First Paleogene fossil mammal from Magallanes, Patagonia, Chile

Abstract

Paleogene fossil mammals marks the initial steps towards the assembly of the highly autochthonous Cenozoic South American faunas, and the rise of evolutionary and ecological novelties within lineages that spread across the continent. Although common to many areas, their greatest diversity and knowledge undoubtedly comes from central Patagonia, where superb exposures in Argentina’s extra-Andean and Atlantic coastal zones have been profusely studied since the second half of the 19th century. In harsh contrast, Paleogene mammals from Chilean Patagonia have only recently begun to be known, based on novel discoveries mostly conducted in the Aysén Region. Here we present the first detailed description of a Paleogene mammal from the austral, Magallanes Region, composed of an isolated lower molar identified as a left m1 or m2. The tooth was exhumed from estuarine deposits of the Upper Member of the Río Turbio Formation in Sierra Baguales, along with thousands of chondrichthyan teeth, and other fossil remains belonging to bony fishes, aquatic mammals, and diverse sauropterygians. The particular morphology and dimension of the tooth allow its recognition as Albertogaudrya unica Ameghino 1901, a large, middle to early late Eocene member of the order Astrapotheria, previously recorded in central Patagonia of the Chubut Province, Argentina, and the Aysén Region, in Chile. The occurrence of A. unica in Sierra Baguales extends by more than 400 kilometers to the south the presence of this iconic Casamayoran through Mustersan? age species, also providing a new element to the biogeographic discussions about the cohesive character of the austral faunas, and the early conformation of the regional provincialism that has characterized Patagonia during much of the Cenozoic. It also helps to constrain a 40-37 Ma age for the fossiliferous levels of the Upper Member of the Río Turbio Formation at Sierra Baguales, consistent with previous biochronologic and geochronologic results. Along with the detailed morphological description of the new specimen, we also comment on taphonomic aspects of the depositional context, its regional chronological and zoogeographic importance, and some particularities of the occlusal morphology of the molar, which shows an interesting and unusual pattern of enamel wear whose development and functionality are also discussed.