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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.

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Abbreviations

FMNH:

Field Museum of Natural History (Chicago, USA)

FUESMEN:

Fundación Escuela de Medicina Nuclear (Mendoza, Argentina)

MACN Pv:

Colección de Paleontología de Vertebrados, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” (Buenos Aires, Argentina)

MLP:

División Paleontología de Vertebrados, Museo de La Plata (La Plata, Argentina)

ROM:

Royal Ontario Museum (Toronto, Canada)

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Acknowledgments

We are grateful to the FUESMEN institute (Fundación Escuela de Medicina Nuclear, Mendoza, Argentina) for access to CT-scanning facilities, and we are particularly indebted to Sergio Mosconi and collaborators for assistance with image processing. We thank A. Kramarz, S.M. Alvarez and L. Chornogubsky (MACN, Buenos Aires, Argentina) and M. Reguero, S.C. Scarano and M.L. de los Reyes (MLP, La Plata, Argentina), who kindly gave access to the specimens under their care. We thank the PaleoFactory Lab (Sapienza Università di Roma, Italy) for access to their facilities, without which this work would not have been possible. We also thank M. Fernández-Monescillo, S. Hernández del Pino and A. Forasiepi (IANIGLA, CCT-CONICET-Mendoza, Argenina) for their useful suggestions. This paper greatly benefited from the careful reading and thoughtful comments by the editor S. Thatje, Prof. G. De Iuliis and other two anonymous reviewers.

Funding

This research was partially funded by ECOS-FonCyT (A14U01).

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Correspondence to Alberto Boscaini.

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Communicated by: Sven Thatje

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Boscaini, A., Iurino, D.A., Billet, G. et al. Phylogenetic and functional implications of the ear region anatomy of Glossotherium robustum (Xenarthra, Mylodontidae) from the Late Pleistocene of Argentina. Sci Nat 105, 28 (2018). https://doi.org/10.1007/s00114-018-1548-y

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Keywords

  • Glossotherium
  • Ground sloth
  • Ear region
  • Bony labyrinth
  • Phylogeny
  • Function