Clevosaurus brasiliensis (Bonaparte & Sues, 2006)

Chambi-Trowell, Sofia A. V., Martinell, Agust ́ ın G., Whitesid, David I., Romo de Viva, Paulo R., Soare, Marina Bento, Schult, Cesar L., Gil, Pamela G., Bento, Michael J. & Rayfiel, Emily J., 2021, The diversity of Triassic South American sphenodontians: a new basal form, clevosaurs, and a revision of rhynchocephalian phylogeny, Journal of Systematic Palaeontology 19 (11), pp. 787-820 : 812-813

Clevosaurus brasiliensis

Our anatomical observations and phylogenetic analysis ( Fig. 16 ) confirm that Clevosaurus brasiliensis is an early diverging clevosaur, sharing several characteristics with Brachyrhinodon taylori , and together with C. bairdii forms a small subclade at the base of Clevosauridae . Unlike later clevosaurs, both C. brasiliensis and B. taylori bear three rows of teeth on their pterygoids ( SAVC-T, pers. obs.), the lateral-most of which is reduced to just two to three teeth. In addition, both taxa possess robust jaws, short snouts, broad skulls, a remarkably elongated retroarticular process, two to three large additional teeth on the maxilla, and a high coronoid process. Unfortunately, the nature of the implantation, tooth form, and number – features that mark C. brasiliensis as distinct from other Clevosaurus – cannot be verified in B. taylori as these characteristics are not preserved.

The teeth of C. brasiliensis differ from those of later clevosaurs in morphology and implantation, appearing to be acrodont but nested deep within the jaw bones so that only a small portion of the tooth is visible. Rather than forming a jagged, narrow, semi-continuous blade, posteriorly the teeth of C. brasiliensis are closely packed and fused together, and both the cusps and dentary are worn at an angle as the animal ages. This means that the anterior hatchling teeth form a continuous narrow cropping beak-like structure. Most Clevosaurus species show additional dentary teeth with a mesiodistally elongated saddle shape and the cusp asymmetrically placed posteriorly on the tooth displaying large anterolateral flanges ( Chambi-Trowell et al. 2020). However, the additional teeth of C. brasiliensis are mostly conical, with only the ultimate tooth – which can be up to two to three times bigger than any of the other teeth – elongated mesiodistally, and with its cusp placed anteriorly rather than posteriorly (two features unique to C. brasiliensis amongst clevosaurs, but also shared with Planocephalosaurus ). In contrast to other clevosaurs, the teeth on the dentary of C. brasiliensis do not appear to bear flanges at all. But despite these differences, C. brasiliensis is otherwise very similar to later clevosaurs, with a short robust snout, broad skull, supratemporal bone, premaxillary beak and an elongated retroarticular process.

Some of the skull traits of C. brasiliensis suggest that it must have been capable of a strong bite, such as its robust and short snout that seems to be further reinforced by its brace-like premaxillae and the slot-like facet on the prefrontal so that the maxilla is supported both medially and laterally. A similar slot-like facet is observed between the postorbital and jugal, with the bone interdigitating along their meeting edge rather than the simpler overlapping facets seen in later clevosaurs.

Figures

Figure 16. Phylogeny of Rhynchocephalia, recovered from: A, strict consensus tree from the maximum parsimony analysis with Bremer values above 1 labelled beside the nodes; and B, 50% majority rule consensus tree from the Bayesian-inference analysis with clade credibility values (decimal proportions) labelled beside nodes.