Ungual phalanges analysis in Pleistocene ground sloths (Xenarthra, Folivora)

Ungual phalanges (the most distal bone within a limb) and claws (the overlying corneous sheath) from the third digit of the forefoot of selected Pleistocene ground sloths (Lestodon armatus, Glossotherium robustum, Scelidotherium leptocephalum and Megatherium americanum) are analysed, as well as those of some living xenarthrans for actualistic comparison, aiming at testing hypotheses of substrate usage and locomotor behaviour. The third digits were chosen for this study because of its size and nearly perfect bilateral symmetry, which increases the possibilities of revealing functional differences between taxa. The analyses performed were of inner and external curvature, the strength indicator and the mechanical advantage. The mechanical advantage indicates that the four ground sloths' species were well adapted for strenuous activities, such as digging, in which force rather than velocity is optimised. Their strength indicator shows expected values for their body size, while in Mylodon darwinii the value obtained was lower than expected. In the two curvature analyses L. armatus, G. robustum and M. americanum fall within the group of armadillos that dig, whereas S. leptocephalum does not, this might be due to a difference in the movements performed while performing an activity such as digging or similar to it.