New Small Bunodont Metatherian from the Late Eocene of the Argentinean Puna

We describe here a new small bunodont metatherian from Priabonian deposits of the Geste Formation exposed in the Puna of Catamarca Province (Argentina). The material, an isolated right M2?, was recovered byand allows us to erect the new speciesApeirodon sorianoi. The combination of diagnostic characters includes triangular occlusal pattern, bunoid, with an equally developed stylocone and cusp D, absence of cusp C, protoconal base wide, preparacrista and postmetacrista short, well-developed paracingulum, presence of double rank postvallum-prevallid shear, and presence of small paraconule and larger metaconule that is distolingually located. The overall morphology ofApeirodonmore closely resemblesGlasbius(particularlyG.) than any other Cretaceous-Paleogene American metatherian, supporting the largely accepted hypothesis proposing a common origin for some North and South American metatherian lineages.Apeirodonalso resemblesReigia punae(another small metatherian recovered from the same stratigraphic levels),Chulpasia,Palangania, and specimens referred toBobbschaefferia. Tentatively, we propose the inclusion ofApeirodonandReigiaas early divergent members of Polydolopimorphia based on the presence of bunodonty, enlarged stylocone and cusp D, and a distolingually-shifted metaconule. These features, incipiently developed in these late Eocene taxa, are emphasized in later representatives of the order. The presence of a generalized putative polydolopimorphian in Priabonian deposits of northwestern Argentina highlights the key temporal and spatial position of the region in the context of the evolution of Paleogene South American mammals.