Into the head of a ground sloth: the digital cranial endocast of Glossotherium robustum (Owen, 1842) (Xenarthra, Pilosa) from the Pleistocene of Argentina

BOSCAINI, ALBERTO; IURINO, DAWID A.; BILLET, GUILLAUME; HAUTIER, LIONEL; SARDELLA, RAFFAELE; TIRAO, GERMAN; GAUDIN, TIMOTHY J.; PUJOS, FRANÇOIS

Anagni

Congreso; Paleodays 2017; 2017

The South American species Glossotherium robustum (Owen, 1842) was one of the most widespread ground sloths during the Pleistocene. This iconic species was discovered by Charles Darwin during his travels on the HMS Beagle, and subsequently studied by Richard Owen, who started the long series of studies on this taxon´s anatomy, as well as on the whole xenarthran family Mylodontidae. This tradition continues until present, with recent studies concerning many aspects of the paleobiology of Glossotherium, such as its digging abilities, hearing capabilities, dietary preferences and food intake. However, anatomical data on its neurocranial features are scanty and out of date, with the only information available on the morphology of its brain cavity that dates back to the latest 19th century.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, allowing to collect measurements and morphological data of all the anatomical features that had previously remained obscure. Therefore, we performed a CT scan on a partial cranium of G. robustum from the Late Pleistocene of the Buenos Aires province (Argentina), and obtained bi- and three-dimensional images of the cranial cavities, 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 3D models of the extant representatives of the suborder Tardigrada (= Folivora, Phyllophaga): the two-toed and the three-toed sloth genera Choloepus and Bradypus. For many characteristics, especially those related to paranasal pneumaticity and the external brain morphology, a close similarity between Glossotherium and Choloepus is observed, in accordance with the most widely accepted phylogenetic scenarios. Other features, peculiar to Glossotherium, are probably indicative of some behavioural aspects of this species, and can give further support to some previously hypothesized morphofunctional perspectives. The current work represents the first exhaustive study of the inner skull anatomy of a fossil ground sloth, involving a full range of 3D imaging techniques, and highlights the importance of the application of these new methodologies for elucidating the evolutionary history of this peculiar mammalian clade.