''Tetracynodon'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

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

. Fossils of ''Tetracynodon'' have been found in the Karoo Basin of South Africa. Two species are known: the

type species In 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 species that contains the biological type specime ...

''T. tenuis'' from the

Late Permian Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effect, ...

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 Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which is a un ...

. 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''. 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 (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

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 '' 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 ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

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