Extinct genus of therapsids from Early Triassic South Africa
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.
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", "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.
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 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:
References.
 ;templatestyles src="Reflist/styles.css" / ;