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

BOSCAINI, A.; IURINO, D. A.; BILLET, G.; HAUTIER, L.; SARDELLA, R.; TIRAO, G.; 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 widespreadground sloths during the Pleistocene of South America, and is one of the mostwell studied species of the family Mylodontidae. Known since the first half ofthe 19th century, this species was discovered by Charles Darwinduring 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. Inrecent times, studies focusing on this species have clarified many aspects ofits paleobiology, such as its digging abilities, hearing capabilities, dietarypreferences and food intake. However, anatomical studies of its neurocranialfeatures are scanty and out of date. The studies of its external ear region are20 to 40 years old, whereas the scarce information about the morphology of itsbrain cavity dates back to the latest 19th century. Novel techniquesof CT scanning and digital reconstructions allow non-destructive access to theinternal cranial features of both extinct and extant vertebrates, and increaseour knowledge of anatomical features that had previously remained obscure.Therefore, we performed CT scans on a Glossotheriumrobustum specimen from the Late Pleistocene of the Buenos Aires province(Argentina), and created a digital reconstruction of the endocast, which allowedus to document many previously unknown structures. These include not only thebrain cavity itself, but also the paranasal sinuses, the inner ear anatomy andthe trajectory of several cranial nerves and blood vessels. All these featureshave 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 paranasalpneumaticity and the brain cavity, a close similarity between Glossotherium and Choloepus is observed, in accordance with the most widely acceptedphylogenetic scenarios. Other features, such as the small and lissencephalicbrains and the volumes occupied by the paranasal sinuses, are shared only bythe extant sloths, but these are probably related to convergent factors andallometric effects. Geometric morphometric analyses also reveal that the innerear anatomy of Glossotherium more closelyresembles that of the giant ground sloth Megatherium,extant anteaters and armadillos, than that of Bradypus and Choloepus,further demonstrating the striking morphological convergence of the extantsloths.