The status of Peripantostylops and Othnielmarshia (Mammalia: Notoungulata: Henricosborniidae) from the early-middle Eocene of Patagonia (Argentina)

The Vacan fauna was defined by the fossil mammalian content of the Vacan Member (Sarmiento Formation) at Canadon Vaca fossil locality (Chubut Province, Argentina), although several other local faunas from nearby areas (e.g. Rio Chico oeste) were also attributed to the Vacan age estimated ca. 45-42 Ma time interval. This fauna contains poorly understood native South American ungulates, such as the Henricosborniidae. This group includes Henricosbornia, Peripantostylops, Othnielmarshia, Simpsonotus, Orome, and Nanolophodon, but both its definition and phylogenetic relationships remain unclear. In particular, Othnielmarshia and Peripantostylops were established as guide taxa of the Vacan fauna, but their knowledge has remained obscure since more than 50 years. The re-study of the Ameghino's collection and the specimens collected by the Scarritt Pocket Expeditions in Canadon Vaca and other nearby localities allows us to 1) provide an update on the systematic status of Othnielmarshia and Peripantostylops; 2) emend their diagnoses; 3) establish comparisons with other related taxa from the same period, proposing synonymies; and 4) recognise both taxa in several local faunas from the Sarmiento and Las Flores formations. We also highlight the variability in certain controversial characters of henricosborniids, which should be evaluated for the definition of the group.