Maizotemnus archaeios gen. 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)

Abstract

The presence of mammals in the Maíz Gordo Formation (Salta Province) has been restricted with doubts to a single specimen assigned to the basal notoungulate Simpsonotus. However, the stratigraphic origin of this specimen is uncertain. Here, we present the first mammalian fossil remain from the Maíz Gordo Formation (late Paleocene–early Eocene) collected with precise geographical and stratigraphic control, originating from the uppermost levels of this unit that are chronologically dating at 56 Ma (Thanetian–Ypresian boundary). This specimen represents the first South American mammal record confidently contemporaneous with the Paleocene–Eocene Thermal Maximum. IBIGEO-P 143 is a poorly preserved left maxillary fragment with M2–3 and represents the new notoungulate Maizotemnus archaeios gen. nov. sp. nov., which differs from the other “isotemnids” by, mainly, the following combination of features: M2–3 with low crown height, being wider than long, with a subtriangular contour; transversally shallow parastylar groove with a poorly-developed parastyle, not labially projected; the metacone column is more pronounced in M3 than in M2; weak both mesial and distal cingula and without lingual and labial cingula; the central fossette is oblique, being more transverse and narrower than that of the other “isotemnids”; and M3 with a sub-rounded metastyle and with its distal wall interrupted by a transversally and longitudinally deep distal notch. 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, but no sister taxon of this new taxon can be defined. The discovery of M. archaeios gen. nov. sp. nov. increases the diversity of basal toxodontians, extends the geographic and stratigraphic distribution of Toxodontia, and represents (based on dates) the oldest record for this suborder.