First record of Toxodontia for the Itaboraí Basin (Paleogene, Rio de Janeiro, Brazil) and comments on the evolution of basal toxodonts (Pan-Perissodactyla, Mammalia)

Abstract

Although most studies on Paleogene notoungulates have been made focusing on the Patagonian record during the 20th century, the São José de Itaboraí Basin (Brazil), representing a mid-latitude fossil-bearing locality, has yielded a very remarkable set of Paleogene species for that order. In this contribution, we present a new notoungulate species for these outcrops, Itaboraitemnus macrodon, based on material collected from levels of the ‘Itaboraí II’ assemblage (Ypresian, early Eocene). The new taxon clearly differs from previous Itaboraí species (Nanolophodon tutuca, Colbertia magellanica, Camargomendesia pristina, Itaboraitherium atavum) given its relatively larger size, enlarged canines, and presence of multiple free cristae between protoloph and crochet in upper molars. In turn, Itaboraitemnus is similar to the stock of basal toxodonts, and particularly those species traditionally regarded as ‘isotemnids’. We describe the material assigned to the new taxon and conduct a phylogenetic analysis focused on a wide variety of notoungulate species, which supports the inclusion of Itaboraitemnus as a toxodont. Although its position among other Paleogene forms was not completely resolved, at least part of our results suggest that some ‘isotemnids’, including the new taxon, may represent a natural group. This is the first toxodont notoungulate recorded for the Itaboraí Basin, and its record increases the diversity of Eocene species of the suborder. Considering that the bulk of the early Eocene diversity of basal Toxodontia is represented by extra-Patagonian taxa, it is evident that the exploration and study of Paleogene mid-latitude outcrops in South America may provide a significant amount of information regarding the early radiation of these notoungulates.