Exceptionally preserved Microgomphodon oligocynus specimens elucidate ecomorphological adaptations in derived therocephalians (Synapsida: Therapsida)

Baurioid therocephalians survived the Permo-Triassic mass extinction event and gave rise to globally widespread lineages that increasingly evolved derived ‘mammal-like’ characteristics. The last surviving members of the clade belong to the Early-to-Middle Triassic family Bauriidae, which are represented in the Karoo Basin of South Africa by two taxa: Bauria cynops and Microgomphodon oligocynus. Here, we provide novel ecomorphological data on one of the most morphologically specialized therocephalians from the Cynognathus Assemblage Zone of South Africa, M. oligocynus. We examine three specimens (NMQR 3189, SAM-PK-K11601, CGP/1/44) using a combination of X-ray microcomputed tomography (µCT), synchrotron X-ray CT scanning, and classical osteohistology producing a complete postcranial and an updated osteohistological description of Microgomphodon. SAM-PK-K11601 represents the largest and most complete skeleton of Microgomphodon to date and represents a subadult individual. CGP/1/44 represents one of the smallest skeletons and is also the first therocephalian skeleton preserved in situ within its burrow. Among others, the most salient features that characterize the postcranial skeleton of Microgomphodon are four sacral vertebrae, an enigmatic scapular flange, and a scapulocoracoid foramen positioned between the scapula and procoracoid. Other postcranial and microanatomical features of the forelimb—alongside the presence of CGP/1/44 in a terminal burrow cast—offer compelling evidence for fossoriality in the genus. Overall, this study details nearly every facet of the anatomy of Microgomphodon, producing one of the most in depth paleobiological investigations of a bauriid therocephalian to date.