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

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.

Evaluation of the microgrinding procedure for the microscopic analysis of temporal bones.

INTRODUCTION: The microgrinding technique is used to study cochlear implant electrode positioning and cochlear trauma. It may be argued that this technique might cause damage to inner cochlear structures even without a cochlear implant insertion and thus it should not be recommended. Most papers do not explain how microgrinding is performed, referring to older papers for its description. Properly describing the technique and re-evaluating its safety may reassure researchers of their findings when studying trauma after cochlear implant insertion. OBJECTIVE: To accurately describe the microgrinding technique and re-evaluate its safety to assess intracochlear trauma by studying non-implanted temporal bones. METHODS: Four fresh temporal bones were removed before 24 hours postmortem and frozen at -20°C. Two were prepared for microgrinding before 24 hours of freezing and the others after 6 months. A descriptive analysis of the microscopic anatomy was performed, as well as a comparison between the bones processed within 24 hours of freezing and the bones frozen for 6 months. RESULTS: A total of 80 surfaces was evaluated. Preservation of even the most delicate intracochlear and vestibular structures was observed, such as the crista ampullaris, Reissner's and basilar membranes, permitting an adequate micro-anatomical study. Artifacts were rare and did not interfere with the analysis. Bones studied before 24 hours postmortem exhibited better quality than those frozen for 6 months. CONCLUSIONS: The microgrinding technique accurately preserves the inner ear's membranous microscopic anatomy and thus it is useful to study cochlear implant electrode positioning and trauma inside the cochlea. Studies that aim to evaluate inner ear microanatomy should be performed with fresh bones or bones frozen for less than 24 hours since they exhibit a better micro-anatomical quality.