The Great American Interchange was an important zoogeographic event about three million years ago. Land and freshwater fauna migrated between North America and South America.
The migration happened during the Pliocene, 3.6–2.6 million years ago (mya). The volcanic Isthmus of Panama rose from the seafloor and bridged the two continents.
The land bridge in what is now Panama joined the Neotropic (roughly South America) and Nearctic (roughly North America) ecozones to form the Americas.
The interchange is visible from both stratigraphy and nature. Its most dramatic effect is on the distribution of mammals, but weak-flying or flightless birds, reptiles, amphibians, arthropods and even freshwater fish also migrated.
The differences in the fauna of North and South America had been known for some time. Both Humboldt and Darwin discussed it. The interchange as a concept was first fully laid out in 1876 by the "father of biogeography", Alfred Russel Wallace. Wallace had spent 1848–1852 exploring and collecting specimens in the Amazon Basin. Others who made significant contributions to understanding the event in the century that followed include Florentino Ameghino and George Gaylord Simpson.
Similar interchanges occurred earlier in the Cainozoic, when the former Gondwana continents of India and Africa made contact with Eurasia, about 50 and 30 mya respectively.
Plate tectonics
When the global continent Pangaea broke up early in the Jurassic period, the great southern supercontinent Gondwana was formed. It separated from Laurasia 200–180 mya.
In stages, Gondwana also broke up, eventually into half-a-dozen continents we recognise today. They were Australasia, Indian subcontinent, Africa, Madagascar, Antarctica and South America.
South America drifted west from Africa, starting about 130 mya in the Lower Cretaceous. There was open sea between them by 110 mya. The last connection of South America with any part of Gondwana was a link with West Antarctica, which broke in the Oligocene, only 30 mya.
Relevance to biogeography
When Gondwana was separated from Laurasia, evolution of most animals and plants went on separately in the two great supercontinents. Later, as Gondwana broke up, most of its fauna and flora also evolved separately.
This is not obvious in Africa and India, since they moved north and joined the Eurasian continent long ago. Animals and plants moved north and south there also, but the original pattern is now hard to see. But in Australasia and South America the pattern is clear.
The mammals are a good example. Eutherian mammals evolved in Laurasia but few got to Gondwana before it was separated. Earlier groups of mammals did get to Gondwana. They were the marsupials, the monotremes and other (now extinct) therian mammals.
Thus, when South America was completely separated, it had only the earlier types of mammals, including some early eutherians such as the Xenarthra. The groups which evolved later, and which dominate the northern continents, only got to South America by the Great American Interchange.
South America's endemic fauna
After the late Mesozoic breakup of Gondwana, South America spent most of the Cainozoic era as an island continent.
Its "splendid isolation" allowed the fauna to evolve into many forms found nowhere else on earth, most of which are now extinct.
Early mammals
The endemic mammals initially consisted of metatherians (including marsupials, xenarthrans, and a diverse group of South American ungulates.
Marsupials appear to have travelled from South America through Antarctica to Australasia in the late Cretaceous or early Tertiary.
Ratites (relatives of South American tinamous) probably migrated by this route about the same time, in the direction from South America towards Australia/New Zealand.
Other taxa that may have dispersed by the same route (if not by flying or rafting) are parrots, chelid turtles and (extinct) meiolaniid turtles.
One living South American marsupial, the tiny Monito del Monte, is more closely related to Australian marsupials than to other South American marsupials. Since it is the most 'basal' (= primitive) australidelphian we know, its group probably evolved in South America and then colonized Australia.
A 61 mya old platypus-like monotreme fossil from Patagonia may be an Australian immigrant.
Marsupials in South America included didelphimorphs (opossums), shrew opossums and several other small groups.