A basal Hegetotheriidae (Mammalia, Notoungulata) from the early Miocene of Patagonia

Hegetotheriidae represents one of the most derived clades within the Order Notoungulata. Already diversified in the late Oligocene, its members, many of them with gliriform appearance, are conspicuous components of the terrestrial South American mammalian faunas up to the Pleistocene. However, the knowledge of their diversity during the Colhuehuapian South American Land Mammal Age (early Miocene) is still imperfect. Here it is described a new hegetotheriid, Hegetotheriopsis sulcatus n. gen. et sp., represented by a skull fragment with partial dentition from Colhuehuapian beds of the Sarmiento Formation at Bryn Gwyn, central Patagonia. Many mandibular fragments with teeth from this locality and other putative coeval localities at Northern Patagonia are referred tentatively to this new taxon. Hegetotheriopsis sulcatus shows a unique combination of cranial and dental characters, some previously known for archaeohyracid notoungulates (interpreted as the sister group of Hegetotheriidae) and others that typify to hegetotheriids. The phylogenetic analysis, based on a data matrix from previous studies, points out that Hegetotheriopsis sulcatus represents the earliest diverging hegetotheriid known so far, thus it cannot be classified within neither Hegetotheriinae nor Pachyrukhinae, the two subfamilies in which all hegetotheriids are traditionally included The occurrence in Colhuehuapian levels of this taxon, more basal than others known for late Oligocene beds (Deseadan), adds at least a 4 million years ghost lineage to the paleontological record of this mammals.