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Osteology and Functional Morphology of the Axial Postcranium of the Marine Sloth Thalassocnus (Mammalia, Tardigrada) with Paleobiological Implications

Abstract

The gross morphology of the axial postcranium of Thalassocnus is presented here, completing the description of the skeleton of the genus. Thalassocnus is characterized by a low spinous process on C7, a cranially shifted position of the diaphragmatic vertebra, a great number of caudal vertebrae, the morphology of their transverse processes, and the conservation of the craniocaudal length of their centra up to Ca19. Additionally, the late species of Thalassocnus feature cranial articular surfaces of the atlas that are oriented cranioventrally and thoracolumbar vertebrae with spinous processes that are more inclined caudally, shorter craniocaudally, and have a smaller apex than in earlier species. In the late species, the thoracolumbar vertebrae are also characterized by zygapophyseal articulations that are more conspicuously concavo-convex, and by ribs that are affected by osteosclerosis and pachyostosis. Thalassocnus yaucensis additionally differs from the earlier species of the genus in featuring thoracolumbar vertebral centra that are shortened craniocaudally. The morphology of the axial postcranium of Thalassocnus is consistent with a reduced amount of time spent in a terrestrial habitat. Furthermore, the overall body size and extensive and extreme osteosclerosis of Thalassocnus suggest that bottom-walking was part of its modes of swimming. The tail was probably involved in diving and equilibration but did not contribute to propulsion. A downturned position of the head is inferred for the late species of Thalassocnus, and is probably related to grazing activity on the seafloor. The stabilized vertebral column may be related to the digging behavior purported in Thalassocnus. The aquatic functions of the entire skeleton of Thalassocnus are reviewed.

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Abbreviations

C:

Cervical

T:

Thoracic

L:

Lumbar

S:

Synsacrum

Ca:

Caudal (not included in the synsacrum, see below)

LACM:

Natural History Museum of Los Angeles County, Los Angeles, California, USA

MCL:

Museu de Ciencias Naturais da Pontifícia Universidade Católica de Minas Gerais, Belo Horizonte, Brazil

MNHN:

Muséum national d’Histoire naturelle, Paris, France

MUSM:

Museo de Historia Natural de la Universidad Nacional Mayor de San Marcos, Lima, Peru

UW:

Burke Museum, University of Washington, Seattle, Washington, USA

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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. Specimens of the MNHN included in the present work as well as in the preceding installments of the series (Amson et al. 2014a, b) were collected by Robert Hoffstetter and Christian de Muizon with funds of the CNRS (Centre National de la Recherche Scientifique), the MNHN, and the IFEA (Institut Français d’Études Andines). François Pujos (CCT-CONICET-Mendoza) and Rodolfo Salas-Gismondi (MUSM) are acknowledged for the assistance they rendered regarding general xenarthran 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. Christian Lemzaouda and Philippe Loubry (CNRS) are thanked for taking the photographs that illustrate this paper. We are grateful to Justine Jacquot-H for the realization of the life restoration. Malcolm T. Sanders (MNHN) is thanked for making the line drawing of the articulated skeleton of MNHN.F.SAS734 (T. natans, holotype). Finally, we thank the two anonymous reviewers for the considerable improvement they brought to the manuscript. The editor, John Wible, should also receive our gratitude for the amelioration brought to the manuscript as well as to those of the preceding installments of the series.

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Amson, E., Argot, C., McDonald, H.G. et al. Osteology and Functional Morphology of the Axial Postcranium of the Marine Sloth Thalassocnus (Mammalia, Tardigrada) with Paleobiological Implications. J Mammal Evol 22, 473–518 (2015). https://doi.org/10.1007/s10914-014-9280-7

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Keywords

  • Aquatic adaptation
  • Axial postcranium
  • Functional anatomy
  • Marine mammal
  • Megatheria
  • Pisco Formation
  • Thalassocnus