Intracranial Anatomy of Oxyodontherium zeballosi (Litopterna, Macraucheniidae) from the Río Quinto Formation, San Luis Province, Argentina

Utilizing segmental data of MHIN-UNSL-GEO-V465, weextend and reevaluate previous observations on the morphology of the macraucheniid Oxyodontheriumzeballosi comparing with additional micro and medicalCT evidence from published data on Litopterna and other South American NativeUngulates (SANU). This new anatomical information includes qualitative and quantitativedata on cranial pneumatization, nasal and encephalic cavities, and body massestimates. The calculated Encephalization Quotients forMHIN-UNSL-GEO-V 465 range between 0.32 and 0.43, which are relatively low comparedto other macraucheniids. The neopallial height ratio indicates a less expandedneopallial surface than in Huayqueriana andother Miocene SANU, and a higher ratio than several early ungulates. Our datareveal that O. zeballosi shares characteristics with the protolipternid cf. Miguelsoria parayirunhor,such as a shallow rostral tympanic process and a bulkier pars canalicularis ofthe inner ear. Although O. zeballosi shows less pneumatic spaces than othermacraucheniids, it does show an important degree of pneumatization in theanterior portion of the caudal cranium, especially affecting frontals,parietals, and the alisphenoid. Finally, the estimated body mass of O.zeballosi is around 122-180 kg. This studyincreases the knowledge of the internal cranial anatomy of litopterns providingpotentially valuable characters for future phylogenetic analyses.