Functional anatomy of two megalonychid sloths (Xenarthra, Mammalia)

CHRISTINE, ARGOT; FRANÇOIS, PUJOS

Mainz, Alemania

Congreso; 73 Jahrestagund der Paläontologischen Gesellschaft; 2003

Living sloths, represented today by two genera, Choloepus (the two-toed sloth) and Bradypus (the three-toed sloth), are highly specialized Neotropical mammals which use inverted suspensory behavior during their active periods. Their particular mode of life seems to have appeared at least twice in the history of Xenarthra, since Bradypus (Bradypodidae) is tentatively allied with megatheriids whereas Choloepus is usually recognized as a Megalonychidae.     The appearance and evolution of the suspensory locomotion beneath branches in the living species is not understood yet, because of the lack of functional data on extant animals, and lack of fossils that exhibit adaptations toward such a peculiar mode of life. Although fossil sloths have usually been considered as “ground sloths” (in contrast to the modern “tree sloths”), a yet undescribed Peruvian Pleistocene megalonychid might have been one of the most arboreal fossil sloths according to some postcranial adaptative features. However, these features suggest that it did not use an inverted suspensory behavior in contrast to Choloepus. A few thousand years separate this fossil taxon from Choloepus, without fossils in between as far as is known.     It is noteworthy that most of peculiar postcranial features known in Mio-Plio-Pleistocene sloths, like peculiar sesamoids at the knee joint or the attachment of the clavicle with a strongly developed acromiocoracoid bridge, are kept in the highly specialized modern sloths. The functional consequences of such features are investigated, using especially computer tomography and cineradiography. Even if Choloepus evolved a highly specialized mode of locomotion which may sometimes obscure comparisons, the functional understanding of the adaptive features characterizing the Recent taxon helps to reduce the scope of the locomotor capabilities in the fossil.