Great American Interchange

The Great American Interchange was a very important paleozoogeographic event in which land and freshwater animal faunas migrated from Central America to South America and vice versa, during the collision of both continents. The migration peaked dramatically around 3 Million years ago (Piacenzian, first half of the Upper Pliocene).

It resulted in the joining of the Neotropic (roughly South America) and Nearctic (roughly North America) definitively to form the Americas. The interchange is visible from observation of both stratigraphy and nature (neontology). Its most dramatic effect is on the zoogeography of mammals but it also gave an opportunity for non-flying arthropods and freshwater fish, like carps, to migrate.

South America was characterised by a strange endemic fauna, consisting only of xenarthrans, notoungulates (the "alternative ungulates"), litopterns and marsupials, like armadillos, sloths (like the giant ground sloth, Megatherium) and anteaters. The marsupials present in South America resembled opossums, but many larger forms also existed, like the Miocene sabre-toothed marsupial Thylacosmilus. The notoungulates and the litopterns occupied ungulate ecological niches and had many strange forms, like Macrauchenia. Both groups started evolving in the Lower Paleocene, dwindled before the great interchange, and went extinct in the Pleistocene.

The North American fauna was a pretty typical boreoeutherian one.

The interchange already started around 30 Mya (late Oligocene), when rodents started invading South America through island-hopping and (at least one fertilised female, more commonly a group of animals) accidentaly "rafting" (on driftwood for instance) southwards. Rodents gave – among others – rise to capybaras, chinchillas and viscachas. A little later apes followed. The apes capable of migrating had to be small. These gave rise to the New World monkeys (Platyrrhini).

Around 7 Mya, raccoons invaded South America.

The last and most conspicious wave, the great interchange, around 3 Mya, caused the immigration of llamas (also ungulates), mastodons, tapirs, felines (like pumas and sabre-toothed cats), canids, bears and horses.

In general, the netto-migration was symmetrical. Later on, however, the Neotropic species proved far less succesful than the Nearctic, witness the relatively low number of xenarthrans and marsupials in North America. This "bad luck" happened bothways: northwardly migrating animals were not able to compete for resources as well as the Nearctic species already occupying the same ecological niches. The southwardly migrating boreoeutherians are thought to have caused the extinction of some of the South American mammals. The presence of armadillos and opossums in the United States is explained by the Great American Interchange. Opossums were by far the most succesful northward migrants, reaching as far as Canada. Generally speaking, however, the rodents' dispersal and consequent adaptive radiation through South America was much more succesful (both spatially and by number of species).