Cranial anatomy of the baurioid therocephalian Choerosaurus dejageri with comments on secondary palate convergence in therocephalians and cynodonts

Abstract

Permian members of the therocephalian clade Baurioidea remain much more poorly known than the extremely mammal-like Triassic representatives. Here, we describe the cranium of the single known specimen of the lycideopid baurioid Choerosaurus dejageri , based on high-resolution CT-imaging and provide comparisons with closely related taxa. Besides examining external anatomical characters like its characteristic cranial and dentary bosses, we provide important details on the previously obscure palatal morphology. The reconstruction of the holotype shows that this species represents an important transitional stage in the evolution of the therocephalian secondary palate. Complete secondary palates evolved independently three times in therapsids (incl. baurioid therocephalians and cynodonts). In Choerosaurus , the vomer plays the role of an important bracing element in the palate through a long suture with the maxillary shelves, known as the maxillo-vomerine bridge. This differs from the early cynodont condition and highlights different evolutionary origins of therocephalian and cynodont secondary palates. Moreover, similarities in the palatal morphology with Lycideops support the hypothesis of a close relationship between these two taxa.