Systematics and stratigraphical range of the hegetotheriids <i>Hegetotheriopsis sulcatus</i> and <i>Prohegetotherium sculptum</i> (Mammalia: Notoungulata)

Hegetotheriidae is one of the most advanced clades within the endemic South American placental Order Notoungulata. The species <i>Hegetotheriopsis sulcatus</i> Kramarz &amp; Paz, 2013 differs from all other known hegetotheriids in having a peculiar combination of cranial and dental features, some of them shared with Archaeohyracidae (sister group of Hegetotheriidae). A previous cladistic analysis supported <i>H. sulcatus</i> as the earliest diverging hegetotheriid. Nevertheless, a more recent analysis, based on a different data matrix, concluded that this species is an advanced form within the family, with the little-known <i>Prohegetotherium sculptum</i> Ameghino, 1897 (late Oligocene) being the most basal taxon. Here we present a revision of the concept and the content of <i>P. sculptum</i> based on a re-examination of the type specimens. We find that this material exhibits several dental and cranial characters not recognized in all previous studies; we conclude that no specimen other than the types can be assigned to <i>P. sculptum</i>, and that all the synonymies previously proposed (i.e. <i>P. shumwayi</i> and <i>P. crassus</i>) are unfounded. A new cladistic analysis, combining characters from the two aforementioned analyses, confirms the position of <i>H. sulcatus</i> as the earliest diverging hegetotheriid. The taxon was originally described based on remains from early Miocene deposits in central and northern Patagonia (Sarmiento, Chichinales and Cerro Bandera formations). Here we report new material from the late Oligocene of Cabeza Blanca (central Patagonia), and reassign other material previously attributed to <i>Prohegetotherium</i> from presumably equivalent levels at Quebrada Fiera (central West Argentina). These remains extend the occurrence of <i>H. sulcatus</i> back to the late Oligocene, and fill an important gap of the early record of Hegetotheriidae.