Ontogeny and associated changes of the extinct sloth Simomylodon uccasamamensis (Xenarthra, Mylodontidae) from the Pliocene of the eastern Puna, Argentina.

Within Xenarthra (Eocene-Recent), Folivora developed (late Eocene-Recent) a remarkable diversity with respect to ecology and taxonomy over its evolutionary history. Knowledge of the diversity achieved by members of this clade in high-altitude areas of South America (i.e., Altiplano and Puna regions of Peru, Bolivia, and northwestern Argentina) has been improved in recent decades. A particular example involves the late Neogene Mylodontidae Simomylodon uccasamamensis, known mostly from multiple specimens recovered from the Bolivian Altiplano. Although several anatomical descriptions of this ground sloth have been published, almost nothing is known about its ontogenetic development and the associated morphological changes. Here we describe and compare new specimens of S. uccasamamensis from the upper level of the Tafna Formation (Pliocene) in the eastern Puna (ca. 3800 masl), Argentina, representing the southernmost record of this species. The new material is represented by specimens showing different ontogenetic stages, from infant to adult. One subadult specimen reached an estimated body mass of ca. 232 kg. The comparative study of external and internal morphology (the latter obtained from CT scans and radiography) shows remarkable changes in the mandible and molariforms associated with ontogeny; in addition, evidence suggests that the mfs2-3 are the first functional teeth, followed by mf1 and cf1. Based on our body mass estimates (ca. 232 kg.), we inferred an average lifespan of 14 years, 9-month gestation time, and sexual maturation at 4.1 years, quite similar to the values we obtained based on estimated body masses of adult specimens from Bolivia published by previous authors. Along its latitudinal distribution (ca. 14° S-21° S) S. uccasamamensis co-occurred with other ground sloths (e.g., Megatheriinae, Thalassocninae, and Scelidotheriinae), suggesting niche partitioning. The presence of this medium-sized ground sloth is consistent with the similarity between the faunas of eastern Puna and the Bolivian Altiplano during the Pliocene, which is also concordant with what was observed in other clades, such as Rodentia and Notoungulata.