A new toxodont (Mammalia, Panperissodactyla, Notoungulata) from the Oligocene of Patagonia, Argentina, and systematic considerations on the paraphyletic ‘Notohippidae’

Abstract

Notoungulates were the most diverse group of South American native ungulates during the Cenozoic. They ranged from small rodent-like to heavy rhino-like forms and, despite this morphological diversity, their monophyly is widely accepted. Within Notoungulata, Typotheria and Toxodontia (the two major clades) are well supported, whereas phylogenetic relationships between early diverging lineages (formerly grouped as ‘Notioprogonia’) are still a matter of debate. Within Toxodontia, ‘Notohippidae’ sensu Simpson (1967) is a paraphyletic group lacking systematic consensus and represented by small to medium-sized taxa from the late Eocene to early/middle Miocene, but that were especially diverse during the late Oligocene. In this context, we describe a new toxodont (Teushentherium camaronensis gen. et sp. nov.), found at Barrancas Blancas, an Oligocene locality in the south-east of Chubut (Patagonia, Argentina). It is represented by the anterior portion of the skull (palate and anterior snout) and the associated horizontal rami of the mandible. A phylogenetic analysis places the new taxon as closely related to taxa traditionally regarded as ‘Notohippidae’. In addition, extensive taxonomic sampling allowed us to assess phylogenetic relationships among other representatives. As expected, only Homalodotheriidae, Leontiniidae and Toxodontidae were recovered as monophyletic groups, whereas neither ‘Notohippidae’ nor ‘Isotemnidae’ were supported. We also identified a clade (sister to Homalodotheriidae) for which we propose the name Eutoxodontia. It includes ‘Notohippidae’ sensu Simpson, Leontiniidae and Toxodontidae, and provides a more accurate definition for middle and later diverging toxodonts, avoiding terminology such as ‘advanced’ or ‘derived’. We hope to contribute to the stabilization of toxodontian taxonomy, to highlight the main gaps in this respect, and to promote future contributions dealing with this intricate issue.