Cenozoic South American Land Mammal Ages (SALMAs) have historically been correlated to the geologic time scale using 40Ar/39Ar dating and magnetostratigraphy. At Gran Barranca (68.7°W, 45.7°S)—one of South America’s key areas for constraining SALMAs—existing radioisotopic ages have uncertainties of up to 4 m.y. To better constrain the ages of mammalian assemblages, we employed high-precision (±<40 k.y.) U-Pb dating using single zircon crystals. We dated nine tuffs from the Sarmiento Formation containing middle Eocene–early Miocene faunas (Barrancan, Mustersan, Tinguirirican, Deseadan, Colhuehuapian, and “Pinturan”). The new dates span from 39.861 ± 0.037 Ma to 19.041 ± 0.027 Ma. The La Cancha Tuff, occurring within the Tinguirirican faunal level yielded an age of 33.581 ± 0.015 Ma, confirming that the Vera Member contains the only fossiliferous geologic section encompassing the Eocene–Oligocene transition in the Southern Hemisphere. The pre-Deseadan fauna, La Cantera, is ≤30.77 Ma, the age of the Colhuehuapian is expanded to 21.1–20.1 Ma, and the Pinturan may be as old as ca. 19 Ma.

The new U-Pb dates confirm that atmospheric temperatures and vegetation remained constant across the Eocene–Oligocene transition in Patagonia and that hypsodonty occurred in South American ungulates much earlier than on any other continent. Additionally, refinement of the SALMA boundaries will eventually provide the context necessary to compare faunal transitions across continents, although currently too much data are missing to allow such comparisons. Finally, the new ages provide a high-resolution age model from which hypotheses about rates of environmental and evolutionary change at Gran Barranca can be tested.

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