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

Abstract

Several 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 this mammalian clade. Few studies of the inner ear anatomy in these peculiar animals were conducted historically, but these are increasing 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 sloth Glossotherium robustum and provide a description of the external features of the ear region and the endocranial side of the petrosal bone, coupled with the first data on the anatomy of the bony labyrinth. Some features observable in the ear region of G. robustum (e.g., the shape and size of the entotympanic bone and the morphology of the posteromedial surface of the petrosal) are highly variable, both intraspecifically and intraindividually. The form of the bony labyrinth of G. robustum is also described, providing the first data from this anatomical region for the family Mylodontidae. The anatomy of the bony labyrinth of the genus Glossotherium is here compared at the level of the superorder Xenarthra, including all available extant and extinct representatives, using geometric morphometric methods. In light of the new data, we discuss the evolution of inner ear anatomy in the xenarthran clade, and most particularly in sloths, considering the influence of phylogeny, allometry, and physiology on the shape of this highly informative region of the skull. These analyses show that the inner ear of Glossotherium more closely resembles that of the extant anteaters, and to a lesser extent those of the giant ground sloth Megatherium and euphractine armadillos, than those of the extant sloths Bradypus and Choloepus, further demonstrating the striking morphological convergence between the two extant sloth genera.

Cranial Anatomy and Paleoneurology of the Extinct Sloth Catonyx tarijensis (Xenarthra, Mylodontidae) From the Late Pleistocene of Oruro, Southwestern Bolivia

“…The pterygoid/squamosal suture extends posteriorly into the ear region ( Figures 2B,C). The pterygoid of C. tarijensis appears less inflated basally than is the case in either M. darwinii or G. robustum (Patterson et al, 1992;Boscaini et al, 2018a). In the latter two taxa, a pair of inflated areas, lateral and medial, are observed at the base of the pterygoid, whereas in C. tarijensis, a single low pterygoid inflation is present ( Figure 2B).…”

“…This is particularly true for the groove for the hypoglossal nerve (XII), which is the only nerve leaving the skull from the hypoglossal foramen. In Catonyx tarijensis the hypoglossal foramen is roughly equivalent in size to the jugular foramen (Figures 2, 6), whereas in G. robustum the hypoglossal foramen is larger (Boscaini et al, 2018a(Boscaini et al, , 2020. The hypoglossal nerve, which innervates the glossal musculature, controls most of the intrinsic and extrinsic lingual musculature (Evans and de Lahunta, 2013).…”

“…187). In contrast, the hypoglossal foramen is larger than the jugular in Mylodon and Glossotherium (Patterson et al, 1992;Boscaini et al, 2018a).…”

“…However, the hypoglossal foramen is distinctive in a number of aspects. First, in MNHN-Bol V 13364 it is nearly identical in size to the jugular foramen (Figure 3), whereas in Mylodon and Glossotherium the hypoglossal foramen is larger than the jugular (Patterson et al, 1992;Boscaini et al, 2018a). Moreover, it nearly abuts the jugular foramen, the two being separated by a sharp ventral crest (Figure 3).…”

“…Moreover, it nearly abuts the jugular foramen, the two being separated by a sharp ventral crest (Figure 3). In other mylodontids, the hypoglossal foramen is located farther posteriorly (Patterson et al, 1992;Boscaini et al, 2018a).…”