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

Abstract

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.

Keywords:

• Paleobiology
• Xenarthra
• biomechanics
• Pleistocene
• ground sloth

Acknowledgements

We would like to thank Marcelo Reguero (Museo de la Plata), Itatí Olivares (Museo de la Plata), Andrés Rinderknecht (Museo Nacional de Historia Natural), Enrique González (Museo Nacional de Historia Natural) and Sabrina Riverón (Colección de Vertebrados de la Facultad de Ciencias) for giving us access to the collections under their care. We are also indebted to Gregory McDonald for providing us with his photos and X-rays of Nothrotheriops claws from the Vertebrate Paleontology Department of the Natural History Museum of Los Angeles County and with Pip Brewer (Natural History Museum of London, Palaeontology Collection) and Dan Sykes (Natural History Museum of London, Imaging and Analysis Centre) for providing us with the images of M. darwinii claw.