Appendicular morphology of Mesenosaurus efremovi and the earliest occurrence of a calcaneal tuberosity

Abstract

The Richards Spur locality in Oklahoma, U.S.A. is renowned for its diverse assemblage of early Permian terrestrial tetrapods, including the varanopid Mesenosaurus efremovi. Recent quarrying operations yielded numerous skeletal remains attributed to this taxon, including partially articulated cranial and postcranial remains with the latter being especially important given the comparatively minimal description in previous studies. The specimens used in this study, including a superbly preserved hindfoot, all represent approximately the same growth stage, and collectively reveal the morphology of the entire pelvis and hindlimb in unprecedented detail. CT data, analysed using the segmentation software Avizo, was utilised to examine the hindfoot specimen. As in other varanopids, the limb proportions of this predator are gracile, the bones being more slender than in the similarly well-known taxon Varanops. In addition, M. efremovi exhibits unique ankle morphologies including a highly robust astragalus with a massive transverse ridge and a medially oriented tibial articular surface, a pronounced lateral calcaneal tuberosity and an enlarged fourth tarsal. These likely diagnostic features of Mesenosaurus point to specialised lower limb design for rapid locomotion, and further confirms its close relationship to the middle Permian M. romeri from Russia and consequent temporal longevity of this varanopid genus.