Digital cranial endocasts of the extinct sloth Glossotherium robustum (Xenarthra, Mylodontidae) from the Late Pleistocene of Argentina. Description and comparison with the extant sloths

The internal cranial morphology of the terrestrial sloth Glossotherium robustum is described here based on a neurocranium from the late Pleistocene of the Pampean region of Buenos Aires, northeastern Argentina. The first published data on the morphology of the brain cavity of this species date back to the latest nineteenth century. The novel techniques of CT scanning and digital reconstructions enable non-destructive access to the internal cranial features of both extinct and extant vertebrates, and thus improve our knowledge of anatomical features that had previously remained obscure. Therefore, we performed CT scans on the posterior half of a skull of G. robustum and created digital models of the endocasts and internal structures. The results reveal the morphology of the brain cavity itself, as well as the paranasal sinuses and the trajectory of several cranial nerves and blood vessels. These features have been compared with the two extant folivoran genera, the two-toed sloth Choloepus and the three-toed sloth Bradypus. For many characteristics, especially those related to the paranasal pneumaticity and the brain cavity, a closer similarity between Glossotherium and Choloepus is observed, in accordance with the most widely accepted phylogenetic scenarios. However, other features are only shared by the two extant genera, but are probably related to allometric effects and the convergence that affected the two modern lineages. This study, which represents the first exhaustive analysis of digital endocasts of a fossil sloth, reveals the importance of the application of new methodologies, such as CT scans, for elucidating the evolutionary history of this peculiar mammalian clade.