Tetracynodon Temporal range: Late Permian - Early Triassic
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Synapsida
Clade:	Therapsida
Clade:	†Therocephalia
Family:	†Lycideopidae
Genus:	†Tetracynodon Broom and Robinson, 1948
Species
†T. tenuis Broom and Robinson, 1948 †T. darti Sigogneau, 1963

Tetracynodon is an extinct 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 to reptiles, its braincase is very primitive and more resembles that of modern amphibians and reptiles than of mammals.^[1]

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, Ictidosuchoides 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.^[1]

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.^[1]

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.^[2]

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 Lycideops and Choerosaurus, designated as the family Lycideopidae. 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:^[1]

v t e Therocephalia
Kingdom: Animalia Phylum: Chordata Clade: Synapsida Clade: Therapsida Clade: Eutheriodontia Synapsida see Synapsida Therocephalia see below↓
Therocephalia Therocephalia Gorynychus Porosteognathus Cynodontia? Lycosuchidae Lycosuchus Simorhinella Scylacosauridae Alopecodon Alopecognathus Biarmosuchoides? Eutheriodon Glanosuchus Hyorhynchus Julognathus Koksharovia Maraisaurus Pardosuchus Pristerognathus Scylacosaurus Eutherocephalia Caodeyao Kotelcephalon? Nanictidops Perplexisaurus Purlovia Scylacosuchus? Cynodontia? Chthonosauridae Chthonosaurus Ichibengops Akidnognathidae Akidnognathus Annatherapsidus Cerdosuchoides Euchambersia Jiufengia Moschorhinus Notaelurodon Olivierosuchus Promoschorhynchus Shiguaignathus Zorillodontops Whaitsioidea Ophidostoma Hofmeyriidae Gorochovetzia Hofmeyria Ictidostoma Mirotenthes Whaitsiidae Dalongkoua Ictidochampsa Megawhaitsia Microwhaitsia Moschowhaitsia Theriognathus Viatkosuchus Baurioidea see below↓
Baurioidea Baurioidea Homodontosaurus Icticephalus Ictidodon Ictidodraco Ictidognathus Macroscelesaurus Nanictocephalus Polycynodon Rhigosaurus Scalopocephalus Scalopodon Scalopolacerta Silphictidoides Silpholestes Yikezhaogia Ictidosuchidae Ictidosuchoides Ictidosuchops Ictidosuchus Malasaurus Muchia? Scalopodontes Scaloporhinus? Scaloposuchus Karenitidae Karenites Mupashi Lycideopidae Choerosaurus Lycideops Tetracynodon Regisauridae Regisaurus Urumchia Bauriamorpha Hazhenia Nothogomphodon Notictoides Ordosiodon Scaloposaurus Ericiolacertidae Ericiolacerta Pedaeosaurus? Silphedosuchus Bauriidae Antecosuchus Bauria Microgomphodon Scalopognathus Traversodontoides
Nomina dubia Alopecorhinus Arnognathus Blattoidealestes Dongusaurus Hyaenasuchus Ictidoparia Ictidosaurus Scymnosaurus Tamboeria Trochorhinus Trochosaurus Trochosuchus
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