Great American Interchange

The Great American Biotic Interchange (commonly abbreviated as GABI), also known as the Great American Interchange and the Great American Faunal Interchange, was an important late Cenozoic paleozoogeographic biotic interchange event in which land and freshwater fauna migrated from North America to South America via Central America and vice versa, as the volcanic Isthmus of Panama rose up from the sea floor, forming a land bridge between the previously separated continents. Although earlier dispersals had occurred, probably over water, the migration accelerated dramatically about 2.7 million years ( Ma) ago during the Piacenzian age. It resulted from the joining of the Neotropic (roughly South American) and Nearctic (roughly North American) biogeographic realms definitively to form the Americas. The interchange is visible from observation of both biostratigraphy 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. Coastal and marine biota were affected in the opposite manner; the formation of the Central American Isthmus caused what has been termed the Great American Schism, with significant diversification and extinction occurring as a result of the isolation of the Caribbean from the Pacific.

The occurrence of the interchange was first discussed in 1876 by the "father of biogeography", Alfred Russel Wallace. Wallace had spent five years 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. The Pliocene timing of the formation of the connection between North and South America was discussed in 1910 by Henry Fairfield Osborn.

Analogous interchanges occurred earlier in the Cenozoic, when the formerly isolated land masses of India and Africa made contact with Eurasia about 56 and 30 Ma ago, respectively.

Before the interchange

Isolation of South America

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 known as the Meridiungulata: notoungulates (the "southern ungulates"), litopterns, astrapotheres, pyrotheres and xenungulates. A few non-therian mammals – monotremes, gondwanatheres, dryolestids and possibly cimolodont 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.

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 (Ameridelphia); however, it is the most basal australidelphian, meaning that this superorder arose in South America and then dispersed to Australia after the monito del monte split off. ''Monotrematum'', a 61-Ma-old platypus-like monotreme fossil from Patagonia, may represent an Australian immigrant. Paleognath birds (ratites and South American tinamous) may have made a similar migration around the same time to Australia and New Zealand. Other taxa that may have dispersed by the same route (if not by flying or oceanic dispersal) are parrots, chelid turtles, and the extinct meiolaniid turtles.

Marsupials remaining in South America included didelphimorphs (opossums), paucituberculatans (shrew opossums) and microbiotheres (monitos del monte). Larger predatory relatives of these also existed, such as the borhyaenids and the saber-toothed '' Thylacosmilus''; these were sparassodont metatherians, which are no longer considered to be true marsupials. As the large carnivorous metatherians declined, and before the arrival of most types of carnivorans, predatory opossums such as '' Thylophorops'' temporarily attained larger size (about 7 kg).

Metatherians and a few xenarthran armadillos, such as ''Macroeuphractus'', 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 and giant opossums shared the ecological niches for large predators with fearsome flightless "terror birds" ( phorusrhacids), whose closest living relatives are the seriemas. North America also had large terrestrial predatory birds during the early Cenozoic (the related bathornithids), but they died out before the GABI in the Early Miocene, about 20 million years ago. Through the skies over late Miocene South America (6 Ma ago) soared one of the largest flying birds known, ''Argentavis'', a teratorn that had a wing span of 6 m or more, and which may have subsisted in part on the leftovers of ''Thylacosmilus'' kills. Terrestrial sebecid ( metasuchian) crocodyliforms with ziphodont teeth 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 was present, including pampatheres, the ankylosaur-like glyptodonts, predatory euphractines, 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, such as ''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 semibipedal, clawed browser like a chalicothere, and horned ''Trigodon'' looked like a rhinoceros. 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 typical boreoeutherian one, supplemented with Afrotherian proboscids.

Pre-interchange oceanic dispersals

The invasions of South America started about 40 Ma ago (middle Eocene), when caviomorph rodents arrived in South America. Their subsequent vigorous diversification displaced some of South America's small marsupials and gave rise to – among others – capybaras, chinchillas, viscachas, and New World porcupines. The independent development of spines by New and Old World porcupines is another example of parallel evolution. This invasion most likely came from Africa. The crossing from West Africa to the northeast corner of Brazil 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) and westward oceanic currents. Crossings of the ocean were accomplished when at least one fertilised female (more commonly a group of animals) accidentally floated over on driftwood or mangrove rafts. Hutias (Capromyidae) would subsequently colonize the West Indies as far as the Bahamas, reaching the Greater Antilles by the early Oligocene. Over time, some caviomorph rodents evolved into larger forms that competed with some of the native South American ungulates, which may have contributed to the gradual loss of diversity suffered by the latter after the early Oligocene. By the Pliocene, some caviomorphs (e.g., ''Josephoartigasia monesi'') attained sizes on the order of or larger.
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Later (by 36 Ma ago), primates followed, again from Africa in a fashion similar to that of the rodents. Primates capable of migrating had to be small. Like caviomorph rodents, South American monkeys are believed to be a clade (i.e., monophyletic). However, although they would have had little effective competition, all extant New World monkeys appear to derive from a radiation that occurred long afterwards, in the Early Miocene about 18 Ma ago. Subsequent to this, monkeys apparently most closely related to titis island-hopped to Cuba, Hispaniola, and Jamaica. Additionally, a find of seven 21-Ma-old apparent cebid teeth in Panama suggests that South American monkeys had dispersed across the seaway separating Central and South America by that early date. However, all extant Central American monkeys are believed to be descended from much later migrants, and there is as yet no evidence that these early Central American cebids established an extensive or long-lasting population, perhaps due to a shortage of suitable rainforest habitat at the time.

Fossil evidence presented in 2020 indicates a second lineage of African monkeys also rafted to and at least briefly colonized South America. '' Ucayalipithecus'' remains dating from the Early Oligocene of Amazonian Peru are, by morphological analysis, deeply nested within the family Parapithecidae of the Afro-Arabian radiation of parapithecoid simians, with dental features markedly different from those of platyrrhines. The Old World members of this group are thought to have become extinct by the Late Oligocene. '' Qatrania wingi'' of lower Oligocene Fayum deposits is considered the closest known relative of ''Ucayalipithecus''.

Remarkably, the descendants of those few bedraggled " waifs" that crawled ashore from their rafts of African flotsam in the Eocene now constitute more than twice as many of South America's species as the descendants of all the flightless mammals previously resident on the continent ( 372 caviomorph and monkey species versus 136 marsupial and xenarthran species).

Many of South America's bats may have arrived from Africa during roughly the same period, possibly with the aid of intervening islands, although by flying rather than floating. Noctilionoid bats ancestral to those in the neotropical families Furipteridae, Mormoopidae, Noctilionidae, Phyllostomidae, and Thyropteridae are thought to have reached South America from Africa in the Eocene, possibly via Antarctica. Similarly, free-tailed bats (Molossidae) may have reached South America from Africa in as many as five dispersals, starting in the Eocene. Emballonurids may have also reached South America from Africa about 30 Ma ago, based on molecular evidence. Vespertilionids may have arrived in five dispersals from North America and one from Africa. Natalids are thought to have arrived during the Pliocene

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