Unexpected pampatheriid from the early Oligocene of Peruvian Amazonia: insights into the tropical differentiation of cingulate xenarthrans

Cingulates (armoured xenarthrans) constitute one of the most peculiar endemic clades of South American mammals. Their first occurrence in the fossil record can be traced back to the early Eocene. However, their Palaeogene remains are scarce, especially in tropical regions. Here, we report an isolated molariform tooth of xenarthran from the early Oligocene of Peruvian Amazonia. This tooth is small and bilobed, with slightly marked apicobasal sulci, more compatible with a cingulate dental pattern than that of folivorans (the only toothed pilosans). High-resolution microtomography of this specimen reveals an inner layer highly vascularised suggestive of osteodentine, encircled by a middle layer of orthodentine with densely S-curved dentinal tubules, and a thin outer layer. There is no clear evidence regarding the presence of cement, otherwise typically present in tree sloths. Although its external appearance cannot fully discard an assignment to a sloth, the microstructural pattern, such as well-developed osteodentine arranged in a row without ramifications, suggests that this tooth belongs to an early pampatheriid. The presence of Oligocene Pampatheriidae in Western Amazonia, previously suggested by isolated osteoderms, is substantiated by this dental remain, which underlines the importance of tropical areas for the differentiation of cingulate xenarthrans.