Paleogeographic influences on freshwater fish distributions in northeastern Brazil

Geological and climatic events are widely understood to affect biodiversity, especially in the species-rich rain-forests of Greater Amazonia, yet similar studies have lagged in less diverse regions of tropical South America. Here we assess the roles of paleogeographic and paleoenvironmental conditions on the formation of freshwater fish assemblages in the semi-arid regions of northeastern Brazil, in coastal rivers of the states of Maranhao and Piaui. This study combines data from the geographic distributions of freshwater fish species, the geological history of the region, and simulations studies of riverine connectivity patterns arising from sea-level fluctuations over the past 10 million years. We identify two main types of paleogeographic events that structured the regional ichthyofauna: uplift of mountains and structural arches by tectonic reactivation of intraplate lineaments (faults), and changes in shorelines (marine transgressions/regressions) due to eustatic sea-level fluctuations. These events altered the courses of large coastal rivers, changed the connectivity patterns within and among rivers basins through river captures at the headwaters and along the coastal plain, and profoundly affected the processes of dispersal, speciation and extinction that produced the unique fish fauna of the Maranhao region.