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

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.