AEOLOSAURUS COLHUEHUAPENSIS SP. NOV. (SAUROPODA, TITANOSAURIA), FROM THE BAJO BARREAL FORMATION, UPPER CRETACEOUS OF ARGENTINA. A new sauropod, Aeolosaurus colhuehuapensis. sp. nov., based on an articulated caudal series recovered from an emergent island in Lago Colhue Huapi, Chubut Province is described. This series is characterized by neural arches located very anteriorly above the centrum and inclined forward; well-developed prezygapophyses projecting forward and upward, and wide articular facets in the anterior caudals. Aeolosaurus colhuehuapensis sp. nov. is unique by the presence of a marked and deep fossa between the transverse process and the base of the neural spine in the most anterior caudal vertebrae; a slender lamina that connects the base of the prespinal lamina with the lower part of the articular facet in the medial part of the prezygapophysis, defining a fossa on both sides of the prespinal lamina; posterior condyle centrally placed in the entire caudal series. The genus Aeolosaurus is recorded in the Campanian-Maastrichtian in Argentina and Brazil, and is considered part of a faunal association with hadrosaurids, Madtsoiinae, Chelidae, Sudamericidae, and non-tribosphenic Theria in floodplain or littoral environments. Its presence in the Upper Member of the Bajo Barreal Formation, along with other paleontological records from the same locality, and geologic observations allow us to assign preliminarily a Campanian-Maastrichtian? age for the upper levels of this stratigraphic unit outcropping in the southeast of Lago Colhue Huapi, Chubut Province.