Clevosaurus Swinton, 1939

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 : 44-46

Genus Clevosaurus Swinton, 1939

Type species: Clevosaurus hudsoni Swinton, 1939 ; see below.

Clevosaurus hudsoni Swinton, 1939 Material. —Referred specimen, NHMUK PV R36832, is a near complete skull, with much of the left side of the skull still articulated.

Description.—The scans of Clevosaurus hudsoni (NHMUK PV R36832) showed a few bones that had been incompletely segmented or overlooked, including the right postorbital, both quadrates, the right pterygoid, the braincase, and the coronoid process.

Postorbital: The right postorbital ( Fig. 2A ) had been segmented but not identified. Its morphology was a near perfect mirror of the left postorbital (which is described in O’Brien et al. 2018), but it has a damaged postero-ventral edge and a better-preserved ventral process. The ventral process bends anteriorly at a roughly 90° angle and extends medially to form a shelf that would have partially wrapped around the jugal when articulated, and would have formed the postero-ventral rim of the orbit.

Squamosal Both squamosals have been identified Fig. 2B, C ), with the left squamosal being better preserved than the right. The left squamosal had been correctly identified by O’Brien et al. (2018), though it was later labelled as a left pterygoid flange; notably, the left squamosal had undergone more rotation than any of the other bones in the skull, nearly ~180°, which probably led to this misidentification. Its identity as a squamosal was confirmed based on its differences from the complete right pterygoid, including being much thicker than the delicate pterygoid flange, its matching mirrored morphology with a similar bone that we identify as the right squamosal, and its defined postorbital facet, quadrate process and medially recurved posterior process. The anterior process is damaged for both sides, but part of the postorbital facet on the left squamosal is well defined, the ventral process is long and would have extended three-quarters of the height of the quadrate to which it articulated. The posterior process curves posteromedially towards the posterior process of the pterygoid and bears a rough uneven surface that would have likely formed an articulation site for the supratemporal.

Quadrate: The left quadrate of NHMUK PV R36832 Fig. 2D ) was found to be more complete than previously described, and its morphology appears to be typical of C. hudsoni . The right quadrate ( Fig. 2E ) was also located out of place, thrust upwards and out of position by the forelimb bones that were found within the skull ( O’Brien et al. 2018); the posterior region of the “lateral plate” appears to have been damaged. The condyles for both quadrates are preserved and they are separated by a deeply concave cotyle. Dorsally, the quadrate extends into two connected structures either side of the condyles, the “lateral plate” which anteriorly bears the quadrato-jugal foramen, and the medially placed “vertical strut”, which is a tall columnar structure with expanded ends. Both converge anteriorly, and extend to form the quadrato-pterygoid process, which is a thin flange, reinforced by a robust ventral ridge.

Palatine: On re-examination of NHMUK PV R36832, we find the left palatine possesses nine teeth ( Fig. 2F ), with a thin surface of bone running beneath them all, possibly representing the raised lateral ridge typically found on the palatine of C. hudsoni , but at the resolution in this part of the CT scans, it is hard to discern bone from matrix. In O’Brien et al. (2018), these teeth are segmented but some were suggested to be pterygoidal teeth, though the left pterygoid is absent. We confirm that the teeth are from the palatine. Nine palatine teeth is more than the five to six teeth noted for C. hudsoni by Fraser (1988: 137), and suggests there may be a greater range in morphology for this species than previously described.

Pterygoid: The right pterygoid, already partially segmented, was re-examined, and we found the thin quadrato-pterygoidalflange( Fig.2G )andlateralectopterygoidal-pterygoidal process still articulated with the right ectopterygoid. The complete pterygoid matches the description given for this species by Fraser (1988: 138, fig. 16). The anterior region of the pterygoid bearing two rows of teeth is described in O’Brien et al. 2018. The two tooth rows of the left pterygoid, and its lateral process were also located, still articulated with the left ectopterygoid, but the resolution in this region of the scans was too low to discern anything more.

Parabasisphenoid: The basipterygoid processes in Clevosaurus hudsoni (NHMUK PV R36832) had previously been segmented but had been left unlabelled, whereas the parasphenoid, basisphenoid and left pila antotica process had not been segmented. The basisphenoid and parasphenoid are fused with no sutures discerned between them ( Fig. 2H ). Anteriorly, the basisphenoid forms the paired basipterygoid processes, both of which were preserved intact, having been previously segmented but left unidentified by O’Brien et al. (2018). Located between the two basipterygoid processes, the cultriform process is preserved ( Fig. 2H ) and is elongate compared to that of previous reconstructions of C. hudsoni ( Fraser 1988: fig. 17B). Also, its base is nearly level with the basipterygoid processes rather than positioned anteriorly, as in C. brasiliensis ( Bonaparte and Sues 2006) .

Prootic: The left pila antotica process was identified from amongst previously partially segmented bone. It is a tall, flattened slender process that has an antero-posteriorly expanded dorsal head ( Fig. 2H ).

Coronoid: The left coronoid bone is preserved ( Fig. 2I ) and is reported here for the first time in Clevosaurus hudsoni . The coronoid bone had become vertically and medially displaced from the coronoid process and was segmented as part of the dentary in O’Brien et al. 2018, artificially raising its height in the original model. It is a small latero-medially compressed bone that forms the medial portion of the coronoid eminence.

Remarks.—Examination of the Clevosaurus hudsoni dentaries at the NHMUK revealed two possible morphotypes. The additional dentary teeth of NHMUK PV R36832 and the syntypes from W.E. Swinton’s original description (NHMUK PV R5939a and R5939b) have a simple structure, where the tooth shape is triangular in labial profile with the “extensive antero-lateral flange” described by Fraser (1988:144) reduced or absent in the syntypes. There is also little or no overlap between teeth and with the reduced antero-lingual escape structures, together forming a straighter cutting edge ( Fig. 3A–C ), which could be related to ontogenetic change that is beyond the scope of this study. However, we note that the C. hudsoni specimen NHMUK PV R36832, which, although smaller, shares a similar tooth morphology to the syntypes and is reported to be an adult specimen ( O’Brien et al. 2018: 191; it also lacks any remnant hatchling dentition). Furthermore, although a similar change occurs in some of the most heavily worn of the dentaries of C. convallis , with a reduction in anterolateral flange length and tooth overlap, it only affects the anteriormost of the teeth, and not the entire row ( Säilä 2005). Several of the dentaries in the P.L. Robinson collection NHMUK PV R37270, R37271, and R37272) show a greater resemblance to the reconstructions made by Fraser (1988: 142), with extensive antero-lateral flanges, noticeable overlap between teeth and large antero-lingual escape structures, so that the cutting edge of the teeth was en echelon ( Figs. 3D–F ).

The syntype palatine in W.E. Swinton’s collection ( NHMUK PV R5939 c) bears four (possibly a fifth broken) lateral teeth that scale down in size posteriorly and bear posterolateral wear facets, and the maxilla ( NHMUK PV R9249 ) bearing four additional teeth and a morphology closely resembling that of NHMUK PV R36832 (see SOM 4 and 5). Together, this information confirms that our specimen is conspecific with the syntype specimens of C. hudsoni .

Stratigraphic and geographic range. —Upper Triassic (Rhaetian) fissure infill within Lower Carboniferous Limestone, Cromhall Quarry, near Cromhall Village, Gloucestershire, South-West England, UK (NGR ST 704916).

Figures

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. 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.