Fusion of anterior thoracic vertebrae in Pleistocene ground sloths

ABSTRACT

A previous study reported the fusion of spinous processes of thoracic vertebrae one and two in the ground sloth Glossotherium, proposing an injury or a congenital deformation as possible causes. In this paper we examined members of four families of Pleistocene sloths (Mylodontidae, Megatheriidae, Nothrotheriidae and Megalonychidae) to document the presence of this condition in different genera. We found that of nine genera analyzed, Glossotherium and Megalonyx were the only that present fusion of the spinous processes, which consistently occur between T1 and T2. When only identified T1 and T2 were considered, the incidence of the fusion was 87.5% in the case of Glossotherium and 40% in the case of Megalonyx. The high incidence of fusion in almost only one genus almost certainly would preclude the possibility of having been caused by an injury. We discuss the possibility that the observed fusions could be related to a pathological condition, or a mechanical response due to repetitive movements or particular habits.

KEYWORDS:

• Ground sloths
• Pleistocene
• paleopathologies
• vertebrae

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Acknowledgments

We are grateful to Andrés Rinderknecht (Museo Nacional de Historia Natural), Alejandro Kramarz (Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”), Marcelo Reguero (Museo de La Plata) and Amanda Millhouse (National Museum of Natural History) for access to the collections under their care. We also thank Sebastián Vázquez for the X-Ray images of MPAC 349 and MPAC 1060; Sue Ware, Kevin Fitzgerald, Benjamin Young, Kris Newquist, Adam Isack and Jessica Alford from Alameda East Veterinary Hospital, Denver, Colorado for the X-Ray image of DMNS EPV.6491; and Amanda Millhouse, National Museum of Natural History, for providing the photograph of NMNH 24590 for Figure 3. Two anonymous reviewers made critical suggestions and corrections that greatly improve the final manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was funded by Agencia Nacional de Investigación e Innovación (ANII, grants: FCE_1_2014_1_104572, POS_NAC_2014_1_102504).