Endocranial structures of a juvenile Glossotherium robustum (Xenarthra, Mylodontidae) and preliminary considerations on their ontogeny

BOSCAINI, ALBERTO; IURINO, DAWID A.; SARDELLA, RAFFAELE; MACPHEE, ROSS D.E.; GAUDIN, TIMOTHY J.; SOTO, IGNACIO M.; PUJOS, FRANÇOIS

Buenos Aires

Congreso; XII Congreso de la Asociación Paleontológica Argentina; 2021

In paleontology, studies based on computed tomographies (CT) and posterior digital reconstructions allow the observation of the internal cranial cavities of vertebrate skulls in a non-destructive way. In recent years, these studies have often been applied to xenarthrans and, among them, several extinct terrestrial sloths have been analyzed using these techniques. Here, we present the first 3D digital endocranial models of a juvenile specimen of the mylodontine Glossotherium robustum (Owen, 1842), housed and CT-scanned at the American Museum of Natural History (collection number: AMNH 11270). The early ontogenetic stage of this specimen is clearly suggested by its small size, open cranial sutural contacts, and conical dentition. The digital reconstructions of the inner ear, brain cavity, and paranasal sinuses are compared with the homologous structures from an adult representative of the same species. Among these anatomical areas, the inner ear appears to be the most conservative, with the labyrinthine morphology being almost identical in the juvenile and the adult. The brain cavity of the juvenile G. robustum lacks the olfactory bulbs, due to preservation issues. Nonetheless, the general shape of the brain endocast of AMNH 11270 appears more anteroposteriorly compressed than the adult conformation. The greatest juvenile-adult differences concern paranasal pneumatization. In the juvenile G. robustum, paranasal sinuses are limited to the frontal, whereas pneumatization in the adult extends far posteriorly, invading almost the entire basicranium. The pattern observed in the pneumatization of the juvenile G. robustum is strongly reminiscent of that of juvenile and adult members of Scelidotheriinae, the sister group of Mylodontinae, and other earlier non-mylodontid sloths, such as Hapalops. This suggests that the impressive sinus organization seen in the adult G. robustum may be the consequence of an extended development of this trait during late life stages (i.e., peramorphosis). Analyzing trait evolution during the ontogeny of extinct sloth lineages may untangle interesting heterochronic patterns and further illuminate the evolutionary history of the group.