Maizotemnus archaeiosgen. nov. sp. nov. the oldest Toxodontia (Mammalia, Panperissodactyla, Notoungulata) and the first South American mammal from the Paleocene–Eocene Thermal Maximum (Maíz Gordo formation, Salta Province, Argentina)

The presence of mammals in the Maíz Gordo Formation (late Paleocene–early Eocene), Salta Province (Argentina) has been restricted with doubts to a single specimen, which stratigraphic origin is also uncertain, assigned to the basal notoungulate Simpsonotus. Here, we report the first confidently stratigraphically controlled mammalian fossil from this unit, collected from its uppermost levels dated at 56 Ma (Thanetian–Ypresian boundary). The specimen IBIGEO-P 143 is a poorly preserved left maxillary fragment with M2–3 that constitutes a new notoungulate, Maizotemnus archaeios gen. nov. sp. nov., and it represents the first South American mammal record confidently contemporaneous with the Paleocene–Eocene Thermal Maximum. According to the cladistic study, M. archaeios gen. nov. sp. nov. is closely related to the three middle Eocene ‘isotemnids’: both species of Pampatemnus (Pa. deuteros and Pa. infernalis) from Northwestern Argentina and Anisotemnus distentus from Argentine Patagonia, though no direct sister taxon was identified. The discovery of M. archaeios gen. nov. sp. nov. increases the diversity of basal toxodontians, expands the known geographic and stratigraphic distributions of Toxodontia, and represents (based on dates) the oldest record for this suborder.