The manus of mylodon darwinii Owen (Tardigrada, Mylodontidae) and its phylogenetic implications

Abstract

The first nearly complete and articulated manual material of the ground sloth Mylodon darwinii, found in Upper Pleistocene strata from Argentina, is reported here. It shares cuneiform similarities with Mylodonopsis ibseni from Brazil, including pisiform facet distinct, facet for ulna obliquely concave, and prominent distolateral process, as well as metacarpal III gracile. It shares a flattened pisiform with Glossotherium robustum. The trapezoid is unique in the obliquely elongate proportions of its dorsal surface. The lack of fusion of epiphyses in metacarpals, as well as the scarce development of muscular attachment scars indicates a subadult ontogenetic stage. Shapes of the articular facets indicate different functions in digits II and III, with the former having greater motion range and the latter greater stability at the joints. Clear arboreal and fossorial adaptations are absent. A phylogenetic analysis was conducted; it recovers M. darwinii as closely related to M. ibseni. The analysis mainly agrees with larger phylogenetic analyses of sloth relationships based on craniomandibular evidence, except in the position of Thinobadistes segnis. When both data sets are analyzed together, the recovered position is that of the larger craniomandibular analysis. Our data of hand structure supports more than two events of mylodontine dispersal to North America.