First record of Toxodontia for the Itaboraí Basin (Paleogene, Rio de Janeiro, Brazil) and an assessment on the evolution of isotemnid notoungulates (Pan-Perissodactyla, Mammalia)

The traditional taxonomy of notoungulates divides this order in two main groups, Typotheria and Toxodontia. Within the latter, isotemnids encompass some of the earliest and more generalized species, radiating mostly during the Eocene and reaching also the Oligocene. Although somewhat conservative, the history of this group is marked by some morphological changes, and isotemnids presented a diversity of body sizes during the Paleogene. In Brazil, the Itaboraí Basin have yielded several paleontological records mainly represented by mammals. Its chronological context, in the first half of the Paleogene, places this basin as an important source of information on the evolution of South American vertebrates (particularly mammals). In this contribution we describe a new isotemnid species, based on a partial skull and isolated maxillary and dental material, recognized for levels of the Itaboraí Formation cropping out in São José de Itaboraí, Rio de Janeiro, Brazil. A phylogenetic analysis was performed in order to evaluate the relationships of this new taxon, which results indicate (in part) that Isotemnidae represent a natural group, with some of the characters being related to the development of canines. Considering the evolutionary history of the family in this context, we recognized a series of radiation pulses during the Paleogene, and a possible correlation with Eocene climatic events that apparently affected the evolutionary rates of these notoungulates. This new record enhances our knowledge on the fossil diversity within the context of the Itaboraí Basin, whose vertebrates stand as important exponents of the early evolution of the South American biota.