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Osteology and Functional Morphology of the Forelimb of the Marine Sloth Thalassocnus (Mammalia, Tardigrada)

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Abstract

Thalassocnus is a genus of “ground sloths” known from Neogene deposits, for the great majority of specimens, of the Pisco Formation (Peru). Five species are recognized, their description being currently restricted, for the most part, to the skull, mandible, and dentition. The bones of the forelimb are here described, and compared among the species of Thalassocnus and to other pilosans. The main characteristics of the forelimb of Thalassocnus relative to other sloths are the shortness of the humerus and radius, and the specialized digits. Moreover, the late species of the genus are characterized by the development of the pronator ridge of the radius, stoutness of the ulna, widening of the proximal carpal row, and shortening of the metacarpals. Analogies with extant tetrapods are proposed in order to infer plausible aquatic functions of the forelimb of Thalassocnus. In addition to paddling, it is argued that the forelimb of Thalassocnus was involved in bottom-walking, a function similarly found in extant sirenians. However, the function of the forelimb of Thalassocnus differs drastically from that of the latter, since it was likely involved in an activity related to obtaining food such as uprooting seagrass rhizomes.

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Acknowledgments

We are indebted to Rodolfo Salas-Gismondi (MUSM), Samuel McLeod and Vanessa Rhue (both LACM), Castor Cartelle (MCL), and Géraldine Veron (MNHN), who allowed access to the collections under their care. Rodolfo Salas-Gismondi (MUSM) and Mario Urbina (MUSM) are thanked for collecting numerous specimens of Thalassocnus. François Pujos (CCT-CONICET-Mendoza) and Rodolfo Salas-Gismondi (MUSM) are acknowledged for the assistance they brought regarding general xenarthrans questions. We thank Colas Bouillet (MNHN), Batz Le Dimet (MNHN), Philippe Richir (MNHN), and Renaud Vacant (CNRS) for preparing and/or helping prepare some of the fossils included in this study. Finally, Christian Lemzaouda and Philippe Loubry (CNRS) are thanked for taking some of the photographs that illustrate this publication. Finally, we thank the two anonymous reviewers and the editor, John Wible, for their numerous comments and suggestions that conspicuously improved the quality of both the content and form of the manuscript.

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  1. Département Histoire de la Terre, Muséum national d’Histoire naturelle, Centre de Recherche sur la Paléobiodiversité et les Paléoenvironnements (CR2P: CNRS, MNHN, UPMC-Paris 06; Sorbonne Universités), 57 rue Cuvier, CP38, 75005, Paris, France

    Eli Amson, Christine Argot & Christian de Muizon

  2. Park Museum Management Program, National Park Service, 1201 Oakridge Drive, Fort Collins, CO, 80525, USA

    H. Gregory McDonald

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  1. Eli Amson
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Correspondence to Eli Amson.

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Specimen numbers of Thalassocnus forelimb material. Abbreviations: (R), right; (L), left; (R&L), right and left; (R or L), right or left (unknown laterality). (XLS 37 kb)

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Amson, E., Argot, C., McDonald, H.G. et al. Osteology and Functional Morphology of the Forelimb of the Marine Sloth Thalassocnus (Mammalia, Tardigrada). J Mammal Evol 22, 169–242 (2015). https://doi.org/10.1007/s10914-014-9268-3

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