A new chronology for middle Eocene-early Miocene South American Land Mammal Ages

Abstract

Cenozoic South American Land MammalAges (SALMAs) have historically beencorrelated to the geologic time scale using40Ar/39Ar dating and magnetostratigraphy.At Gran Barranca (68.7°W, 45.7°S)?one ofSouth America?s key areas for constrainingSALMAs?existing radioisotopic ages haveuncertainties of up to 4 m.y. To better constrainthe ages of mammalian assemblages,we employed high-precision (±<40 k.y.) U-Pbdating using single zircon crystals. We datednine tuffs from the Sarmiento Formationcontaining middle Eocene?early Miocenefaunas (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 CanchaTuff, occurring within the Tinguiriricanfaunal level yielded an age of 33.581 ± 0.015Ma, confi rming that the Vera Member containsthe only fossiliferous geologic sectionencompassing the Eocene?Oligocene transitionin the Southern Hemisphere. The pre-Deseadan fauna, La Cantera, is ≤30.77 Ma,the age of the Colhuehuapian is expanded to21.1?20.1 Ma, and the Pinturan may be asold as ca. 19 Ma.The new U-Pb dates confi rm that atmospherictemperatures and vegetationremained constant across the Eocene?Oligocene transition in Patagonia and thathypsodonty occurred in South American ungulatesmuch earlier than on any other continent.Additionally, refi nement of the SALMAboundaries will eventually provide the contextnecessary to compare faunal transitionsacross continents, although currently toomuch data are missing to allow such comparisons.Finally, the new ages provide a highresolutionage model from which hypothesesabout rates of environmental and evolutionarychange at Gran Barranca can be tested.