The late Oligocene mammalian fauna of Quebrada Fiera is one of the most diverse of the Deseadan SALMA. We describe its endemic xenarthran assemblage, represented by 13 species, including two stem sloths and 11 armored cingulates. The rare folivoran remains complete the knowledge of Octodontotherium, one of the early known mylodontids, and suggest the existence of a new small non-megalonychid megatherioid Similhapalops nivis gen. et sp. nov. The lumbar vertebrae of Octodontotherium sp. provide the oldest evidence of xenarthry for the order. The presence at the knee joint of an ossified meniscus and cyamo-fabella strongly suggests a plantigrade ancestral condition in Mylodontidae. Armored xenarthrans are represented by armadillos Eutatini (Meteutatus sp., M. lagenaformis, and Stenotatus ornatus), an undetermined Euphractinii, the “horned” peltephilids Peltephilus sp. and P. undulatus, and the palaeopeltid Palaeopeltis inornatus; an undetermined Glyptodontidae, the glyptatelines Glyptatelus cf. tatusinus and Clypeotherium sp., and an undetermined propalaeohoplophorine. The xenarthrans represent a third of the mammals of the Quebrada Fiera fauna, one of the most varied at the end of the Paleogene in South America. This vertebrate fauna is represented by a mix of endemic taxa with other species of high latitude (Patagonian) origin, but few from low latitude faunas. A detailed study of this mammalian assemblage complemented by detailed geological and petrographical studies, suggests the existence of strong volcanic activity, the presence of braided rivers, lagoonal environments in open habitats under a temperate or hot climate with savannas and/or grasslands and isolated forests during the late Oligocene at the foot of the proto-Andes.