MODERNIZATION OF SOUTH AMERICAN FRESHWATER FISHES AND THEIR HABITATS

Direct fossil evidence shows that by the middle Miocene (ca. 15 Ma) the South American freshwater fish fauna was essentially modern across wide taxonomic and ecological ranges. There is a growing number of Cenozoic records of living fish clades including river sting rays, lungfishes, osteoglossids, a great variety characins and catfishes, electric eels, cichlids and percichthyids. Late Cretaceous fossils provide the earliest direct indications of a few extant groups. However, placing these fossils into their phylogenetic and global biogeographic contexts indicates a much deeper time frame in the Mesozoic for origins of some Neotropical groups. Overall then, it is clear that late Miocene though Holocene Earth history events and biotic factors played little or no role in creating the great diversity of Neotropical fishes at the levels of family, genus and species group. Fish diversification is no doubt ongoing, but since the Miocene this has been at low taxonomic levels.

During the Cretaceous and earliest Cenozoic several "archaic" fishes completely disappeared from South America. In contrast, the fossil record contains few documented extinctions of distinct lines of fishes that are phylogenetically close to living Neotropical groups. There are, however, many cases of late and post-Miocene regional extirpation of modern groups from areas peripheral to the large, lowland tropical rivers. Local extinctions are related to tectonic isolation of faunas in small watersheds and probably climatic change.

The distributions of both modern and fossil Neotropical fishes are consistent with geological and biological evidence for landform change that altered or created the courses and some aquatic habitats of the major drainage systems such as the Amazon, Paraná, Orinoco, Magdalena and Maracaibo.