The lower deciduous dentition of protodidelphids (Mammalia, Marsupialia) from the Itaboraí Basin (lower Eocene), Brazil, and the validity of "Riolestes capricornicus" (Goin et al., 2009).

The Itaboraí Basin, Southeastern Brazil, preserved one of the most diverse mammalian faunas from the Paleogene, with Metatheria comprising more than 60% of its diversity. Among identified metatherian clades, it has been argued that Paucituberculata is represented by "Riolestes capricornicus". This species was described based on a putative first lower molar (m1). However, its morphological pattern is remarkably similar to that of deciduous third lower premolars (dp3s) recovered in this basin, differing mostly by its more inflated crown. These similarities led to a previous hypothesis that this specimen represents a deciduous tooth. In this contribution, we review the dental anatomy of "R. capricornicus" and describe a dp3 of Protodidelphis vanzolinii. To test the hypothesis that "R. capricornicus" is a dp3, we performed a principal component analysis (PCA) to distinguish, for the first time, the morphology of deciduous third lower premolars (dp3s) from that of lower molars. Our results indicate that the morphological pattern of the holotype of "R. capricornicus" is occlusally compatible with that of an upper deciduous third premolar (DP3) of the protodidelphine Robertbutleria mastodontoidea, confirming its deciduous (dp3) nature and, thus, "R. capricornicus" is a junior synonym of Ro. mastodontoidea. Therefore, Paucituberculata is no longer recorded in the Itaboraí Basin, restricting its occurrence during the early Eocene to southern Patagonia (Austral biogeographic zone). Further, an Austral ancestral area for Paucituberculata and Australidelphia is suggested.