Clevosaurus, SWINTON, 1939

O, Aileen, Brien, Whiteside, David I. & Marshall, John E. A., 2018, Anatomical study of two previously undescribed specimens of Clevosaurus hudsoni (Lepidosauria: Rhynchocephalia) from Cromhall Quarry, UK, aided by computed tomography, yields additional information on the skeleton and hitherto undescribed bones, Zoological Journal of the Linnean Society 183, pp. 163-195 : 168-170

GENUS: CLEVOSAURUS SWINTON, 1939

Type species: Clevosaurus hudsoni Swinton, 1939

Included species: Clevosaurus minor Fraser, 1988 ; C. latidens* Fraser, 1993 ; C. bairdi Sues, Shubin & Olsen, 1994 ; C. wangi Wu, 1994 , C. mcgilli Wu, 1994 and C. petilus Wu, 1994 ; C. convallis Säilä, 2005 ; C. brasiliensis Bonaparte & Sues, 2006 and C. sectumsemper Klein et al., 2015 .

Remarks: The most recent cladistic analysis by Hsiou et al. (2015) resolved a Clevosauridae clade with the following apomorphic features: antorbital region forming one-quarter of the skull length (reversed to between one-third to one-quarter in C. brasiliensis , C. wangi and C. petilus ); a narrow and elongated dorsal process of the jugal; palatine teeth forming a single row, plus one isolated tooth. Hsiou et al. (2015) define the clade as ‘all taxa more closely related to Clevosaurus than to Sphenodon ’ ( Hsiou et al., 2015: 4). * Clevosaurus latidens is positioned outside of Clevosauridae in this analysis.

Characters that occur in clevosaurs, but are not restricted to the genus include: a lateral forked flange of the premaxilla preventing contact between the maxilla and the external naris [horizontal posterior flange not present in C. convallis ( Säilä, 2005) ]; a dorsally expanded lateral process of the premaxilla; suborbital fenestra bounded solely by the ectopterygoid and palatine; a high, steeply inclined coronoid process of the dentary; flanged teeth; a broad maxillary-jugal contact ( Säilä, 2005; Bonaparte & Sues, 2006; Jones, 2006; Hsiou et al., 2015; Klein et al., 2015).

Diagnosis: Based on Swinton (1939), Robinson (1973) and Fraser (1988), specimen NHMUK PV R36832 can be diagnosed as C. hudsoni . The diagnosis is based on the following principal features, identified in the specimen:

• Acrodont dentition. The maxilla bears four large additional teeth that increase in size posteriorly; the teeth are conical in form and have posterolingual flanges. Three smaller conical teeth occur posteriorly to that additional set on the maxilla. The dentary has four large, conical, additional teeth which increase in size posteriorly; these additional teeth have anterolateral flanges. • Teeth are evident on the pterygoid and palatine. Two rows of teeth are present on the pterygoid. A single lateral row of large teeth is present on the palatine but because of incomplete preservation of the medial region of the palatine, the small isolated tooth medially offset from

170 A. O’BRIEN ET AL.

the lateral row, typical of clevosaurs, is not recorded.

• Incomplete lower temporal bar. There is a gap between the posterior process of the jugal and the quadrate on this specimen. Fraser (1988) notes that in some specimens of C. hudsoni , weak contact is made between the jugal and quadratojugal. There is no contact on this specimen.

• A flattened, plate-like quadrate present.

• Large pineal foramen.

• Postorbital triangular in shape.

• Supratemporal present.

Specimen NMHUK PV R36846 is designated the hind limb of C. hudsoni based on its great similarity to C. hudsoni , figured and described by Fraser (1988: fig. 35). Identifiable features on this specimen include:

• Size of tibia falls within size range of elements measured by Fraser (1988).

• Hooked fifth metatarsal.

• Fifth tarsal fused to fifth metatarsal to form a single tarsometatarsal.

A single astragalocalcaneum, formed by fusion of ankle bones, is considered diagnostic of clevosaurs but on this specimen the elements appear to be separate (see description of NMHUK PV R36846 below).