| Great American Interchange |
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| biogeography | |
| natural history of central america | |
| natural history of the americas | |
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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 Arthropod s, Reptile s, Amphibian s and even freshwater Fish to migrate. South America was characterised by a strange Endemic fauna, consisting only of Xenarthra ns, Notoungulate s (the "alternative Ungulate s"), Litoptern s and Marsupial s, like Armadillo s, Sloth s (like the Giant Ground Sloth , ''Megatherium'') and Anteater s. 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 Borhyaenid s. The notoungulates and the litopterns occupied ungulate Ecological Niche s 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 Boreoeutheria n one. The interchange already started around 30 Mya (late Oligocene ), when Rodent s 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 Capybara s, Chinchilla s and Viscacha s. A little later Primate s followed. The primates capable of migrating had to be small. These gave rise to the New World Monkey s (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 , which evolved into the Bear -like Chapmanalia . Around 7 Mya, Raccoon s invaded South America. The last and most conspicuous wave, the great interchange, around 3 Mya, caused the immigration of Llama s (also ungulates), Mastodon s, Tapir s, Feline s (like Puma s and Saber-toothed Cats ), Canid s, Bear s and Horse s. 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). SOUTH AMERICAN SPECIES THAT MIGRATED NORTH... NORTH AMERICAN SPECIES THAT MIGRATED SOUTH...
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