A cladistic investigation of the phylogenetic relationships among 21 extinct and extant genera of sloths (Mammalia, Xenarthra, Tardigrada) was performed on the basis of characteristics of the bony anatomy of the auditory region. This study was undertaken in order to evaluate specific hypotheses of relationship within the group. Questions of particular interest include the relationship among the three traditional family groupings of extinct ground sloths and the monophyletic or diphyletic origin of the two genera of extant tree sloths. Eighty-five discrete morphological characters were analyzed using the computer program PAUP. Characters were polarized via comparisons to the following successive outgroups, all members of the supraordinal grouping Edentata: the Vermilingua, or anteaters; the Cingulata, or armadillos and glyptodonts; the Palaeanodonta; and the Pholidota, or pangolins. Three most parsimonious trees result (for 21 ingroup taxa and 5 outgroup taxa; Length = 304 steps, CI = 0.405, RI = 0.712). The results of this analysis provide characters which support the monophyly of the Xenarthra as a whole, as well as its dichotomous division into the clades Cingulata and Pilosa. In addition, it strongly corroborates the monophyly of the Vermilingua and the Tardigrada. Within tardigrades, the living three-toed sloth Bradypus is suggested to represent the sister-taxon to all other sloths. The results support the monophyly of the three traditional ground sloth families Megatheriidae, Megalonychidae, and Mylodontidae (the latter two quite strongly), and suggest that the subfamily Nothrotheriinae may represent a paraphyletic stem group for the remaining sloths. A novel relationship between the families Megalonychidae and Mylodontidae is proposed which contrasts with previous hypotheses of a close relationship between megalonychids and megatheriids. Lastly, this analysis strongly supports the derivation of the extant two-toed sloth Choloepus from within the West Indian megalonychid ground sloths, contradicting the traditional systematic grouping of the extant tree sloths into a monophyletic family Bradypodidae and suggesting that the living tree sloths represent one of the most remarkable examples of convergent evolution known among mammals.
