The Scelidotheriinae Proscelidodon gracillimus (Xenarthra: Mylodontidae) from the Cerro Azul Formation (late Miocene), Argentina: Ontogenetic variability and taxonomy

The subfamily Scelidotheriinae (Xenarthra, Folivora) is a clade of extinct sloths recorded from the middle Miocene to the Pleistocene/Holocene transition in South America, namely Argentina, Bolivia, Brazil, Colombia, Chile, Peru, Ecuador, Uruguay, and Paraguay. Quaternary taxa are the best known, but the knowledge of the late Neogene representatives has been greatly improved in recent years, particularly in Argentina. New materials of Scelidotheriinae recovered from the Cerro Azul Formation (late Miocene; Huayquerian) in La Pampa and Buenos Aires provinces are here described and identified as Proscelidodon gracillimus Rovereto, and ontogenetic variations are established for the first time in this group of sloths. The comparative study has led to the taxonomical revision of other species such as P. almagroi Rovereto (from Catamarca Province) and Elassotherium altirostre Cabrera (from western Buenos Aires Province), whose type specimens are reinterpreted as subadult and juvenile individuals, respectively, of P. gracillimus; thus, the former two become synonymous names of the latter. As the type material from Mendoza, the remains of P. gracillimus from Buenos Aires and La Pampa provinces have been recovered from Huayquerian levels, being absent from the older Chasicoan levels, within the late Miocene. In turn, the materials from Catamarca (type of P. almagroi) have less chronological precision, and could range from late Miocene to early Pliocene.