Digital cranial endocast of the ground sloth Glossotherium robustum (Owen, 1842) (Xenarthra, Pilosa) from the Pleistocene of Argentina

BILLET, G.; TIRAO, G.; BOSCAINI, A.; IURINO, D. A.; HAUTIER, L.; SARDELLA, R.; GAUDIN, T. J.; PUJOS, F.

Calgary

Congreso; 77th AMSVP; 2017

The Society of Vertebrate Paleontology

The species Glossotherium robustum (Owen, 1842) was one of the most widespread ground sloths during the Pleistocene of South America, and is one of the most well studied species of the family Mylodontidae. This species was discovered by Charles Darwin during his travels on the HMS Beagle, and subsequently studied by Richard Owen, becoming one of the most iconic taxa of the South American extinct megafauna. In recent times, studies focusing on this species have clarified many aspects of its paleobiology, such as its digging abilities, hearing capabilities, dietary preferences and food intake. However, anatomical studies of its neurocranial features are scanty and out of date. The studies of its external ear region are 20 to 50 years old, whereas the scarce information about the morphology of its brain cavity dates back to the latest 19th century. Novel techniques of CT scanning and digital reconstructions enable non-destructive access to the internal cranial features of both extinct and extant vertebrates, as well as improve our knowledge of anatomical features that had previously remained obscure. Therefore, we performed CT scans on a partial cranium of G. robustum from the Late Pleistocene of the Buenos Aires province (Argentina), and created a digital reconstruction of the endocast, which allowed us to document many previously unknown structures. These include not only the brain cavity itself, but also the paranasal sinuses, the inner ear anatomy and the trajectory of several cranial nerves and blood vessels. All these features have been compared with the extant representatives of the suborder Tardigrada (= Folivora, Phyllophaga): the two-toed sloth genus Choloepus and the three-toed sloth genus Bradypus. For many characteristics, especially those related to paranasal pneumaticity and the brain cavity, a close similarity between Glossotherium and Choloepus is observed, in accordance with the most widely accepted phylogenetic scenarios. Geometric morphometric analyses also reveal that the inner ear anatomy of Glossotherium more closely resembles that of the giant ground sloth Megatherium, extant anteaters and armadillos, than that of Bradypus and Choloepus, further demonstrating the striking morphological convergence between the two extant sloth genera. The current work represents the first exhaustive study of a digital endocast of a fossil ground sloth, and reveals the importance of the application of these new methodologies for elucidating the evolutionary history of this peculiar mammalian clade.