A tiny-sized Herpetotheriidae (Mammalia, Metatheria) from the Itaboraí basin (early Eocene), Brazil: Paleobiogeographic and systematic implications for Herpetotheriidae

Abstract

The Itaboraí Basin, located in Itaboraí, Rio de Janeiro, Brazil, has preserved one of the most important early Eocene vertebrate assemblages of the Southern Hemisphere. Among its representatives, Metatheria comprises its most diverse group, with a minimum of 29 genera and 43 species. Considering this diversity, tiny and very small taxa are represented by Jaskhadelphyidae and closely related taxa, “Peradectoidea”, Sternbergiidae, Derorhynchidae, and an undescribed taxon with uncertain affinities, previously referred to as “cf. Armintodelphys”. Here, this last taxon is named Thylacopygmaeus oliveirai gen. et sp. nov.; it is distinguished by its extremely small size (inferred body mass less than 15 g) and in that its lower molars show the metaconid placed distally to the protoconid and a developed postentocristid. Two PCA analyses of T. oliveirai and other tiny-sized taxa from Itaboraí indicate that the new taxon exhibits distinctive morphometric and morphofunctional patterns compared to other sympatric tiny-sized taxa. The inclusion of T. oliveirai gen. et sp. nov. in a phylogenetic analysis recovered it as the sister taxon of Rumiodon, an early Oligocene herpetotheriid from Peru, based on the presence of a labiolingually compressed metaconid on m3 and a labiolingually compressed protoconid on m4. We group these two taxa within the Herpetotheriidae in a new clade, the Rumiodontinae nov. Contrary to a previous hypothesis, Thylacopygmaeus is not closely related to Armintodelphys. The results confirm the Paleogene occurrence of Herpetotheriidae in South America.