''Tetracynodon'' is 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 ...

genus of therocephalian. Fossils of ''Tetracynodon'' have been found in the Karoo Basin of South Africa. Two species are known: the type species ''T. tenuis'' from the Late Permian and the species ''T. darti'' from the Early Triassic. Both species were small-bodied and probably fed on insects and small vertebrates. Although ''Tetracynodon'' is more closely related to mammals than it is to reptiles, its braincase is very primitive and shares more in common with modern amphibians and reptiles than it does with 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'' and ''

Promoschorhynchus ''Promoschorhynchus'' is a genus of akidnognathid therocephalians from the Late Permian and Early Triassic of South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded ...

''. 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 The Lilliput effect is a decrease in body size in animal species which have survived a major extinction. There are several hypotheses as to why these patterns appear in the fossil record, some of which are: the survival of small taxa, dwarfing of la ...

in which small species are more common immediately after a mass extinction.

Phylogeny

''Tetracynodon'' was once classified among a group of therocephalians called

scaloposaurid 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. S. ...

s, 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 ''Lycideops'' is an extinct genus of therocephalians from the Late Permian of South Africa. The type species is ''Lycideops longiceps'', named in 1931 by South African paleontologist Robert Broom. Fossils of ''Lycideops'' come from the ''Dicynod ...

'', 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 analysis of therocephalians in 2012 and was placed in clade Baurioidea along with many other prior scaloposaurids. It was found to nest in a smaller clade with ''

'' and '' Choerosaurus'', designated as the family

Lycideopidae Lycideopidae is an extinct family of therocephalians from the Late Permian and Early Triassic The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between Ma and Ma (million yea ...

. ''Tetracynodon tenuis'' and ''T. darti'' were found to be sister taxa, meaning that they were a valid phylogenetic grouping and ''T. tenuis'' could not be a juvenile of ''Lycideops''. Below is a cladogram from the study:

References

{{Taxonbar, from=Q5428956 Lopingian synapsids of Africa Baurioids 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