Rethinking therapsid phylogeny through Bayesian and cladistic approaches

Therapsids are early-diverging synapsids that thrived during the Permian and Triassic periods, and ultimately gave rise to mammals. They include six major groups, which already exhibited considerable diversity at their first appearance in the fossil record. Their early to middle Permian origins remain poorly understood due to a scarcity of fossils with clear therapsid morphology in the early Permian and the presence of an interval of poor fossil preservation near the early to middle Permian boundary (Olson’s Gap). This limits our insight into the phylogenetic relationships among the main middle Permian groups. This study presents a novel approach to therapsid phylogeny, using probabilistic frameworks. In performing a phylogenetic analysis based exclusively on cranial characters, using both RevBayes and MrBayes, this study challenges understanding of the early diversification of Therapsida. We recover Neotherapsida, with Anomodontia branching as the sister taxon to Theriodontia. Biarmosuchia and Dinocephalia form a clade. The positions of taxa such as Sinophoneus and Biseridens are reconsidered. The application of Fossilized Birth-Death models suggests that while Therapsida originated in the early Permian, currently known therapsid groups diversified around 281 to 272 million years ago and subsequently underwent rapid radiation into five clades: Biarmosuchia, Dinocephalia, Anomodontia, Gorgonopsia, and Eutheriodontia.