LAUSR

Abstract The fossils of Hegetotheriinae notoungulates studied herein were recovered from the Collon Cura Formation at Cerro Zeballos (Chubut) and Comallo (Rio Negro), which allow to report the first record of Hemihegetotherium from the Middle to Late Miocene of Patagonia (Argentina) and to describe a new species into this genus. The new taxon Hemihegetotherium tantillum sp. nov. is characterized by having a small size, an overlapped C labial to the P1, a squared and less imbricate P3–4, a not molarized P4, p1 positioned labial to p2, a lingually concave and labially bilobed p2, and more acute and pronounced mesial and distal ends on lower molars. The new species is morphologically similar to H. trilobus Croft and Anaya from the Middle Miocene fauna of Quebrada Honda (Bolivia), both sharing three synapomorphies. On the other hand, the holotype of Propachyrukhos robustus Roth is recognized as Hemihegetotherium cf. tantillum sp. nov., which implies that the genus, and tentatively the new species, are also present in the type locality of the Collon Cura Formation at Neuquen Province from where the holotype of P. robustus came from. This new report is relevant not only because it broadens the knowledge of Hegetotheriinae from the Collon Cura Formation and Middle to Late Miocene of Patagonia, but also because it involves the southernmost record of Hemihegetotherium in South America and a close affinity between the faunas of Patagonia and Quebrada Honda.