Clevosaurus cambrica Keeble, Whiteside and Benton, 2018

Chambi-Trowell, Sofia A. V., Whiteside, David I. & Benton, Michael J., 2019, Diversity in rhynchocephalian Clevosaurus skulls based on CT reconstruction of two Late Triassic species from Great Britain, Acta Palaeontologica Polonica 64 (1), pp. 41-64 : 46-57

Clevosaurus cambrica Keeble, Whiteside and Benton, 2018

Material.—Referred specimens include a partially disarticulated near complete skull and at least five cervical vertebra, NHMUK PV R 37014, and a left forelimb associated with shoulder elements and dorsal vertebra, NHMUK PV R 37013.

Description.—The original report of Clevosaurus cambrica ( NHMUK PV R 37014; Keeble et al. 2018) identified the scanned materials as a few cervical vertebrae and the skull below the palate level, comprising portions of both maxillae, mandibles, palatines and pterygoids. A close inspection of these scans showed that the left side of the partial skull of C. cambrica is near complete ( Fig. 4 ), though missing several of the postero-lateral bones, including the jugal, postfrontal, postorbital, quadrate, squamosal and prefrontal. The right side of the skull has sustained more physical damage, with several bones (e.g., left maxilla, dentary, prefrontal, and postorbital) fragmented. The bones are all closely associated, and in a few cases show some articulation. The whole skull was ventro-dorsally compressed post-mortem, but was only minimally deformed, and the bones appear to have fallen between and on either side of the mandibles before fossilisation took place. There are also five articulated elements originally identified as cervical vertebrae by Keeble et al. (2018), though they considered that the most anterior element might be a basioccipital, which we confirm here. Much of the braincase of C. cambrica is also present. The only missing bones, or at least not visible at the resolution of the scans, are the jugals and stapes.

The left forelimb (humerus, ulna, and radius) and partial pectoral girdle (scapulae and coracoids) of this same specimen were also resegmented to refine the resolution ( Fig. 5). In addition to the scapula already identified, two previously uncertain elements were identified as a second scapula and a coracoid, and a fourth previously unsegmented bone was identified as a second coracoid. Further dorsal vertebrae were identified running in a column beneath the forelimb, forming a total of six articulated vertebrae. The carpal mentioned by Keeble et al. (2018) could not be located in this reassessment of the CT scans.

Premaxilla:The left and right premaxillae of Clevosaurus cambrica ( NHMUK PV R 37014) are preserved, with the right showing a more complete dorsal process ( Fig. 6A ). The two premaxillae are found in close association with the anterior edge of the maxilla. The internarial process is relatively short but robust and tapers to a point. The dorsal process makes the posterior edge of the nasal openings, and there the maxillary facet is a posterior concavity that bifurcates to form two flattened processes which extend beneath the medial surface of the maxilla, as in most Clevosaurus . The exception is C. convallis which lacks the bifurcating dorsal maxillary process and is more robust ( Säilä 2005: 820). The unexpectedly short length of both the internarial and dorsal maxillary processes preserved here suggests they might have been broken but the internarial process of C. convallis is similarly short ( Säilä 2005: 820). The portion of the dorsal maxillary process ventral to the start of the bifurcation is relatively longer than in C. hudsoni ( Fraser 1988: fig. 5) and C. sectumsemper ( Klein et al. 2015: fig. 3A, B). The number of teeth present on the premaxilla is not clear, either due to the resolution of the scans or perhaps indicating that the worn premaxilla teeth are already encased in secondary bone and dentine so that they form a single conical structure, much like that observed in other species of Clevosaurus ( Fraser 1988; Sues et al. 1994; Säilä 2005; Bonaparte and Sues 2006). However, in C. sectumsemper this overgrowth is not observed and this is a diagnostic feature of the species ( Klein et al. 2015: 409).

A1 A 2 5 mm

Maxilla: While the holotype maxilla ( NHMUK PV R 37016) has been described ( Keeble et al. 2018), the maxillae in NHMUK PV R 37014 were incompletely segmented Fig. 6B ). The right maxilla is complete and bears six acrodont teeth. The left maxilla has broken in two, at approximately two-thirds along its length, and its anterior portion bears four acrodont teeth. The posterior portion remains articulated with the left ectopterygoid, and it appears to be damaged and teeth cannot be clearly discerned. The maxilla is as described by Keeble et al. (2018), but bears a small sixth additional tooth, adding to the ‘five prominently flanged teeth’ of Keeble et al. (2018), and is approximately 12 mm in length. The marginal tooth row is offset posteromedially so that it is inset from the lateral margin of the maxilla posteriorly, as in C. bairdii ( Sues et al. 1994: 329) . However, unlike C. bairdii , the premaxillary process is pronounced in C. cambrica . A lip of secondary bone appears above the 2 nd to 5 th maxillary tooth ( Fig. 6B 2 ) and no hatchling teeth appear to be present. The maxilla of C. cambrica is similar in shape to that of C. hudsoni ( Fraser 1988: fig. 6; O’Brien et al. 2018: fig. 5A), although superficially the suborbital margin formed by the dorsal process of C. cambrica is more recurved, resembling that of C. bairdii ( Sues et al. 1994: fig. 1B). It is also posteriorly longer, with the facial process of the maxilla being less than half the length of the suborbital length, suggesting larger orbits, and this morphology is similar to C. brasiliensis albeit with a shallower dorsal process ( Hsiou et al. 2015: fig. 2C).

Nasal: Nasals are rarely preserved in clevosaurs as they are fragile ( Fraser 1988: 131–132), but here both nasals are present as large, thin, concave and simple structures ( Fig. 6C ). The left nasal has some damage to its lateral edge where the left prefrontal has been thrust upwards through it. It is likely both nasals were damaged, as the lateral and anterior edges of both appear to be incomplete. The nasals in C. brasiliensis are steep and fused to the frontals ( Bonaparte and Sues 2006: fig. 1A, B; Hsiou et al. 2015: 7), but here we find both nasals isolated from the frontals and their length suggests that they could not have been angled as steeply or fused to the frontals.

Frontal: Both frontals are present ( Fig. 6D ) and they are broken apart at an asymmetrical angle so that the left frontal posteriorly is still sutured with a portion of the right frontal, while a larger portion of the right frontal has become displaced and lies beneath the left frontal. While some detail like the prefrontal facets is preserved, finer detail such as the sutures between the two frontals does not appear in these scans. When their positions are corrected, the two frontals together form an hour-glass shape in dorsal view.

Parietal: Both parietals ( Fig. 6E ) are preserved in close association, but there appears to be some asymmetry in the parietal processes, and it is possible that the left parietal sustained damage that is not clear on the scans. The parietals have disassociated from the frontals, which appear to have been moved anteriorly with the rest of the skull. The left parietal process is positioned at a ~90° angle to the left parietal table horizontally, and is vertical and flat, and this is likely the result of a break. The right parietal process is angled further backward at about 120° horizontally and slopes ventrally at a shallow angle vertically; the right parietal process also bears a bump on its ventral surface where it joins the right parietal. There are distinct slot-like facets for the supratemporals on the posterior-most ends of both parietal processes. The pineal foramen is not visible at current resolution, but there may have been damage in this region. As with the frontals, the lateral parietal margins are much more robust than the medial ones. As in Clevosaurus hudsoni and C. brasiliensis ( Fraser 1988: 174; Hsiou et al. 2015), the parietal table of C. cambrica lacks a true parietal crest, unlike Sphenodon .

Prefrontal: The left prefrontal ( Fig. 7A ) has broken into at least three pieces, perhaps when the prefrontal was forced upwards through the left nasal. The largest fragment is the keeled ventral portion of the prefrontal, which would have formed the anterior margin of the orbital rim, a large flat bone that is angled at roughly 90° at its keel. The second fragment is slightly smaller and comprises the posterodorsal process of the prefrontal, which forms the anterodorsal edge of the orbital rim. The third fragment is a simple slight concave surface that would have formed the dorsal portion of the prefrontal and would have articulated with the nasal in life. When reconstructed, the prefrontal is large and semi-lunate in shape. However, we note that the damaged prefrontal of Clevosaurus cambrica might have been deeper and more robust than reconstructed here, meaning the skull overall might have been broader.

Postfrontal: The left postfrontal ( Fig. 7B ) is a small triradiate bone with a slightly recurved medial surface and it is closely associated with the left frontal and is nearly articulated with the dorsal process of the postorbital.

Postorbital: The left postorbital ( Fig. 7C ) is a large triradiate bone with a broad and elongated ventral jugal process that extends nearly as far as the maxilla and bears a broad jugal facet on its medial surface ( Fig. 7C 2 ). It was found to have broken into at least two pieces, with the dorsal portion still articulated with the postfrontal but having been displaced ventrally beneath the lower portion of the postorbital, and the dorsal portion appears to have been broken a second time so that the postfrontal process is at an angle to the rest of the element, which was corrected by digitally rebreaking the bone at this point and rotating it dorsally. Unlike C. brasiliensis ( Hsiou et al. 2015: 8) , C. cambrica has both a long ventral and dorsal process, which encompasses almost the entire posterior orbital rim.

Squamosal: The preserved left squamosal lacks the anterior flange that would have articulated medially with the postorbital ( Fig. 7D ), but the tapering ventral quadrate process, which would have extended approximately half the height of the quadrate, as in C. brasiliensis and C. wangi ( Bonaparte and Sues 2006; Jones 2006), and the medially recurved posterior supratemporal process are both well preserved.

Supratemporal: Both supratemporals appear to be preserved. The left supratemporal is still articulated with the left parietal, and it is a thin medially convex plate-like bone ( Fig. 7E ). As in Clevosaurus hudsoni ( Fraser 1988: 134) , it is broadest dorsally and narrows ventrally, and this broad extension would have articulated with the squamosal in life. The possible right supratemporal is part of a narrow, flattened process that appears to extend from the anterior surface of the braincase (see Fig. 4A 2 ). Its identity is inferred from its position relative to the braincase, but it could be a fragment of bone from elsewhere in the skeleton. Another possible candidate for a right supratemporal is a fragment of bone overlying the right opisthotic (see Fig. 4A 1 ). The shape of the supratemporal differs from that of C. brasiliensis , which is elongate ( Hsiou et al. 2015) but similarly tapers anteriorly to contact the parietal process.

Vomer : Despite their rare preservation in clevosaurs because of their fragility ( Fraser 1988: 136), remarkably both vomers are present and intact in C. cambrica ( Fig. 8B ). The two vomers are in close association and located close to where they would have been in life. They are thin convex structures with robust medial and anterior margins. Vomerine teeth are hard to discern at the resolution of the scans, but at least two appear along the posteromedial edge, in the same position as in C. hudsoni ( Fraser 1988: fig. 14) and C. brasiliensis , which latter species bears three teeth in this medial position ( Hsiou et al. 2015: 10).

Palatine: Both palatines are present in Clevosaurus cambrica , with the right palatine well preserved and with the whole maxillary process intact and barely disarticulated Fig. 8A ), forming much of the boundary of the choanae. The palatines have a robust lateral raised ridge bearing six conical teeth, as in the C. hudsoni specimens described by Fraser (1988: 137), while medially the palatine forms a thin flattened flange that widens anteriorly and becomes more concave, bearing a single medio-anteriorly placed tooth. The palatine is distinct from that of C. minor as it possesses a raised lateral tooth ridge and concave vomerine facet ( Fraser 1988: 159–160) not found in that species. The maxillary process is robust and bifurcates at the maxilla to form an extensive but thin surface of contact. The six laterally positioned teeth in C. cambrica , are the same as in C. hudsoni , C. brasiliensis , and C. sectumsemper ( Fraser 1988; Hsiou et al. 2015; Klein et al. 2015), but greater in number than the five in C. convallis ( Säilä 2005) or the four in C. minor ( Fraser 1988) , and fewer than the nine in C. hudsoni NHMUK PV R 36832 or the seven in C. wangi ( Wu 1994; Jones 2006). The lateral tooth row is also much straighter than in the Chinese clevosaurs C. wangi and C. petilus ( Wu 1994; Jones 2006).

Epipterygoid: Both epipterygoids may be present. The left epipterygoid ( Fig. 8C ) is a tall, columnar and slender element that does not appear to have the medio-lateral flattening seen in the Clevosaurus hudsoni of O’Brien et al. (2018). It is positively identified as it is still articulated with the left pterygoid, albeit it has collapsed antero-posteriorly so that it lies parallel to the long axis of the skull, and the cephalic region appears to have been broken. There is a possible left epipterygoid but at the current resolution this cannot be confirmed, and its identity is debatable (see Fig. 4 ). It is a flattened rod-like bone. The shape of the left epipterygoid is much narrower and more columnar than in Sphenodon . The epipterygoid of C. bairdii is similar to that of C. cambrica in being “rod-like dorsally” and flattening ventrally towards its attachment above the basipterygoid joint ( Sues et al. 1994); the epipterygoid has not be described in any other species of Clevosaurus .

Ectopterygoid: Both ectopterygoids are present and intact, and the left ectopterygoid is still articulated with the left pterygoid, palatine and a fragment of the maxilla. The ectopterygoid ( Fig. 8D ) is formed of a posteromedial robust rod-like bone that bifurcates at either end, with a broad lateral process that is more elongate anteriorly and would have had an extensive contact with the medial surfaces of maxilla and the jugal, as in Clevosaurus brasiliensis ( Bonaparte and Sues 2006) and C. bairdii ( Sues et al. 1994) . The medial process by contrast is longer posteriorly, and this section forms a triangular facet that overlaps and articulates with the anteroventrally facing facet on the lateral posterior pterygoid flange, while anteriorly it forms a flattened short surface where the palatine would have articulated. Overall, the ectopterygoids appear very similar to the C. hudsoni specimens described by Fraser (1988).

Pterygoid: Pterygoids are present on both sides of the skull, though the right appears to be better preserved ( Fig. 8D ). Pterygoid teeth cannot be definitively distinguished at the resolution of the scans, but several small conical bumps are present on the flattened palatal shelf that appear to form two parallel rows, which differs from the three rows found in Clevosaurus brasiliensis ( Hsiou et al. 2015) . The transverse process that runs between the palatal shelf and the posterior quadrate flange is short, as in C. hudsoni ( Fraser 1988: 137–138; NHMUK PV R36832). The quadrate flange is still articulated with the pterygoid flange of the left pterygoid. The anteriormost portion of the left pterygoid has broken off but remains articulated anteriorly with the right pterygoid. The pterygoids bear robust transverse ectopterygoid processes with triangular facets that articulate with the medial surface of the ectopterygoids. While the tooth number cannot be discerned, the overall shape of the pterygoids is as described for C. hudsoni by Fraser (1988) with no medial “neck” between the ectopterygoid processes and the basipterygoid facet as in most other Clevosaurus but differing in C. wangi where this region is described as elongate ( Wu 1994; Jones 2006).

Quadrat e: The left quadrate is preserved and appears to be complete ( Fig. 8E ), though at the CT scan resolution some detail is unclear. The medially placed vertical strut is recurved medially, and thickens towards both the cephalic and articular condyles, but notably more so for the articular condyle where it has a pronounced ventrally convex lateral keel that attaches to the lateral plate-like structure of the quadrate. The condyle is very similar to that described for the Clevosaurus hudsoni specimens of Fraser (1988: 136), with the anterior-posterior length much shorter than the width, and the cotyle being medio-laterally deeply concave. The medial edge of the condyle projects strongly ventrally much as in C. brasiliensis ( Hsiou et al. 2015) , but differing from C. hudsoni ( Fraser 1988: 13 a, 17a; NHMUK PV R36832). The thin blade-like pterygoid-quadrate process is preserved, extending medio-anteriorly from the vertical strut, and as in C. hudsoni , is reinforced by a more robust ventral edge, as also seen in C. hudsoni (NHMUK PV R36832; Fig. 2D, E ). The medially recurved lateral side of the quadrate is very thin and poorly resolved here, but a quadratojugal foramen appears to be present. Relatively the quadrate of C. cambrica is much shorter than in C. hudsoni (NHMUK PV R36832) but is only moderately shorter than that of the C. hudsoni of Fraser (1988). The quadratojugal cannot be discerned from the quadrate, and we assume that the bones are fused.

Basioccipital and exoccipitals: The basioccipital remains articulated with the basisphenoid and parabasisphenoid ( Fig. 9 ). The shape appears similar to that of Clevosaurus sectumsemper and C. hudsoni ( Fraser 1988; Klein et al. 2015), with a lunate occipital condyle, and two robust and rounded basal tubera that project postero-laterally. The left and right exoccipitals, in close association with the basioccipital, appear to have sustained some damage and have broken apart asymmetrically; together they would have formed the lateral edges of the foramen magnum.

Parabasisphenoid: Aside from the basioccipital, the floor of the braincase is primarily formed by the fusion of the parasphenoid and basisphenoid ( Fraser 1988: fig. 18; Hsiou et al. 2015: 12; figs. 9C, D) and is relatively much broader in Clevosaurus cambrica than in C. hudsoni ( Fraser 1988; O’Brien et al. 2018) but similar to that of C. brasiliensis ( Hsiou et al. 2015: fig. 2). Anteriorly, the basisphenoid forms the paired basipterygoid processes, but in C. cambrica (NHMUK PV R37014) these two processes have been clearly broken off ( Fig. 9A 3 ). We located what we infer to be the left basipterygoid process located ventral to the basipterygoid facet on the left pterygoid. It appears as a small but robust bone that is roughly tear-shaped. Like the basipterygoid processes, the cultriform process also appears to have been broken off ( Fig. 9A 3 ), though it might be preserved, but as it is a very delicate thin structure ( Fraser 1988: fig. 17b; NHMUK PV R36832), it is not visible at the resolution of the CT scans used here.

Supraoccipital, opisthotics, and prootics: The supraoccipital in Clevosaurus cambrica is still fused to both left and right prootics, which are in turn both fused to their respective opisthotics ( Fig. 9 ). As has been observed in the C. hudsoni specimens of Fraser (1988: 140), these elements are often fused. Posteriorly the supraoccipital forms the dorsal margin of the foramen magnum, but the bone is so thin here (as in C. hudsoni ; NHMUK PV R36832), that if it is not damaged then it is not visible at this resolution. The dorsal surface is convex and broad with no sign of a dorsal crest, but the area is poorly resolved and may have been damaged. The paroccipital processes of the opisthotics are robust and short, with a concave ventral surface, and they project posterolaterally. Suture lines between these bones cannot be distinguished, and though there clearly has been some displacement and breakage, digital reconstruction is limited. The left pila antotica, formed from the prootic ( Fraser 1988: 141), is preserved and appears similar to that of C. hudsoni (NHMUK PV R36832), though relatively dorsoventrally shorter, and it appears to have been broken off the main body of the prootic. These elements have not been well preserved in other species of Clevosaurus and cannot therefore be compared here.

Dentary: Both dentaries have been partially described ( Keeble et al. 2018: 104–105). In the amended scans, both dentaries were found to be complete, although the left dentary appears to have sustained some damage along its lateral and ventral margins, and we identified the anterior fragment which in the original description had been tentatively suggested to be part of the hyoid. The right dentary is intact, while the left dentary has a break in the anteriormost third of its length. The anterior third is still articulated with the right dentary in the symphyseal region, while the posterior two-thirds of the left dentary have been displaced laterally. The anterior fragment bears no teeth and forms a dorso-anteriorly concave structure that inclines anteriorly very much like that of Clevosaurus hudsoni ( Fraser 1988: fig. 19). The symphyseal contact is ovoid, gracile and positioned at a shallow angle, which is like the condition in C. sectumsemper (Klein et al. 2018: fig. 4), but differs from the oval near-vertical symphyseal contact of C. brasiliensis ( Hsiou et al. 2015) or the stout symphyseal contact of C. convallis ( Säilä 2005) . There are six acrodont teeth on the right dentary, with pronounced wear facets, which are further described by Keeble et al. 2018), and below them is a pronounced lateral ridge visible indicating the development of secondary bone ( Fig. 10A 1 ). The penultimate tooth is longest mesio-distally, but roughly the same height as the ultimate tooth. The Meckelian groove is present but not pronounced, expanding posteriorly below the tooth row. The region of dentary immediately behind the posteriormost tooth tapers into a narrow upwardly slanted neck, which then expands dorsally to form the coronoid process, where it resumes running parallel to the anterior portion of the dentary. Both left and right dentaries are still articulated with the surangular, angular, articular and prearticular, as well as both being in close association with their respective coronoids, so a complete articulated mandible was easy to reconstruct ( Fig. 10A ). The dentary of C. cambrica is slighter than those of C. convallis ( Säilä 2005) ,

C. brasiliensis ( Bonaparte and Sues 2006) and C. bairdii ( Sues et al. 1994) . It is most similar in morphology to those of C. hudsoni and C. sectumsemper ( Fraser 1988; Klein et al. 2015) which, although they have only four additional teeth, are more gracile and possess teeth with prominent anterolateral flanges. The C. cambrica dentary is more slender and has a lower coronoid process than the C. hudsoni specimen NHMUK PV R36832 ( Fig. 10B ).

Coronoid: Previously undocumented in any species of Clevosaurus from the British Isles, though observed in North American and Chinese clevosaurs ( Sues et al. 1994; Wu 1994; Jones 2006), both left and right coronoids are present and complete, with only slight disarticulation. Both bear clear facets for the surangular on their posterior surfaces in the form of a corresponding convex structure ( Fig. 10A 2 ). The coronoid in life would have articulated with the antero-medial surface of the coronoid process. The shape is similar to that of C. hudsoni (NHMUK PV R36832) that we describe here, with both bearing a smoother and rounder lateral profile than in Sphenodon . Similar to C. bairdii ( Sues et al. 1994: 334) , the coronoid bone projects slightly above the coronoid process. The coronoid is roughly an inverted tear-drop in shape, with a rounded, broader and wider dorsal margin that corresponds with the medio-dorsal edge of the coronoid process of the dentary. Ventrally the coronoid tapers and flattens.

Articular, prearticular, surangular and angular: The posterior bones of the mandible are present on both sides of the skull of Clevosaurus cambrica . As mentioned by Keeble et al. (2018: 109), the angular cannot be separated from the prearticular, which in turn is fused to the articular complex, but it can be inferred as it extends beyond the anterior edge of coronoid eminence and reaches as far as the penultimate tooth. The articular and surangular are fused to form the articular complex, as in C. hudsoni ( Fraser 1988) and C. convallis ( Säilä 2005) . Both prearticular and surangular are latero-medially compressed and are fused posteriorly to the angular, forming the ventral and dorsal edges respectively of the adductor fossa ( Fig. 10A ). The prearticular runs parallel to the length of the dentary and is anteriorly fused to the angular. The surangular runs at a shallow angle towards the apex of the coronoid process, forming its posterodorsal edge, and anteriorly articulating with the coronoid. The articular is robust, forming the articular condyle before rapidly tapering into an elongate medially recurved retroarticular process which remains at a constant width along its length ( Fig. 10A ). The mandibular foramen cannot be discerned. The mandible terminates posteriorly in an elongated retroarticular that is similar in relative length to that of C. hudsoni (NHMUK PV R36832; Fig. 10B ) and C. brasiliensis ( Bonaparte and Sues 2006; Hsiou et al. 2015).

Teeth: Maxillary and dentary teeth in Clevosaurus cambrica were already well described by Keeble et al. (2018), and the main amendments we make is that there is an anterior small sixth tooth (probably not flanged) on the right maxilla and that the dentary teeth appear to be bounded ventrally by secondary bone or dentine with wear facets of individual maxillary teeth typical of Clevosaurus . We also add that the trailing flanges that recurve posteromedially on the teeth have large prominent medially placed escape structures on the additional teeth of the dentary. The additional dentary and maxillary teeth of C. cambrica bear prominent flanges, and “saddle-shaped” additional teeth on the dentary with prominent anteromedially placed escape structures; this morphology also resembles that in C. hudsoni as described by Fraser (1988) and in C. sectumsemper ( Klein et al. 2015) . This latter species has gaps with no overlap between the teeth on the dentary, a feature which C. cambrica appears to share ( Keeble et al. 2018: fig. 5C; Fig. 6B ), but the dentary of C. cambrica differs markedly from both in having 5–6 teeth rather than 4 (including C. minor ; Fraser 1988). Furthermore, unlike C. hudsoni and C. minor , it is not the ultimate but the penultimate dentary (or the antepenultimate) tooth that is the largest in C. cambrica , similar to C. convallis ( Säilä 2005) . However, the dentition of C. cambrica differs from that of C. convallis in that hatchling teeth do not remain visible in adult specimens, and the tooth number is between 5–6, rather than 6–10 (maximum 8 on the dentary and 10 on the maxilla; Säilä 2005: 820). C. convallis also lacks the pronounced escape structures or antero-lateral flanges on its dentary teeth which often overlap and lack any gap between them.

Atlas/axis complex: The atlas (in NHMUK PV R37013) has undergone some fragmentation, and the antero-posteriorly short neural arch has broken in half and has disarticulated from the atlas centrum ( Fig. 11A ) and shares a similar morphology to that of Clevosaurus sectumsemper ( Klein et al. 2015: fig. 5A, B). The centrum of the atlas itself is small, rounded and appears to be fused to the front of the axis centrum, forming the odontoid complex. There are no visible intercentra, but many small fragments of bone in the region could be parts of atlantal intercentra. The neural spine of the axis may be partially incomplete, as the expected axial neural spine (seen in NHMUK PV R36832, O’Brien et al. 2018) is seemingly absent, damaged or too thin to be visible at the resolution of the CT scans, and instead we see a medially placed notably high pointed spike at the antero-dorsal surface of the neural arch that rises and then appears to drop sharply ( Fig. 11A 2 ). The zygosphene-zygantrum articulation is preserved between the axis and all successive cervical vertebra. No proatlas was identified, but it might be present, as the resolution of the scans is poor in this region.

Cervical vertebrae: Excluding the atlas and axis, there are three articulated cervical vertebrae in the scan ( Fig. 11B ), making five cervical vertebrae altogether ( Fig. 11D ) in NHMUK PV R37013. These have shorter neural spines than the axis. They appear typical of sphenodontians, with an hourglass-shaped centrum, which is both notochordal and amphicoelous. The neural arches appear to be complete and are all fused to their respective centra. Intercentra for these vertebrae have either been lost from this specimen or at not visible at the resolution of the CT scans. These cervical vertebrae appear typical of Clevosaurus ( Fraser 1988) , with shorter spines than in Sphenodon .

Posterior cervical, and dorsal, vertebrae: As noted by Keeble et al. (2018), we found two isolated but articulated vertebrae close to the left scapula (in NHMUK PV R37013), but we add a further four, making a total of six closely associated vertebrae running beneath the forelimb ( Fig. 11E ). Their close association with the shoulder elements, and the number of cervical vertebrae already counted (five), suggests these are dorsal, or at least some are dorsal, vertebrae, as in C. hudsoni which had eight cervical vertebrae ( Fraser 1988; O’Brien et al. 2018). Not much detail can be discerned from the scans, and only the two caudalmost of the six vertebrae can be seen in sufficient detail to be compared with the cervical vertebrae. The dorsal vertebrae both bear amphicoelous notochordal centra, and as in C. hudsoni ( O’Brien et al. 2018) they are wider than the cervical vertebrae ( Fig. 11C ). In close association with the six articulated vertebrae we found three fragments of ribs.

Scapulae and coracoids: Here we identified in addition to the left scapulacoracoid (re-identified here as the left scapula; Fig.11F 1 ), the right scapula ( Fig. 11F 2 ) and both coracoids ( Fig. 11G ), displaced from their original positions in NHMUK PV R37013. Both scapula and coracoids are found separate, suggesting that they were not fused (cf. the Clevosaurus hudsoni of Fraser 1988; O’Brien et al. 2018) as seen in C. sectumsemper , indicating that the animal may have been a sub-adult at the time of death ( Klein et al. 2015). The left scapula is disarticulated but closely associated with the proximal head of the humerus, while the right scapula is found displaced anteriorly of the distal head of the humerus, and Keeble et al. (2018) suggested it might be part of the sternum. The scapula blade here is found to be much shorter than previously described, and more like that of C. hudsoni , with the start of the blade clearly marked by a sharp transverse flexure in the bone. The glenoid protuberance is preserved in close association with the distal head of the humerus. Not much further detail can be discerned from the scans. The two coracoids are found displaced either side of the vertebral column. One coracoid is positioned posteriorly to the distal head of the humerus and had been segmented by Keeble et al. (2018) but was left out of the description. The second coracoid, found on the opposite side of the vertebral column, was segmented here for the first time. The coracoids are both damaged, but their identities can be inferred by their close association with the other shoulder elements and their positions either side of the vertebral column. Further, the robust protuberance found on each is likely to have formed the ventral half of the glenoid fossa.

Humerus: The left humerus (in NHMUK PV R37013) was described by Keeble et al. (2018), and we have refined the scan data to give a smoother surface and remove segmentation artefacts. Some alterations were made to the distal head of the humerus. The length of the humerus is nearly twice the mean length for C. sectumsemper but is around 80% the mean length for C. hudsoni ( Fraser 1988, Klein et al. 2015).

Radius and ulna: These elements have already been described, but here they were segmented separately, allowing us to re-measure them individually, and we find them to be slightly shorter than they had been previously described, at 11 mm each ( Table 1). The distal end of the radius appears to have a recurved head, and while in position for a carpal, no gaps could be observed to indicate it was a separate bone. The length ratio between the length of the ulna to the length of the humerus (in NHMUK PV R37013) falls closest to C. hudsoni measured by Fraser (1988: 157, tables 2, 3; Table 2), rather than C. hudsoni (NHMUK PV R36832), but this may not be a reliable comparison as they are the only two articulated specimens reported. However, the ulna/ humerus length ratios of the two articulated C. hudsoni specimens differ greatly and it is possible that these two specimens belong to two morphotypes. Skull length and forelimb proportions show some variations between the species of Clevosaurus ( Table 3), as Fraser (1988) also noted.

Stratigraphic and geographic range. — Upper Triassic (Rhaetian) fissure infill within Lower Carboniferous Limestone , Pant-y-ffynnon Quarry , near Bonvilston, Vale of Glamorgan, South Wales, UK ( NGR ST 046742 ) .

Figures

Fig. 4. The resegmented bones of lepidosaur rynchocephalian Clevosaurus cambrica Keeble, Whiteside, and Benton, 2018 (NHMUK PV R37014) from Late Triassic, Pant-y-ffynnon, Wales, UK; in dorsal (A1) and ventral (A2) views. Abbreviations: l., left; r., right.

Fig. 6. Cranial bones of lepidosaur rynchocephalian Clevosaurus cambrica Keeble, Whiteside, and Benton, 2018 (NHMUK PV R37014) from Late Triassic, Pant-y-ffynnon, Wales, UK. A. The right premaxilla in medial (A1) and lateral (A2) views. B. The right maxilla in lateral (B1) and medial (B2) views. C. The right (C1) and left (C2) nasal in dorsal view. D. The right (D1, D4) and left (D2, D3) frontals in dorsal (D1, D2) and ventral (D3, D4) views. E. The right (E1, E4) and left (E2, E3) parietals in dorsal (E1, E2) and ventral (E3, E4) views.

Fig. 2. Newly identified cranial bones present in lepidosaur rhynchocephalian Clevosaurus hudsoni Swinton, 1939 (NHMUK PV R36832) from Rhaetian, Cromhall Quarry, England, UK. A. The right postorbital in medial view, identified from amongst unidentified bones from the original surface dataset created by O’Brien et al. (2018). B. The left squamosal in lateral view, re-identified here from its previous suggested identity of a pterygoid flange. C. The right squamosal in lateral view, like A identified here from amongst previously unidentified bones from the original dataset. D. The left quadrate in medial view, partially resegmented from the original dataset to be more complete. E. The right quadrate in medial view, newly segmented and identified here. F. Left palatine in ventral view, partially resegmented from the original data set, note the number of palatine teeth (nine). G. The right pterygoid in medial view, partially resegmented from the original dataset, seen here with a newly segmented pterygoid flange. H. The newly identified pila antotica, and newly segmented para- and basisphenoids in left (H1) and right (H2) lateral; ventral (H3), and dorsal (H4) views. I. The left coronoid in medial view.

Fig. 7. Cranial bones of lepidosaur rynchocephalian Clevosaurus cambrica Keeble, Whiteside, and Benton, 2018 (NHMUK PV R37014) from Late Triassic, Pant-y-ffynnon, Wales, UK. A. The reassembled left prefrontal. B. The left postfrontal. C. The reassembled left postorbital. D. The left squamosal. E. The left supratemporal. In lateral (A1–D1, E) and medial (A2–D2) views.

Fig. 1. The two surface data-sets of lepidosaur rynchocephalians. A. Clevosaurus hudsoni Swinton, 1939 (NHMUK PV R36832) from Rhaetian, Cromhall Quarry, England, UK, the left side of the digitally segmented skull in lateral view, adapted from O’Brien et al. (2018). B. Clevosaurus cambrica Keeble, Whiteside, and Benton, 2018 (NHMUK PV R37014) from Late Triassic, Pant-y-ffynnon, Wales, UK, available prior to this paper, the digitally segmented skull in ventral view, adapted from Keeble et al. (2018).

Fig. 8. Cranial bones present in lepidosaur rynchocephalian Clevosaurus cambrica Keeble, Whiteside, and Benton, 2018 (NHMUK PV R37014) from Late Triassic, Pant-y-ffynnon, Wales, UK. A. The right palatine in ventral view. B. The left and right vomers in ventral view. C. The left epipterygoid in lateral (C1) and posterior (C2) views. D. The left pterygoid and ectopterygoid in ventral view. E. The left quadrate in medial (E1) and posterior (E2) views.

Fig. 9. The reassembled braincase of lepidosaur rynchocephalian Clevosaurus cambrica Keeble, Whiteside, and Benton, 2018 (NHMUK PV R37014) from Late Triassic, Pant-y-ffynnon, Wales, UK. In dorsal (A1), posterior (A2), ventral (A3), and lateral (A4) views. Abbreviations: l., left; r., right.

Fig. 3. Dentaries of lepidosaur rynchocephalian Clevosaurus hudsoni Swinton, 1939 from Rhaetian, Cromhall Quarry, England, UK, including the two syntypes (A, B) belonging to W.E. Swinton’s collection. A. NHMUK ST R5939b. B. NHMUK ST R5939a. C. NHMUK PV R36832. D. R 37270, note the highly worn hatchling teeth. E. NHMUK PV R 37271, note the presence of hatchling teeth. F. NHMUK PV R 37272. In lateral (A1–F1) and dorsal A2–F2) views.

Fig. 10. The reconstructed left mandible of lepidosaur rynchocephalians. A. Clevosaurus cambrica Keeble, Whiteside, and Benton, 2018 (NHMUK PV R37014) from Late Triassic, Pant-y-ffynnon, Wales, UK. B. Clevosaurus hudsoni Swinton, 1939 (NHMUK PV R36832) from Rhaetian, Cromhall Quarry, England, UK. Note: the anterior tip of the dentary is highlighted in blue to indicate where bone is taken from C. cambrica. In lateral (A1, B1), medial (A2, B2), and dorsal (A4, B4) views, simplified reconstruction in medial view (A3, B3).

Fig. 11. The vertebral column and pectoral girdle elements of lepidosaur rynchocephalian Clevosaurus cambrica Keeble, Whiteside, and Benton, 2018 NHMUK PV R37013 and NHMUK PV R37014) from Late Triassic, Pant-y-ffynnon, Wales, UK. A. The axial-atlas complex in dorsal (A1), lateral (A2), ventral (A3) and anterior (A4) views. B. The third cervical vertebra in right lateral (B1) and anterior (B2) views. C. A dorsal vertebra in right lateral (C1) and anterior (C2) views. D. The five cervical vertebrae present in the scans taken of NHMUK PV R37014, in lateral view, with asterisk indicating the cervical vertebra seen in B; note the posterior-most vertebra terminates prematurely due to the extent of the CT scans. E. The partial vertebral column of NHMUK PV R37013 in lateral view, with asterisk indicating the dorsal vertebra seen in C. F. The left (F1) and right (F2) scapula in lateral view. G. The left (G1) and right (G2) coracoids in lateral view.