Clevosaurs are an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

group of

rhynchocephalia Rhynchocephalia (; ) is an order of lizard-like reptiles that includes only one living species, the tuatara (''Sphenodon punctatus'') of New Zealand. Despite its current lack of diversity, during the Mesozoic rhynchocephalians were a diverse g ...

n reptiles from the

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

and

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

periods.

History and definition

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 ''Brachyrhinodon'' (meaning "short nose tooth") is an extinct genus of sphenodontian from the Late Triassic Lossiemouth Sandstone of Scotland. It is related to the tuatara, the only member of its order that is not extinct. ''Brachyrhinodon'' i ...

'', and ''

Polysphenodon ''Polysphenodon'' is an extinct genus of clevosaurine reptile from the Late Triassic Keuper Group of Germany,N. C. Fraser and M. J. Benton. (1989) The Triassic reptiles ''Brachyrhinodon'' and ''Polysphenodon'' and the relationships of the sphe ...

'') 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, the

tuatara Tuatara (''Sphenodon punctatus'') are reptiles endemic to New Zealand. Despite their close resemblance to lizards, they are part of a distinct lineage, the order Rhynchocephalia. The name ''tuatara'' is derived from the Māori language and m ...

(''Sphenodon).'' In 2006, Bonaparte and Sues finally gave "clevosaurs" a formal name and taxonomic rank as the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

Clevosauridae. They defined Clevosauridae as the last common ancestor of ''Clevosaurus, Brachyrhinodon,'' and ''Polysphenodon,'' and all of its descendants. In 2015, the definition of this family was revised to be "all taxa more closely related to ''Clevosaurus'' than to ''Sphenodon''". 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. ''Clevosaurus'' was particularly widespread and diverse, surviving the Triassic-Jurassic extinction and being known from numerous

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

. 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 of ''Clevosaurus'' reacquire 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 long jugal bones which extend back as far as the squamosal bones in the back of the head, forming the entire upper edge of their lower temporal fenestrae in the process. Like other sphenodontians, clevosaurs had several rows of teeth on the roof of the mouth. Their teeth were acrodont, meaning that they grew directly from the bone rather than from tooth sockets. The large outermost row of teeth were attached to the

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The t ...

e bones while the small innermost teeth were clustered in rows on the pterygoid bones. Between the maxillary and pterygoid teeth, clevosaurs characteristically had one row of large teeth on each palatine bone, 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 modern '' Uromastyx''

lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia alt ...

s. It is likely that adult clevosaurs may have been omnivorous or herbivorous, similar to ''Uromastyx'' in ecology.

Classification

Below is a cladogram of the relationships within Clevosauridae based on the phylogenetic analysis of ''Hsiou et al.'' (2015): ''"Clevosaurus" latidens'' was recovered outside of Clevosauridae, as the

sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and t ...

of Opisthodontia.

References

{{Taxonbar, from=Q9193248 Sphenodontia