Miocene long-lived lake Pebas as a stage of mollusc radiations, with implications for landscape evolution in western Amazonia

The Miocene Pebas system was a huge (> 1 million km2) system of long-lived lakes and wetlands that occupied most of western Amazonia between c. 23 and 8 Ma. Remarkable endemic radiations of molluscs and ostracods occurred in the Pebas system. The continuity of many of the endemic lineages between c. 17 and 9 Ma shows that the system was never fully replaced by fluvial or marine settings. Many of the endemic invertebrate groups developed an unusual range of morphologies that reflect adaptation to specific ecological stresses provided by the Pebas system, such as chemical stress, common dysoxia and high predation pressure. Mollusc diversity increased especially during the Middle Miocene. The Pebas system provided pathways for mobile marine organisms to transfer into freshwater biotopes, and at the same time obstructed exchange of terrestrial biota between the tropical Andes and the Guyana region. Short-lived, lowland aquatic corridors over northern shield regions and through the Ecuadorian Andean region almost certainly existed. The Pebas system was terminated just before the establishment of the modern Amazon system (slightly before 8 Ma), possibly coinciding with a single, wide ranging marine incursion into lowland Amazonia. With the termination of the Pebas system, the endemic mollusc fauna became largely extinct. The termination of the Pebas system provided large tracts of land for the establishment and development of terrestrial biota in western Amazonia. Subsequent diversification in lowland Amazonia has been enhanced by the edaphic heterogeneity of the Pebas Formation deposits.