''Tetracynodon'' is 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 ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

therocephalian Therocephalia is an extinct clade of therapsids (mammals and their close extinct relatives) from the Permian and Triassic periods. The therocephalians ("beast-heads") are named after their large skulls, which, along with the structure of their te ...

. Fossils of ''Tetracynodon'' have been found in the

Karoo Basin The Karoo Supergroup is the most widespread stratigraphic unit in Africa south of the Kalahari Desert. The supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Late Carboniferous and Early Jurassic, a per ...

of South Africa. Two species are known: the

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

''T. tenuis'' from the

Late Permian Late or LATE may refer to: Everyday usage * Tardy, or late, not being on time * Late (or the late) may refer to a person who is dead Music * ''Late'' (The 77s album), 2000 * Late (Alvin Batiste album), 1993 * Late!, a pseudonym used by Dave Groh ...

and the species ''T. darti'' from the

Early Triassic The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between 251.9 Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which ...

. Both species were small-bodied and probably fed on insects and small vertebrates. Although ''Tetracynodon'' is more closely related to mammals than to reptiles, its braincase is very primitive and more resembles that of modern amphibians and reptiles than of mammals.

Permo-Triassic survivorship

''Tetracynodon'' was one of the few therapsid genera known to have survived the Permo-Triassic extinction event. Aside from ''Tetracynodon'', the only therocephalian genera known from both sides of the Permo-Triassic boundary are ''

Moschorhinus ''Moschorhinus'' is an extinct genus of therocephalian synapsid in the family Akidnognathidae with only one species: ''M. kitchingi'', which has been found in the Late Permian to Early Triassic of the South African Karoo Supergroup. It was a larg ...

'', ''

Ictidosuchoides ''Ictidosuchoides'' is an extinct genus of ictidosuchid therocephalians. Fossils have been found from the Karoo Basin in South Africa. The genus is known to have been one of the few therocephalians to have survived the Permian-Triassic extinct ...

'' and '' Promoschorhynchus''. The Triassic species ''Tetracynodon darti'' would have been part of the extinction's survivor fauna, a low-diversity community of therapsids and other land vertebrates. The Triassic species ''Tetracynodon darti'' is relatively small compared to Permian baurioids. This may be a possible example of the Lilliput effect in which small species are more common immediately after a mass extinction.

Phylogeny

''Tetracynodon'' was once classified among a group of therocephalians called scaloposaurids, all characterized by their very small size. Scaloposauridae is no longer recognized as a valid group because many scaloposaurid features are now thought to be characteristics of juvenile individuals rather than evidence of close evolutionary relationships. In 2008, ''Tetracynodon tenuis'' was proposed to be a juvenile of the larger therocephalian '' Lycideops'', which is known from the same Late Permian strata. Since ''T. tenuis'' is the type species of ''Tetracynodon'' and the type species of ''Lycideops'', ''L. longiceps'', was named earlier than ''T. tenuis'', the species would be a synonym of ''Lycideops longiceps'' and the genus ''Tetracynodon'' would be invalid. Moreover, since the study found that ''Lycideops'' was not directly related to the Early Triassic ''Tetracynodon darti'', it suggested that this lineage of therocephalians did not survive uninterrupted past the Permo-Triassic extinction event. ''Tetracynodon'' was included in a

phylogenetic 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 dat ...

analysis of therocephalians in 2012 and was placed in clade

Baurioidea Baurioidea is a superfamily of therocephalian therapsids. It includes advanced therocephalians such as ''Regisaurus'' and ''Bauria''. The superfamily was named by South African paleontologist Robert Broom in 1911. Bauriamorpha, named by D. M. ...

along with many other prior scaloposaurids. It was found to nest in a smaller clade with '' Lycideops'' and '' Choerosaurus'', designated as the family Lycideopidae. ''Tetracynodon tenuis'' and ''T. darti'' were found to be

sister taxa 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 ...

, meaning that they were a valid phylogenetic grouping and ''T. tenuis'' could not be a juvenile of ''Lycideops''. 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 ...

from the study:

References

{{Taxonbar, from=Q5428956 Lopingian synapsids of Africa Baurioidea Early Triassic synapsids of Africa Fossil taxa described in 1948 Taxa named by Robert Broom Lopingian genus first appearances Changhsingian genera Induan genera Early Triassic genus extinctions