Great American Interchange

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The Great American Interchange was a very important paleozoogeographic event in which land and freshwater faunas migrated from Central America to South America and vice versa, as the volcanic Isthmus of Panama rose up from the sea floor and bridged the continents. The migration peaked dramatically around 3 Million years ago (Piacenzian, the 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 birds, arthropods, reptiles, amphibians and even freshwater fish to migrate.

South America was characterised by a strange endemic fauna, consisting only of xenarthrans (i.e., armadillos, sloths (like the giant ground sloth, Megatherium) and anteaters), notoungulates (the "alternative ungulates"), litopterns and marsupials. The marsupials present in South America were didelphimorphs (opossums and relatives), but many larger forms also existed, like the Miocene saber-toothed marsupial Thylacosmilus and the borhyaenids. The notoungulates and the litopterns occupied ungulate ecological niches and had many strange forms, like Macrauchenia, a litoptern with a small proboscis. Both groups started evolving in the Lower Paleocene, possibly from condylarth stock, diversified, 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) accidentally "rafting" (on driftwood for instance) southwards or northwards. Rodents gave – among others – rise to capybaras, chinchillas and viscachas.

A little later primates followed. The primates capable of migrating had to be small. These gave rise to the New World monkeys (Platyrrhini). But there is a theory that the ancestor of the South American monkeys came by rafting from West Africa to the northeast corner of Brazil; that crossing was much shorter then due to continental drift, and may have been aided by island-hopping (e.g. via St. Paul's Rocks, if they were an inhabitable island at the time).

Another chance early arrival from North America was a Carnivore: the "dog-coati" Cyonasua, which evolved into the bear-like Chapalmalania.

Around 7 Mya, raccoons invaded South America.

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

In general, the net migration was symmetrical. Later on, however, the Neotropic species proved far less successful than the Nearctic, witness the relatively low number of xenarthrans and marsupials in North America. This "bad luck" happened both ways: 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 successful northward migrants, reaching as far as Canada. Generally speaking, however, the rodents' dispersal and consequent adaptive radiation through South America was much more successful (both spatially and by number of species).