Astrapotheria from the Las Flores Formation (Lower Eocene) and the dawn of astrapotheres in Patagonia

Abstract

The oldest records of the South American Order Astrapotheria in high latitudes belong to the early Eocene Kibenikhoria faunal zone (Shecenia ctirneru) and the Ernestokokenia faunal zone (Trigonostylops sp.), both within the Las Flores Formation in central Patagonia. A recent study concluded that all astrapotheres from these assemblages belong to a single taxon determined as Trigonostylopidae aff. Trigonostylops, and that the two units share more astrapothere and other ungulate taxa than previously proposed. Here we re-evaluate the taxonomic status of the specimens previously interpreted as astrapotheres from these units, report new materials from the Ernestokokenia faunal zone, and describe for the first time astrapotheres from the Las Flores Formation at its type area (Yacimiento Las Flores). For the latter, we identify the presence of Tetragonostylops, likely Trigonostylops but still unconfirmed, and a probable new taxon. Based on their stratigraphic position and available radiometric dates (>51.4 Ma), these are the oldest geochronologically calibrated records of Astrapotheria. For the Kibenikhoria faunal zone, we conclude that Shecenia ctirneru is the only documented astrapothere and it is distinct from Trigonostylops and from any other known astrapothere. Also from this unit, the enigmatic Seudenius cteronc (previously regarded as Trigonostylops sp.) is not even an astrapothere. For the Ernestokokenia faunal zone, new specimens confirm the occurrence of Trigonostylops as the only recorded astrapothere. Our assessment of these and other ungulates reveals that the Kibenikhoria and the Ernestokokenia faunal zones share no astrapothere species, and confirms that the latter shares more ungulate taxa with the Casamayoran assemblages.