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

Abstract

Cenozoic South American Land Mammal Ages (SALMAs) have historically been correlated to the geologic time scale using 40 Ar/ 39 Ar 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, confi rming 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 confi rm 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 conti-nent. Additionally, refi nement 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 highresolution age model from which hypotheses about rates of environmental and evolutionary change at Gran Barranca can be tested.

New Paleogene Notohippids and Leontiniids (Toxodontia; Notoungulata; Mammalia) from the Early Oligocene Tinguiririca Fauna of the Andean Main Range, Central Chile

“…Three different collectors (Ameghino, Feruglio, and Simpson) thus independently recovered E. obscurus from the highest levels of beds once considered Mustersan, but that are now regarded as pertaining to the Tinguirirican SALMA (Wyss et al, 1994;Bond et al, 1996Bond et al, , 1997Kay et al, 1999;Flynn et al, 2003;Gelfo et al, 2009). This age inference is supported by recent studies at Gran Barranca that have documented Eomorphippus only at level GBV-4 ("La Cancha"), an interval referred to the Tinguirician SALMA ) (see also : Ré et al, 2010;and Dunn et al, 2013). Moreover, well-sampled deposits of undisputed Mustersan age in Patagonia have not yielded E. obscurus, despite extensive collecting efforts there, and the occurrence of other notohippid species at these localities (M. Bond, personal commun.).…”

Decoupling the spread of grasslands from the evolution of grazer-type herbivores in South America

“…The age model for the phytolith assemblages is based on the improved chronostratigraphic framework provided by Dunn et al 18 (Supplementary Table S3), but our conclusions are not dependent upon it. Ages for phytolith samples were based on stratigraphic position between horizons of known age, as determined from 40 Ar/ 39 Ar 17 and 206 Pb/ 238 U 18 radioistopic dating, or magnetostratigraphic correlation 16,18 . These age estimates assume constant sedimentation rate between dated horizons except where thick tuff units occur.…”

“…The base of the UPA is dated by the La Cantera Tuff (r30.77 Myr ago) 18 , and the upper part occurs below a basalt flow dated at 27.76 ± 0.08 Myr ago 17 . The basalt flow forms an unconformable surface but provides a maximum age for the top of the unit.…”

“…1). Extensive magneto- 16 and chronostratigraphic work 17,18 there provides a robust temporal framework for detailed study of ecosystem change. We analysed 53 representative phytolith assemblages distributed throughout the Sarmiento Formation; five additional assemblages were analysed along the 7-km outcrop of a single prominent ash bed to investigate spatial heterogeneity in vegetation.…”

Patagonian Eocene Archaeopithecidae Ameghino, 1897 (Notoungulata): systematic revision, phylogeny and biostratigraphy

“…Ré et al, 2010;Dunn et al, 2013;Bellosi and Krause, 2014;Krause et al, 2017). The fossiliferous localities with archaeopithecids are geographically located in Chubut Province, Argentina (Figure 1).…”