Phylogenetic and functional implications of the ear region anatomy of Glossotherium robustum (Xenarthra, Mylodontidae) from the Late Pleistocene of Argentina

International audienceSeveral detailed studies of the external morphology of the ear region in extinct sloths have been published in the past few decades,and this anatomical region has proved extremely helpful in elucidating the phylogenetic relationships among the members of thismammalian clade. Few studies of the inner ear anatomy in these peculiar animals were conducted historically, but these areincreasing in number in recent years, in both the extinct and extant representatives, due to wider access to CT-scanning facilities,which allow non-destructive access to internal morphologies. In the present study, we analyze the extinct ground slothGlossotherium robustumand provide a description of the external features of the ear region and the endocranial side of the petrosalbone, coupled with the first data on the anatomy of the bony labyrinth. Some features observable in the ear region ofG. robustum(e.g., the shape and size of the entotympanic bone and the morphology of the posteromedial surface of the petrosal) are highlyvariable, both intraspecifically and intraindividually. The form of the bony labyrinth ofG. robustumis also described, providing thefirst data from this anatomical region for the family Mylodontidae. The anatomy of the bony labyrinth of the genusGlossotheriumis here compared at the level of the superorder Xenarthra, including all available extant and extinct representatives, using geometricmorphometric methods. In light of the new data, we discuss the evolution of inner ear anatomy in the xenarthran clade, and mostparticularly in sloths, considering the influence of phylogeny, allometry, and physiology on the shape of this highly informativeregion of the skull. These analyses show that the inner ear ofGlossotheriummore closely resembles that of the extant anteaters, andto a lesser extent those of the giant ground slothMegatheriumand euphractine armadillos, than those of the extant slothsBradypusandCholoepus, further demonstrating the striking morphological convergence between the two extant sloth genera