FIRST PALEOGENE FOSSIL MAMMAL OF MAGALLANES, PATAGONIA, CHILE

Abstract

Paleogene fossil mammals mark the initial steps towards the assembly of the highly autochthonous Cenozoic South American faunas, and the rise of evolutionary and ecological novelties that spread across the continent. Although common to many areas of the continent, their greatest diversity and knowledge undoubtedly comes from central Patagonia, where superb outcrops in Argentina's extra-Andean and Atlantic coast have been profusely studied since the second half of the 19th century. In harsh contrast, Paleogene mammals from Chilean Patagonian 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 Rio Turbio Formation in Sierra Baguales, along with thousands of chondrichthyan teeth, and other fossil remains including fishes, aquatic mammals, and diverse sauropterygians. The particular morphology and dimension of the tooth allows 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 constrains 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 topography of the molar, which shows an interesting and unusual pattern of enamel wear, whose development and functionality is also discussed.