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

Abstract

International audienceThe anatomy of the skeletal elements of the hind limb of Thalassocnus is described. This genus of “ground sloth” comprises five species represented by Neogene specimens from the coast of Peru and Chile, mostly found in the Pisco Formation. The hind limb of the genus Thalassocnus as a whole is characterized by a small iliac wing, a gracile femur with well-formed femoral neck, teardrop shaped patella, long and slender tibia, triangular tuber calcis, and proximal development of the lateral process of the Mt V. The comparison of the species of Thalassocnus with each other suggests a progressive shift to a particular ecology from the earliest to the latest species of the genus, a conclusion in agreement with those of the studies of craniomandibular, dental, and forelimb gross morphology, and bone internal microstructure. The pedolateral stance, which involves the bearing of the weight on the lateral side of the foot, was practiced by the earliest species of Thalassocnus, as was the case for other Megatheria. This stance was apparently forsaken by the late species of the genus in favor of the acquisition of a secondary plantigrady. A plantigrade hind limb may have been more efficient for paddling and for bottom-walking. Additionally, the late species of Thalassocnus differ from the early ones in the morphology of the pelvis and the slight overall reduction of the hind limb. This suggests the decrease of the support function of the hind limb of these species when compared to that of the early species of Thalassocnus

The Southernmost Occurrence of the Aquatic SlothThalassocnus(Mammalia, Tardigrada) in Two New Pliocene Localities in Chile

“…The shaft of the left fibula presents some features that differ from the general description of the genus given by Amson et al (2015c). The Lomas del Sauce specimen has a straight shaft elliptic in cross-section at the midshaft and distal end but teardrop-shaped at the proximal end.…”

“…In the locality "Arenas de Caldera", attributed to the late Miocene (Walsh and Suárez, 2005) Inglesa Formation were recovered (Pyenson et al, 2014). the entire genus, this attribution was subsequently confirmed (Amson et al, 2015c): the morphology of the medial trochlear ridge of the femur enabled the attribution of the femur to either T. antiquus or T. natans. Thus, the upper age limit of the Cerro Ballena site might be as initially supposed or older (Amson et al, 2015c).…”

“…Manus morphology showed the capability for gripping fixed objects on the seafloor, which may have facilitated Thalassocnus feeding and stabilization (Amson et al, 2015b); moreover, secondary plantigrady in the youngest species, and the development of a strong claw on the third digit of the pes would also have favored this stabilization (Amson et al, 2015c). Forelimb adaptations are also consistent with digging habits (Amson et al, 2015b).…”

“…13-14). The medial condyle of the proximal epiphysis is oval, unlike that of T. antiquus, in which this facet is more circular (Amson et al, 2015c). The articular surface for the astragalus is formed by two concavities separated by a median ridge along the entire anteroposterior depth of the distal surface.…”

“…This is consistent with the morphology present in the youngest species of Thalassocnus (no data for T. carolomartini). The older species feature a wider medial surface (Amson et al, 2015c).…”