Tetracynodon

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Tetracynodon is an extinct genus of therocephalian therapsids from the Late Permian to Early Triassic. Fossils have been found in the Karoo Basin of South Africa. There are two known species: T. tenuis (Late Permian) and T. darti (Early Triassic). Both were small, likely feeding on insects and small vertebrates. Although they are related to mammals, their braincase is very primitive and resembles those of amphibians and reptiles more than mammals.

Tetracynodon was one of the few therapsid groups to survive the Permian–Triassic extinction event. Along with Moschorhinus, Ictidosuchoides, and Promoschorhynchus, it is one of the therocephalian genera known from both sides of the boundary. The Triassic species T. darti would have lived in a sparse survivor community after the extinction.

The Triassic T. darti is smaller than many Permian relatives, which may illustrate the Lilliput effect—where surviving species tend to be smaller after a mass extinction. The manus (hand) of Tetracynodon shows adaptations for scratch-digging, suggesting burrowing was a survival strategy during the harsh post-extinction environment.

Taxonomically, Tetracynodon was once placed in Scaloposauridae (a group now regarded as invalid because its tiny size likely reflects juvenile stages rather than close relationships). In 2008, it was proposed that T. tenuis might be a juvenile of Lycideops, which would make Tetracynodon invalid. However, later analyses concluded that T. tenuis and T. darti are sister species, and Lycideops is not directly their ancestor, indicating this therocephalian lineage did not survive uninterrupted across the Permo–Triassic boundary.

In a 2012 phylogenetic study, Tetracynodon was placed in the clade Baurioidea, within the family Lycideopidae, alongside related genera. T. tenuis and T. darti were found to be sister taxa, supporting their status as distinct species.