Great American Interchange


The Great American Biotic Interchange, also known as the Great American Interchange or Great American Faunal Interchange, was an important late Cenozoic 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. Although earlier dispersals have occurred, probably over water, the migration accelerated dramatically about 2.7 million years ago during the Piacenzian age. It resulted in the joining of the Neotropic and Nearctic biogeographic realms definitively to form the Americas. The interchange is visible from observation of both biostratigraphy and nature. 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. 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 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, xenarthrans, and a diverse group of native ungulates: notoungulates, 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.
, Dromiciops gliroides, South America's only australidelphian marsupial

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. Paleognath birds may made a similar migration around the same time to Australia/New Zealand. Other taxa that may have dispersed by the same route are parrots, chelid turtles, and meiolaniid turtles.
Marsupials present in South America included didelphimorphs and several other small groups; larger predatory relatives of these also existed, such as the borhyaenids and the saber-toothed Thylacosmilus. As the large carnivorous metatherians declined, and before the arrival of most types of carnivorans, predatory opossums such as Thylophorops temporarily attained larger size.
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. Sparassodonts and giant opossums shared the ecological niches for large predators with fearsome flightless "terror birds", whose closest extant relatives are the seriemas. Through the skies over late Miocene South America 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 crocodyliforms 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. They shared their habitat with one of the largest turtles of all time, the 3.3 m Stupendemys.
, Myrmecophaga tridactyla, the largest living descendant of South American's early Cenozoic mammalian fauna
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, 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, 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.

Island-hopping 'waif dispersers'

The invasions of South America started about 40 Ma ago, 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. 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 and westward oceanic currents. Crossings of the ocean were accomplished when at least one fertilised female accidentally floated over on driftwood or mangrove rafts. 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 attained sizes on the order of or larger.
Later 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. 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 nonflying mammals previously resident on the continent.
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, molossid bats may have reached South America from Africa in as many as five dispersals, starting in the Eocene. Emballonurid bats may have also reached South America from Africa about 30 Ma ago, based on molecular evidence. Vespertilionid bats may have arrived in five dispersals from North America and one from Africa. Natalid bats are thought to have arrived during the Pliocene from North America via the Caribbean.
, Chelonoidis carbonaria
Tortoises also arrived in South America in the Oligocene. They were long thought to have come from North America, but a recent comparative genetic analysis concludes that the South American genus Chelonoidis is actually most closely related to African hingeback tortoises. Tortoises are aided in oceanic dispersal by their ability to float with their heads up, and to survive up to six months without food or water. South American tortoises then went on to colonize the West Indies and Galápagos Islands. A number of clades of American geckos seem to have rafted over from Africa during both the Paleogene and Neogene. Skinks of the related genera Mabuya and Trachylepis apparently dispersed across the Atlantic from Africa to South America and Fernando de Noronha, respectively, during the last 9 Ma. Surprisingly, South America's burrowing amphisbaenians and blind snakes also appear to have rafted from Africa, as does the hoatzin, a weak-flying bird of South American rainforests.
The earliest traditionally recognized mammalian arrival from North America was a procyonid that island-hopped from Central America before the Isthmus of Panama land bridge formed, around 7.3 Ma ago. This was South America's first eutherian carnivore. South American procyonids then diversified into forms now extinct. However, all extant procyonid genera appear to have originated in North America. The first South American procyonids may have contributed to the extinction of sebecid crocodilians by eating their eggs, but this view has not been universally viewed as plausible. The procyonids were followed to South America by rafting/island-hopping hog-nosed skunks and sigmodontine rodents. The oryzomyine tribe of sigmodontine rodents went on to colonize the Lesser Antilles to Anguilla.
One group has proposed that a number of large Neartic herbivores actually reached South America as early as 9–10 Ma ago, in the late Miocene, via an early incomplete land bridge. These claims, based on fossils recovered from rivers in southwestern Peru, have been viewed with caution by other investigators, due to the lack of corroborating finds from other sites and the fact that almost all of the specimens in question have been collected as float in rivers without little to no stratigraphic control. These taxa are a gomphothere, peccaries, tapirs and a palaeomerycid, Surameryx. The identification of Amahuacatherium and the dating of its site is controversial; it is regarded by a number of investigators as a misinterpreted fossil of a different gomphothere, Notiomastodon, and biostratigraphy dates the site to the Pleistocene. The early date proposed for Surameryx has also been met with skepticism.
Megalonychid and mylodontid ground sloths island-hopped to North America by 9 Ma ago. A basal group of sloths had colonized the Antilles previously, by the early Miocene. Terror birds may have also island-hopped to North America as early as 5 Ma ago.
The Caribbean Islands were populated primarily by species from South America, due to the prevailing direction of oceanic currents, rather than to a competition between North and South American forms.

The Great American Biotic Interchange

The formation of the Isthmus of Panama led to the last and most conspicuous wave, the great interchange, starting around 2.7 Ma ago. This included the immigration into South America of North American ungulates, proboscids, carnivorans and a number of types of rodents. The larger members of the reverse migration, besides ground sloths and terror birds, were glyptodonts, pampatheres, capybaras, and the notoungulate Mixotoxodon.
In general, the initial net migration was symmetrical. Later on, however, the Neotropic species proved far less successful than the Nearctic. This difference in fortunes was manifested in several ways. Northwardly migrating animals often were not able to compete for resources as well as the North American species already occupying the same ecological niches; those that did become established were not able to diversify much, and in some cases did not survive for long. Southwardly migrating Nearctic species established themselves in larger numbers and diversified considerably more, and are thought to have caused the extinction of a large proportion of the South American fauna. Native South American ungulates did poorly, with only a handful of genera withstanding the northern onslaught. South America's small marsupials, though, survived in large numbers, while the primitive-looking xenarthrans proved to be surprisingly competitive and became the most successful invaders of North America. The African immigrants, the caviomorph rodents and platyrrhine monkeys, were less impacted by the interchange than most of South America's 'old-timers', although the caviomorphs suffered a significant loss of diversity, including the elimination of the largest forms. With the exception of the North American porcupine and several extinct porcupines and capybaras, however, they did not migrate past Central America.
Due in large part to the continued success of the xenarthrans, one area of South American ecospace the Nearctic invaders were unable to dominate was the niches for megaherbivores. Before 12,000 years ago, South America was home to about 25 species of herbivores weighing more than 1000 kg, consisting of Neotropic ground sloths, glyptodonts, and toxodontids, as well as gomphotheres and camelids of Nearctic origin. Native South American forms made up about 75% of these species. However, none of these megaherbivores has survived.
, Didelphis virginiana, the only marsupial in temperate North America
Armadillos, opossums and porcupines are present in North America today because of the Great American Interchange. Opossums and porcupines were among the most successful northward migrants, reaching as far as Canada and Alaska, respectively. Most major groups of xenarthrans were present in North America until the end-Pleistocene Quaternary extinction event. Among the megafauna, ground sloths were notably successful emigrants; four different lineages invaded North America. A megalonychid representative, Megalonyx, spread as far north as the Yukon and Alaska, and might well have invaded Eurasia had a suitable habitat corridor across Beringia been present.
Generally speaking, however, the dispersal and subsequent explosive adaptive radiation of sigmodontine rodents throughout South America was vastly more successful than any northward migration of South American mammals. Other examples of North American mammal groups that diversified conspicuously in South America include canids and cervids, both of which currently have three or four genera in North America, two or three in Central America, and six in South America.

Canid genera by continent

refn|group=n|Including extinct genera, South America has hosted nine genera of cervids, eight genera of mustelids, and 10 genera of canids. However, some of this diversity of South American forms apparently arose in North or Central America prior to the interchange. Significant disagreement exists in the literature concerning how much of the diversification of South America's canids occurred prior to the invasions. A number of studies concur that the grouping of endemic South American canids is a clade. However, different authors conclude that members of this clade reached South America in at least two, three to four, or six invasions from North America.refn|group=n|Canis, e.g. Canis dirus, was present in South America until the end of the Pleistocene.refn|name=canidlisting|group=n

Disappearance of South America's native predators

During the last 7 Ma, South America's terrestrial predator guild has changed from one composed almost entirely of nonplacental mammals, birds, and reptiles to one dominated by immigrant placental carnivorans. It was originally thought that the native South American predator guild, including sparassodonts, carnivorous opossums like Thylophorops and Hyperdidelphys, armadillos such as Macroeuphractus, terror birds, and teratorns, as well as early-arriving immigrant Cyonasua-group procyonids, were driven to extinction during the GABI by competitive exclusion from immigrating placental carnivorans, and that this turnover was abrupt. However, the turnover of South America's predator guild was more complex, with competition only playing a limited role.
In the case of sparassodonts and carnivorans, which has been the most heavily studied, little evidence shows that sparassodonts even encountered their hypothesized placental competitors. Many supposed Pliocene records of South American carnivorans have turned out to be misidentified or misdated. Sparassodonts appear to have been declining in diversity since the middle Miocene, with many of the niches once occupied by small sparassodonts being increasingly occupied by carnivorous opossums, which reached sizes of up to ~8 kg. Whether sparassodonts competed with carnivorous opossums or whether opossums began occupying sparassodont niches through passive replacement is still debated. Borhyaenids last occur in the late Miocene, about 4 Ma before the first appearance of canids or felids in South America. Thylacosmilids last occur about 3 Ma ago and appear to be rarer at pre-GABI Pliocene sites than Miocene ones.
In general, sparassodonts appear to have been mostly or entirely extinct by the time most nonprocyonid carnivorans arrived, with little overlap between the groups. Purported ecological counterparts between pairs of analogous groups neither overlap in time nor abruptly replace one another in the fossil record. Procyonids dispersed to South America by at least 7 Ma ago, and had achieved a modest endemic radiation by the time other carnivorans arrived. However, procyonids do not appear to have competed with sparassodonts, the procyonids being large omnivores and sparassodonts being primarily hypercarnivorous. Other groups of carnivorans did not arrive in South America until much later. Dogs and weasels appear in South America about 2.9 Ma ago, but do not become abundant or diverse until the early Pleistocene. Bears, cats, and skunks do not appear in South America until the early Pleistocene. Otters and other groups of procyonids have been suggested to have dispersed to South America in the Miocene based on genetic data, but no remains of these animals have been found even at heavily sampled northern South American fossil sites such as La Venta, which is only 600 km from the Isthmus of Panama.
Other groups of native South American predators have not been studied in as much depth. Terror birds have often been suggested to have been driven to extinction by placental carnivorans, though this hypothesis has not been investigated in detail. Titanis dispersed from South America to North America against the main wave of carnivoran migrations, being the only large native South American carnivore to accomplish this. However, it only managed to colonize a small part of North America for a limited time, failing to diversify and going extinct in the early Pleistocen] ; the modest scale of its success has been suggested to be due to competition with placental carnivorans. Terror birds also decline in diversity after about 3 Ma ago. At least one genus of relatively small terror birds, Psilopterus, appears to have survived to as recently as about 96,000 years ago.
The native carnivore guild appears to have collapsed completely roughly 3 Ma ago, not correlated with the arrival of carnivorans in South America, with terrestrial carnivore diversity being low thereafter. This has been suggested to have opened up ecological niches and allowed carnivorans to establish themselves in South America due to low competition. A meteor impact 3.3 million years ago in southern South America has been suggested as a possible cause of this turnover, but this is still controversial. A similar pattern occurs in the crocodilian fauna, where modern crocodiles dispersed to South America during the Pliocene and became the dominant member of crocodilian communities after the late Miocene extinction of the previously dominant large native crocodilians such as the giant caiman Purussaurus and giant gharial Gryposuchus, which is thought to be related to the loss of wetlands habitat across northern South America.
Whether this revised scenario with a reduced role for competitive exclusion applies to other groups of South American mammals such as notoungulates and litopterns is unclear, though some authors have pointed out a protracted decline in South American native ungulate diversity since the middle Miocene. Regardless of how this turnover happened it is clear that carnivorans benefitted from it. Several groups of carnivorans such as dogs and cats underwent an adaptive radiation in South America after dispersing there and the greatest modern diversity of canids in the world is in South America.

Reasons for success or failure

The eventual triumph of the Nearctic migrants was ultimately based on geography, which played into the hands of the northern invaders in two crucial respects. The first was a matter of climate. Any species that reached Panama from either direction obviously had to be able to tolerate. Those migrating southward would then be able to occupy much of South America without encountering climates that were markedly different. However, northward migrants would have encountered drier and/or cooler conditions by the time they reached the vicinity of the Trans-Mexican Volcanic Belt. The challenge this climatic asymmetry presented was particularly acute for Neotropic species specialized for tropical rainforest environments, which had little prospect of penetrating beyond Central America. As a result, Central America currently has 41 mammal species of Neotropical origin,

Central American opossum species

refn|group=n|During the Miocene alone, between about 23 and 5 Ma ago, 11 episodes of invasions of North America from Eurasia have been recognized, bringing a total of 81 new genera into North America.refn|group=n|The combination of Africa, Eurasia and North America was termed the "World Continent" by George Gaylord Simpson.refn|group=n|Simpson, 1950, p. 368refn|group=n|According to data on the EQ of fossil ungulates compiled by H. Jerison, North American ungulates showed a trend towards greater EQs going from the Paleogene to the Neogene periods, while the EQs of South American ungulates were static over the same time interval. This analysis was later criticized. Jerison subsequently presented data suggesting that native South American ungulates also lagged in the relative size of their neocortices. Interestingly, the late survivor Toxodon had one of the highest EQ values among native Neotropic ungulates.

Jerison also found that Neogene xenarthrans had low EQs, similar to those he obtained for South American ungulates.refn|group=n|The estimated EQ of Thylacosmilus atrox, 0.41, is high for a sparassodont, but is lower than that of modern felids, with a mean value of 0.87. Estimates of 0.38 and 0.59 have been given for the EQ of much larger Smilodon fatalis.

Late Pleistocene extinctions

At the end of the Pleistocene epoch, about 12,000 years ago, three dramatic developments occurred in the Americas at roughly the same time. Paleoindians invaded and occupied the New World, the last glacial period came to an end, and a large fraction of the megafauna of both North and South America went extinct. This wave of extinctions swept off the face of the Earth many of the successful participants of the GABI, as well as other species that had not migrated.
All the pampatheres, glyptodonts, ground sloths, equids, proboscids, giant short-faced bears, dire wolves, and machairodont species of both continents disappeared. The last of the South and Central American notoungulates and litopterns died out, as well as North America's giant beavers, lions, dholes, cheetahs, and many of its antilocaprid, bovid, cervid, tapirid and tayassuid ungulates. Some groups disappeared over most or all of their original range, but survived in their adopted homes, e.g. South American tapirs, camelids, and tremarctine bears. Others, such as capybaras, survived in their original range, but died out in areas to which they had migrated. Notably, this extinction pulse eliminated all Neotropic migrants to North America larger than about 15 kg, and all native South American mammals larger than about 65 kg. In contrast, the largest surviving native North American mammal, the wood bison, can exceed 900 kg, and the largest surviving Nearctic migrant to South America, Baird's tapir, can reach 400 kg.
and †Glyptodon
, Tapirus bairdii, the largest surviving Nearctic migrant to South America
The near-simultaneity of the megafaunal extinctions with the glacial retreat and the peopling of the Americas has led to proposals that both climate change and human hunting played a role. Although the subject is contentious, a number of considerations suggest that human activities were pivotal. The extinctions did not occur selectively in the climatic zones that would have been most affected by the warming trend, and no plausible general climate-based megafauna-killing mechanism could explain the continent-wide extinctions. The climate change took place worldwide, but had little effect on the megafauna in Africa and southern Asia, where megafaunal species had coevolved with humans. Numerous :File:Five Myr Climate Change.png|very similar :File:Atmospheric CO2 with glaciers cycles.gif|glacial retreats had occurred previously within the ice age of the last several million years without ever producing comparable waves of extinction in the Americas or anywhere else.
Similar megafaunal extinctions have occurred on other recently populated land masses at different times that correspond closely to the first arrival of humans at each location. These extinction pulses invariably swept rapidly over the full extent of a contiguous land mass, regardless of whether it was an island or a hemisphere-spanning set of connected continents. This was true despite the fact that all the larger land masses involved contained multiple climatic zones that would have been affected differently by any climate changes occurring at the time. However, on sizable islands far enough offshore from newly occupied territory to escape immediate human colonization, megafaunal species sometimes survived for many thousands of years after they or related species became extinct on the mainland; examples include giant kangaroos in Tasmania, giant Chelonoidis tortoises of the Galápagos Islands, giant Dipsochelys tortoises of the Seychelles, giant meiolaniid turtles on Lord Howe Island, New Caledonia and Vanuatu, ground sloths on the Antilles, Steller's sea cows off the Commander Islands and woolly mammoths on Wrangel Island and Saint Paul Island.
The glacial retreat may have played a primarily indirect role in the extinctions in the Americas by simply facilitating the movement of humans southeastward from Beringia to North America. The reason that a number of groups went extinct in North America but lived on in South America appears to be that the dense rainforest of the Amazon basin and the high peaks of the Andes provided environments that afforded a degree of protection from human predation.

South American invasions of North America exclusive of Central America

or extinct North American taxa whose ancestors migrated out of South America:
Fish
Amphibians
Birds
Mammals
Extant or extinct Central American taxa whose ancestors migrated out of South America:
Invertebrates
Fish
Amphibians
Reptiles
Birds
Mammals
Extant or extinct South American taxa whose ancestors migrated out of North America :
Amphibians
Reptiles
Birds
Mammals