Clevosaurs

Clevosaurs Temporal range:
Artist's illustration ofClevosaurus hadroprodon
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Rhynchocephalia
Suborder:	Sphenodontia
Family:	†Clevosauridae Bonaparte & Sues,2006
Genera
†Brachyrhinodon?Huene, 1910 †ClevosaurusSwinton, 1939 †Polysphenodon?Jaeckel, 1911

Clevosaursare anextinctgroup ofrhynchocephalianreptiles from theTriassicandJurassicperiods.

History and definition

The skull ofClevosaurus brasiliensis

Although members of this group have been known since 1910, only recently has the group received a formal name. In the late 1990s, Victor-Hugo Reynoso established that three particular genera of Sphenodontia (Clevosaurus,Brachyrhinodon,andPolysphenodon) were closely related to each other. He gave the informal name "clevosaurs" to these three genera, after the most numerous and well-known genus,Clevosaurus.He considered clevosaurs to be members of the family Sphenodontidae, the family of rhynchocephalians containing the only living member of the order, thetuatara(Sphenodon).

In 2006, Bonaparte and Sues finally gave "clevosaurs" a formal name and taxonomic rank as thefamilyClevosauridae.They defined Clevosauridae as the last common ancestor ofClevosaurus, Brachyrhinodon,andPolysphenodon,and all of its descendants.In 2015, the definition of this family was revised to be "all taxa more closely related toClevosaurusthan toSphenodon".However, the erection of this family conflicts with their position within Sphenodontidae, as a taxonomic family cannot be within another family. Sources which use Sphenodontidae as a wide group of sphenodontians do not use the term Clevosauridae, instead continuing to use the informal term 'clevosaurs'. On the other hand, sources which use Clevosauridae do not use Sphenodontidae. Some do not use either family, instead opting for 'clevosaurs' and 'advanced sphenodontians'.

Description

Clevosaurs were among the first major groups of sphenodontians to evolve, and had a worldwide distribution in the Late Triassic and early Jurassic.Clevosauruswas particularly widespread and diverse, surviving theTriassic-Jurassic extinctionand being known from numerousspecies.

Adult clevosaurs are notable among sphenodontians for their short, boxy snouts. The antorbital region of the skull (the portion in front of the eyes) only occupies a quarter of the length of the entire skull in most clevosaurs, although a few species ofClevosaurusreacquire a slightly longer skull. Like other rhynchocephalians, they possessed two pairs of large holes called temporal fenestrae in the back part of the skull. The lower temporal fenestrae (on the sides of the skull) are very large in most clevosaurs, about a quarter the length of the skull.All clevosaurs have very longjugalbones which extend back as far as thesquamosalbones in the back of the head, forming the entire upper edge of their lower temporal fenestrae in the process.

Uromastyx,a present-day equivalent of clevosaurs.

Like other sphenodontians, clevosaurs had several rows of teeth on the roof of the mouth. Their teeth wereacrodont,meaning that they grew directly from the bone rather than from tooth sockets. The large outermost row of teeth were attached to themaxillaebones while the small innermost teeth were clustered in rows on thepterygoidbones. Between the maxillary and pterygoid teeth, clevosaurs characteristically had one row of large teeth on eachpalatinebone, as well as an additional isolated tooth at the inner front corner of each palatine.

Clevosaurs also had a row of teeth on the edge of their dentaries (lower jaws). In young individuals, these teeth were spike-like, well-adapted for consuming insects and other invertebrates. However, as individual clevosaurs grew older, their jaws became shorter and more robust. In addition, both their maxillary and dentary teeth wear down into a sharp cutting edge, creating a "beak" -like jaw structure somewhat similar to the jaws of modernUromastyxlizards.It is likely that adult clevosaurs may have been omnivorous or herbivorous, similar toUromastyxin ecology.

Classification

Below is acladogramof the relationships within Clevosauridae based on thephylogenetic analysisofHsiou et al.(2015):

Clevosauridae	Polysphenodon mulleri Brachyrhinodon taylori Clevosaurus

"Clevosaurus" latidenswas recovered outside of Clevosauridae, as thesister taxonofOpisthodontia.