The Paleontology of Gran Barranca: Evolution and Environmental Change through the Middle Cenozoic of Patagonia

MADDEN, R.H. ; CARLINI AA; VUCETICH MG; KAY, RF

CAMBRIDGE UNIVERSITY PRESS

Lugar: Cambrigge; Año: 2010 p. 448

978-0-521-87241-6

Explosive Plinian volcanism provided a rich local source of fine-grained volcaniclastic sediments to central Patagonia in the middle Cenozoic. The chronology of the accumulation extends from the Middle Eocene Climate Optimum (MECO) to the Middle Miocene Climate Optimum (MMCO) and includes the only southern hemisphere continental record across the Eocene–Oligocene transition (EOT) and into the Oi-1 glaciation of Antarctica. The continuity of sedimentation is broken by hiatuses of different temporal magnitude that appear to correspond chronologically with changing sea level and the intensity of erosion. The Sarmiento Formation at Gran Barranca is subdivided into six members, Gran Barranca (41.6–38.7 Ma), Rosado (38.7 Ma), Lower Puesto Almendra (37 Ma), Vera (35–33.3 Ma), Upper Puesto Almendra (31.1–26.3 Ma), and Colhue-Huapi (20.4–18.7 Ma). The sequence of fossil mammal faunas in the Sarmiento Formation at Gran Barranca is the standard sequence of South American Land Mammal Ages (SALMAs) and their subdivisions. The sequence includes the Barrancan SALM Subage, an intermediate level “El Nuevo,” the Mustersan SALMA, the Tinguirirican SALMA, an early Deseadan level “La Cantera,” the Deseadan SALMA, Colhuehuapian SALMA, and a Pinturan level. The taxonomic composition and most distinctive features of each of these levels at Gran Barranca is summarized, with special attention to the new and rich record of small mammals recovered by intensive sampling by wet washing. Given the climate intimacy between narrow peninsular Patagonia, the southeast Pacific and South Atlantic oceans, and the vast Southern Ocean today, the terrestrial environments of middle Cenozoic Patagonia were probably subject to the influence of global climate change as recorded in the marine sea-floor record. Evidence that the terrestrial biota responded to the trends and events recorded in marine sediments of the southern oceans depends on an understanding of how the magnitude and rate of this climate change drives evolutionary and biotic change, and whether it leaves an imprint on terrestrial sediments and their fossil archives. We summarize the fossil mammal record at Gran Barranca in light of the chronology of climate change, but are cautious in our conclusions as appropriate to the limitations imposed by deep time. During the MECO, phytolith assemblages include graminoid elements (pooids and chloridoids) as components of savanna and/or forest communities. Climate change subsequent to the MECO is recorded at Gran Barranca by the last occurrence of crocodilians and an episode of coincident increase in hypsodonty in several clades of mammalian herbivores. Important change in paleosols, depositional environments, and ichnofacies suggest a general aridification and cooling at the close of the Barrancan and into the Mustersan. Volcanic activity intensified at this time. Pooid and chloridoid grasses become dissociated in the presence of indicators of plant stress conditions or aridification, the oldest evidence for the adaptation of chloridoid grasses to dry environments. The EOT is preserved in the sediments of the Vera Member at Gran Barranca and the mammals of Tinguirirican SALMA. The mammals document an increase in the overall proportion of hypsodont taxa along with the continued persistence of primitive taxa within the same clades, suggesting species richness was responding to a diversification of diets in addition to the rigors of environmental abrasives. It is tempting to attribute this to the massively bedded volcaniclastic loessites, evidence of reworking by more persistent eolian processes that may have accompanied Oi-1 and an intensification of the developing circum- Antarctic flow. The first appearance of rodents in Patagonia at about 30 Ma occurs some 3 m.y. after significant change in the small marsupial fauna at the EOT. The MMCO coincides with the presence of primates in Patagonia between 20.2 and 15.7 Ma. The mammal fauna of the Colhuehuapian is especially rich in small marsupials and rodents. Most climate indicators accord with the view that Patagonian climates at this time were warm and humid. The South American mammal fauna evolved in geographic isolation from other continents through most of the middle Cenozoic. This geographic isolation began with the final separation of South America from Antarctica in the late Eocene prior to the opening of Drake Passage to deep-water circulation. Within Patagonia, a reduction of geographic area and possibly subdivision bymarine barriers, may have accompanied middle Cenozoic marine transgressions, especially prior to the EOT. The arrival of primates and rodents from Africa and their radiations must represent significant perturbations to continental isolation, but their impact has yet to be fully assessed. The evolution of high-crowned teeth was a major feature of mammal evolution in South America throughout the Cenozoic, as a general faunal trend of increasing proportions of taxa with high-crowned teeth, as successive independent radiations of high-crowned taxa within clades, and as events of coincident increase involving diverse clades. The environmental context of these trends and events are explored.