A new genus and species of a giant Dinomyidae (Rodentia, Caviomorpha) from the late Miocene of Uruguay

Abstract

The Dinomyidae composed a family of "giant" rodents, restricted to South America. Nowadays this family includes only the "pacarana", a medium size caviomorph mostly distributed through the Amazonian Basin. Nevertheless, the past diversity of the group was notable with more than 60 described species currently grouped into five extinct subfamilies. Eumegamyinae is one of these subfamilies, characterized by hosting medium to big size forms including the largest known rodents. In this communication we describe the remains of a new giant Eumegamyinae exhumed from the coast of San José Department (southern Uruguay), in sediments of the Camacho Formation, assigned to the late Miocene Huayquerian SALMA. The remains include the posterior region of the skull, the occipital condyles, and the entire right auditory region of a unique animal. In connection with the crania we recovered the atlas and the almost complete dental series. The premolo-molar series are characterized by having the P4, M1 and M2 composed by five transverse lophs of which the first two (anterior ones) remain free, while the three posterior ones are joined together by the lingual (internal) side. The M3 is composed by three anterior free lophs and two posterior ones, joined at the lingual side. The auditive region has a short meatus acusticus externus with the foramen stylomastoideum located below them, and a well developed ectotympanic cavity. Based on the state of development of the foramen stylomastoideum and the presence or absence of the ectotympanic cavity, we describe two morphologies for the Eumegamyinae auditory region. The taxonomic and systematic relevance of the dental series versus the auditory region in Dinomyidae is discussed.