X-ray microcomputed and synchrotron tomographic analysis of the basicranial axis of emydopoid dicynodonts: implications for fossoriality and phylogeny

Emydopoidea is one of the major dicynodont subclades and includes some purported fossorial taxa. Various cranial and postcranial adaptations for fossoriality have long been recognized in cistecephalid emydopoids, but anatomical variation of their braincases remains poorly understood. Here, using laboratory and synchrotron X-ray tomography, we provide detailed anatomical descriptions of the basicranial axis of three emydopoids (Myosaurus, Kawingasaurus and a Malawian cistecephalid DMMM-PK-16-1) and compare them to the basal dicynodont Pristerodon. Cistecephalids show the presence of divergent crests on the posterior aspect of the opisthotic and a nuchal crest on their occipital plate, contrasting with the featureless occipital plate of other dicynodonts. These depressions and crests increase the attachment area of the atlanto-occipital muscles, suggesting that cistecephalids were capable of powerful movements of the head during digging. Additionally, Kawingasaurus has a pneumatized braincase and highly co-ossified basicranium, which is probably linked to the auditory system. We corroborate the hypothesis that cistecephalids, in addition to being forelimb diggers, were likely head-lift diggers, and we highlight some derived adaptations consistent with a quasi-obligate fossorial lifestyle. Furthermore, new basicranial phylogenetic characters and a re-evaluation of emydopoid relationships are proposed. We recovered Rastodon as a basal emydopoid, Thliptosaurus as a non-kingoriid emydopoid and novel interrelationships among cistecephalids.