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The Great American Interchange was an important paleozoogeographic event in which land and freshwater fauna migrated from North America via Central America to South America and vice versa, as the volcanic Isthmus of Panama rose up from the sea floor and bridged the formerly separated continents. The migration peaked dramatically around three million years (Ma) ago during the Piacenzian age. It resulted in the joining of the Neotropic (roughly South America) and Nearctic (roughly North America) ecozones 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 reptiles, amphibians, arthropods, weak-flying or flightless birds, and even freshwater fish to migrate. The occurrence of the interchange was first discussed 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, W. D. Matthew, W. B. Scott, Bryan Patterson, George Gaylord Simpson and S. David Webb. Analogous interchanges occurred earlier in the Cenozoic, when the formerly isolated land masses of India and Africa made contact with Eurasia c. 50 and 30 Ma ago, respectively. ==South America’s endemic fauna== After the late Mesozoic breakup of Gondwana, South America spent most of the Cenozoic era as an island continent whose “splendid isolation” allowed its fauna to evolve into many forms found nowhere else on earth, most of which are now extinct. Its endemic mammals initially consisted primarily of metatherians (marsupials and sparassodonts), xenarthrans, and a diverse group of native ungulates: notoungulates (the “southern ungulates”), litopterns, astrapotheres (e.g. ''Trigonostylops'', ''Astrapotherium''), and pyrotheres (e.g. ''Pyrotherium''). A few nontherian mammals – monotremes, gondwanatheres, and possibly dryolestids and multituberculates – were also present in the Paleocene; while none of these diversified significantly and most lineages did not survive long, forms like ''Necrolestes'' and ''Patagonia'' remained as recently as the Miocene.〔Nicolás R. Chimento, Federico L. Agnolin and Fernando E. Novas (2015). "The bizarre ‘metatherians’ Groeberia and Patagonia, late surviving members of gondwanatherian mammals". Historical Biology: An International Journal of Paleobiology 27 (5): 603–623. doi:10.1080/08912963.2014.903945.〕 Marsupials appear to have traveled via Gondwanan land connections from South America through Antarctica to Australia in the late Cretaceous or early Tertiary. One living South American marsupial, the monito del monte, has been shown to be more closely related to Australian marsupials than to other South American marsupials; however, it is the most basal australidelphian, meaning that this superorder arose in South America and then colonized Australia after the monito del monte split off.〔 A 61-Ma-old platypus-like monotreme fossil from Patagonia may represent an Australian immigrant. It appears that ratites (relatives of South American tinamous) migrated by this route around the same time, more likely 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 floating across the ocean) are parrots, chelid turtles and (extinct) meiolaniid turtles. Marsupials present in South America included didelphimorphs (opossums) and several other small groups; larger predatory relatives of these also existed, like the borhyaenids and the sabertooth ''Thylacosmilus'' (sparassodont metatherians which are no longer considered to be true marsupials). Metatherians were the only South American mammals to specialize as carnivores; their relative inefficiency created openings for nonmammalian predators to play more prominent roles than usual (similar to the situation in Australia). Sparassodonts shared the ecological niches for large predators with fearsome flightless "terror birds" (phorusrhacids), whose closest extant relatives are the seriemas. Through the skies over late Miocene South America (6 Ma ago) soared the largest flying bird known, the teratorn ''Argentavis'', with a wing span of 6 m or more, which may have subsisted in part on the leftovers of ''Thylacosmilus'' kills. Terrestrial ziphodont sebecid crocodilians were also present at least through the middle Miocene and maybe to the Miocene-Pliocene boundary. Some of South America's aquatic crocodilians, such as ''Gryposuchus'', ''Mourasuchus'' and ''Purussaurus'', reached monstrous sizes, with lengths up to 12 m (comparable to the largest Mesozoic crocodyliforms). They shared their habitat with one of the largest turtles of all time, the 3.3 m (11 ft) ''Stupendemys''. Xenarthrans are a curious group of mammals that developed morphological adaptations for specialized diets very early in their history. In addition to those extant today (armadillos, anteaters and tree sloths), a great diversity of larger types were present, including pampatheres, the ankylosaur-like glyptodontids, various ground sloths, some of which reached the size of elephants (e.g. ''Megatherium''), and even semiaquatic to aquatic marine sloths. The notoungulates and litopterns had many strange forms, like ''Macrauchenia'', a camel-like litoptern with a small proboscis. They also produced a number of familiar-looking body types that represent examples of parallel or convergent evolution: one-toed ''Thoatherium'' had legs like those of a horse, ''Pachyrukhos'' resembled a rabbit, ''Homalodotherium'' was a semi-bipedal clawed browser like a chalicothere, and horned ''Trigodon'' looked like a rhino. Both groups started evolving in the Lower Paleocene, possibly from condylarth stock, diversified, dwindled before the great interchange, and went extinct at the end of the Pleistocene. The pyrotheres and astrapotheres were also strange but were less diverse and disappeared earlier, well before the interchange. The North American fauna was a pretty typical boreoeutherian one (supplemented with Afrotherian proboscids). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Great American Interchange」の詳細全文を読む スポンサード リンク
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