Abstract

Emydopoidea is one of the major subclades of dicynodont and includes a number of taxa showing specializations for a fossorial lifestyle. Various cranial and postcranial adaptations for fossoriality have long been recognized in cistecephalid emydopoids, with recent studies adding information from their basicranial anatomy. Here, using laboratory and synchrotron X-ray micro-computed tomography, we provide detailed anatomical descriptions of the basicranial axis of three emydopoids (Myosaurus, Kawingasaurus, and a Malawian cistecephalid PK-16-1) and compare them to the basal dicynodont Pristerodon. Notable features include the presence of divergent crests on the posterior aspect of the opisthotic and a nuchal crest on the occipital plate of various emydopoids, particularly cistecephalids, which contrasts with the relatively featureless occipital plate of other dicynodonts. These depressions and crests increase the attachment area of muscles associated with the atlanto-occipital joint, suggesting that these taxa were capable of powerful movements of the head during digging. Kawingasaurus has a highly derived basicranium, with a vast network of trabecular spaces and highly co-ossified basicranium, which is probably linked to auditory system function. We propose that cistecephalids, in addition to being forelimb-diggers, were likely head-lift diggers, and highlight some derived adaptations consistent with a quasi-obligate fossorial lifestyle. We also propose new basicranial phylogenetic characters and reevaluate the intra- and interrelationships among emydopoids accordingly. Our analysis recovers Rastodon as a basal emydopoid, Thliptosaurus as a non-kingoriid emydopoid, and novel interrelationships among cistecephalids.