On the Origin of Amazonian Landscapes and Biodiversity: A Synthesis

Abstract

In northern South America the Cenozoic was a period of intense tectonic and climatic interaction that resulted in a dynamic Amazonian landscape dominated by lowlands with local and shield-derived rivers. These drainage systems constantly changed shape and size. During the entire Cenozoic, the Brazilian and Guiana Shields were stable mountainous areas. Andean-derived river systems increased in importance especially in the Neogene. A remarkable feature in western Amazonian history is the waxing and waning of large lake systems and embayments. By the Late Miocene (about 11 Ma), the Andes were connected with the Atlantic through an incipient Amazon River, and from c. 7 Ma Andean-derived river systems became fully established in central and eastern Amazonia and the modern landscape configuration had developed. Rainforests already existed in northern South America during the Paleogene, but the modern rainforests - with resemblance to the Present forest - only developed during the Miocene. The western Amazonian Miocene record contains very diverse aquatic faunas (molluscs, ostracods, turtles, crocodiles, fishes) as well as terrestrial mammals. Remarkable gigantic forms thrived in Amazonian ecosystems at the time. Since the Late Miocene, edaphically heterogeneous lands emerged in western Amazonia in areas previously occupied by lake systems. At the same time nutrient-rich deposits spread over central and eastern Amazonia, an event that, based on molecular phylogenetic studies on extant taxa, coincided with diversification of terrestrial taxa. Molecular-based time estimates confirm the steady diversification and mostly pre-Quaternary origin of extant Amazonian taxa. A significant portion of the current species richness is attributed to a combination of relatively constant wet and warm climates and a heterogeneous edaphic substrate. The Quaternary was a time of distribution shifts, but can no longer be considered a time of diversification in Amazonia.