Paleoenvironmental evolution of southern South America during the Cenozoic

Southern South America (SSA) has today a high diversity of climates, environments, biomes, and biotas, as a result of the complex interaction through time of plants and animals with the geological forces (e.g. plate tectonics, sea-level changes, glaciations) that modulated the geography of the continent. Arid biomes are well represented in SSA today, but were and biomes similarly important in the geologic past? How long in time can be found major and biomes in SSA? With the aim of replaying these questions, in this paper we summarized the paleoenvironmental changes of SSA through Cenozoic, emphasizing the relationships between biomes and the geological forces that, through different climatic-environmental factors, have driven its evolution. We define SSA the south of the 15 degrees S area. We prefer this geographical delimitation because with it we can see and follow the history of the biogeographic (historic and ecologic) relationships of Patagonian biota with the rest of the South American biota. Additionally, with this delimitation we possess the most complete Cenozoic South American land-mammal fossil record. We use biomes because biomes are taxon-free analytical units, and their pattern of change can be traced through time independently of the taxa present at different geological periods. Data on plate tectonics, volcanism, sea-level changes, marine pal eo temperatures, and glaciations were taken from literature. To analyse the pattern of change of southern South American climates and environments through the Cenozoic, we used the fossil record of land-mammals as information source. When available, the record of vascular plants were used to contrast the inferences derived from land-mammals. Finally, we used standard geologic divisions (i.e. Epochs) as chronological units. The main conclusion of this paper is that from Early Paleocene to Late Pleistocene, southern South American climatic conditions changed from warm, wet, and non-seasonal, to colder, dryer, and seasonal. Concomitantly, biomes changed from tropical forest to steppes, across a sequence constituted by subtropical forests, woodland savanna, park-savanna, and grassland savanna. During the Quaternary, and as a consequence of glacial cycles, cold and dry conditions were interrupted by warmer and wet periods. Accordingly, several pulses of expansion and retraction of steppes (and, concomitantly, advances and retreats of the northern tropical forests) are recorded. This cyclic pattern of changes produced the provincialism that has characterized the South American biota from Early Pleistocene to the present. (c) 2006 Elsevier Ltd. All rights reserved.