (C) views. Photographs (A1, B1, C1) and explanatory drawings (A2, B2, C2). D. Section of the right mandibular ramus (see Fig. 2 and 4 for actual line of section). E, F. Teeth.

Fig. 9. Pliosaurid plesiosaur Cryonectes neustriacus gen. et sp. nov., MAE2007.1.1(J), holotype; Late Pliensbachian, Early Jurassic, Calvados, France. A. Cervical vertebrae 1 to 5 in right lateral view. B. Vertebrae 1 to 5 in anterior view. C. Vertebrae 1 to 5 in ventral view.