Clevosaurs are an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce 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 speciose g ...

n reptiles from the

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

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 143.1 Mya. ...

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 ''Clevosaurus'' (meaning "Gloucester lizard") is an extinct genus of rhynchocephalian reptile from the Late Triassic and the Early Jurassic periods. Species of ''Clevosaurus'' were widespread across Pangaea, and have been found on all continents ...

Brachyrhinodon ''Brachyrhinodon'' (meaning "short nose tooth") is an extinct genus of sphenodontian from the Late Triassic Lossiemouth Sandstone of Scotland. History of discovery ''Brachyrhinodon'' and its single species ''B. taylori'' originally described ...

'', and ''

Polysphenodon ''Polysphenodon'' is an extinct genus of sphenodontian reptile with a single species ''Polysphenodon mulleri'' from the Late Triassic Keuper Group of Germany.N. C. Fraser and M. J. Benton. (1989) The Triassic reptiles '' Brachyrhinodon'' and ' ...

'') 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 The tuatara (''Sphenodon punctatus'') is a species of reptile endemic to New Zealand. Despite its close resemblance to lizards, it is actually the only extant member of a distinct lineage, the previously highly diverse order Rhynchocephal ...

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

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

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 A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

. 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 The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestra ...

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 In vertebrates, the maxilla (: maxillae ) 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 two maxil ...

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 A palatine or palatinus (Latin; : ''palatini''; cf. derivative spellings below) is a high-level official attached to imperial or royal courts in Europe since Roman Empire, Roman times.

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 Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...

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

Classification

Below is a

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

of the relationships within Clevosauridae based on the

phylogenetic analysis In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...

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 ...

of Opisthodontia.

References

{{Taxonbar, from=Q9193248 Rhynchocephalia