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Great American Interchange
Paleozoographic event resulting from the formation of the Isthmus of Panama
Examples of migrant species in both Americas. Olive green silhouettes denote North American species with South American ancestors; blue silhouettes denote South American species of North American origin.
Metatherians (and a few xenarthran armadillos like 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 extant relatives are the seriemas. (Similar large terrestrial predatory birds, the bathornithids, were found in North America during the early Cenozoic, but they died out in the Early Miocene, about 20 million years ago.) Through the skies over late Miocene South America (6 Ma ago) soared the largest flying bird known, the teratornArgentavis, with a wing span of 6 m or more, which may have subsisted in part on the leftovers of Thylacosmilus kills. Terrestrial ziphodont[n 5]sebecid (metasuchian) 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 (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.
The giant anteater, Myrmecophaga tridactyla, the largest living descendant of South American's early Cenozoic mammalian fauna
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.
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.
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.
Tortoises also arrived in South America in the Oligocene. It was long thought that they had come from North America, but a recent comparative genetic analysis concludes that the South American genus Chelonoidis (formerly part of Geochelone) is actually most closely related to African hingeback tortoises.[n 7] 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 Panamaland bridge formed, around 7.3 Ma ago. This was South America's first eutherian carnivore. South American procyonids then diversified into forms now extinct (e.g. the "dog-coati" Cyonasua, which evolved into the bear-like Chapalmalania). However, all extant procyonid genera appear to have originated in North America. It has been suggested that 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.[n 8] 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 up 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 the "Baudo pathway", an early land bridge that was probably incomplete and required some swimming and island-hopping to traverse. These claims, based on excavations in Amazonian Peru, have been viewed with caution by other investigators, due to the lack of corroborating finds from other sites. The limited evidence for these early immigrants may reflect their presence primarily in the Amazon basin, an area where fewer fossils have been collected. These taxa are: a proboscidean (Amahuacatherium),[n 9]peccaries (Sylvochoerus and Waldochoerus),tapirs and a palaeomerycid (from a family probably ancestral to cervids), Surameryx; the paleomerycids were probably unable to successfully colonize South America.
Similarly, megalonychid and mylodontid ground sloths island-hopped to North America by 9 Ma ago.Megalonychids had colonized the Antilles previously, by the early Miocene. (Megatheriid and nothrotheriid ground sloths did not migrate north until the formation of the isthmus.) 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. This was due to the prevailing direction of oceanic currents, rather than to a competition between North and South American forms. (Except in the case of Jamaica, oryzomyine rodents of North American origin were able to enter the region only after invading South America.)
The Great American Biotic Interchange (GABI)
Illustration focusing on the exchange of megafauna families (mostly now extinct) during the interchange
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. (There were no extinctions in North America plainly linked to South American immigrants.[n 11]) Although terror birds were able to invade part of North America, their success was temporary; this lineage disappeared about 1.8 million years ago (smaller members of the group survived in South America until much more recently). Native South American ungulates also did poorly, with only a handful of genera withstanding the northern onslaught. (It has long been recognized that several of the largest forms, macraucheniids and toxodontids, survived to the end of the Pleistocene. Recent fossil finds indicate that one species of the horse-like proterotheriid litopterns did as well. The notoungulate mesotheriids and hegetotheriids also managed to hold on at least part way through the Pleistocene.)[A] On the other hand, South America's small marsupials 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,[n 12][n 13] including the elimination of the largest forms (e.g. the dinomyids). With the exception of the North American porcupine and several extinct porcupines and capybaras, however, they did not migrate past Central America.[n 14]
The southwardly migrating Nearctic carnivorans took over South American large terrestrial predator niches, displacing phorusrhacids and sparassodonts,[n 15] as well as eliminating Chapalmalania. It has been argued that canids probably played the major role in the extinction of sparassodonts; they are ecologically and morphologically more similar to most of them than other carnivorans, and are also the most diverse family of modern carnivorans on the continent. The paucity of early competition and plentiful prey seems to have allowed short-faced bears to rapidly evolve into the largest known bear or terrestrial mammalian carnivore species; Arctotherium angustidens is estimated to have weighed around 1600 kg. Later species of Arctotherium exhibited a trend towards smaller size and a more omnivorous diet, probably due to increasing competition from later-arriving or evolving carnivores. In contrast, Smilodon showed a trend toward increasing body size that culminated in the appearance of S. populator, at up to nearly 500 kg the most massive felid known.
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.[n 16] Native South American forms made up about 75% of these species. However, none of these megaherbivores have survived.
The Virginia opossum, 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 most successful northward migrants, reaching as far as Canada and Alaska, respectively. Most major groups of xenarthrans were present in North America up until the end-PleistoceneQuaternary extinction event (as a result of at least eight successful invasions of temperate North America, and at least six more invasions of Central America only). 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.
The effect of formation of the isthmus on the marine biota of the area was the inverse of its effect on terrestrial organisms, a development that has been termed the "Great American Schism". The connection between the east Pacific Ocean and the Caribbean (the Central American Seaway) was severed, setting now-separated populations on divergent evolutionary paths. Caribbean species also had to adapt to an environment of lower productivity after the inflow of nutrient-rich water of deep Pacific origin was blocked. The Pacific coast of South America cooled as the input of warm water from the Caribbean was cut off. This trend is thought to have caused the extinction of the marine sloths of the area.
Reasons for success or failure
A north-south climatic asymmetry in the Americas. Tropical climate zones, which are warm year-round and moist at least part of the year (blue zones Af, Am and Aw), cover much of South America and nearly all of Central America, but very little of the rest of North America.
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 moist tropical conditions. 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 (see map on right) presented was particularly acute for Neotropic species specialized for tropical rainforest environments, who had little prospect of penetrating beyond Central America. As a result, Central America currently has 41 mammal species of Neotropical origin,[n 20] compared to only 3 for temperate North America. However, species of South American origin (marsupials, xenarthrans, caviomorph rodents and monkeys) still comprise only 21% of species from nonflying, nonmarine mammal groups in Central America, while North American invaders constitute 49% of species from such groups in South America. Thus, climate alone cannot fully account for the greater success of species of Nearctic origin during the interchange.
Land areas over which ancestors of Neotropic (green) and Nearctic (red) species could wander via two-way migrations during the latter part of the Cenozoic prior to the interchange. The smaller area available for Neotropic species to evolve in tended to put them at a competitive disadvantage.
The second and more important advantage geography gave to the northerners is related to the land area available for their ancestors to evolve in. During the Cenozoic, North America was periodically connected to Eurasia via Beringia, allowing repeated migrations back and forth to unite the faunas of the two continents.[n 21] Eurasia was connected in turn to Africa, which contributed further to the species that made their way to North America.[n 22] South America, on the other hand, was connected only to Antarctica and Australia, two much smaller and less hospitable continents, and only in the early Cenozoic. Moreover, this land connection does not seem to have carried much traffic (apparently no mammals other than marsupials and perhaps a few monotremes ever migrated by this route), particularly in the direction of South America. This means that Northern Hemisphere species arose over a land area roughly six times greater than was available to South American species. North American species were thus products of a larger and more competitive arena,[n 23] where evolution would have proceeded more rapidly. They tended to be more efficient and brainier,[n 24][n 25] generally able to outrun and outwit their South American counterparts, who were products of an evolutionary backwater. These advantages can be clearly seen in the cases of ungulates and their predators, where South American forms were replaced wholesale by the invaders.
The greater eventual success of South America's African immigrants compared to its native early Cenozoic mammal fauna is another example of this phenomenon, since the former evolved over a greater land area; their ancestors migrated from Eurasia to Africa, two significantly larger continents, before finding their way to South America.
Against this backdrop, the ability of South America's xenarthrans to compete effectively against the northerners represents a special case. The explanation for the xenarthrans' success lies in part in their idiosyncratic approach to defending against predation, based on possession of body armor and/or formidable claws. The xenarthrans did not need to be fleet-footed or quick-witted to survive. Such a strategy may have been forced on them by their low metabolic rate (the lowest among the therians). Their low metabolic rate may in turn have been advantageous in allowing them to subsist on less abundant and/or less nutritious food sources. Unfortunately, the defensive adaptations of the large xenarthrans would have offered little protection against humans armed with spears and other projectiles.
At the end of the Pleistocene epoch, about 12,000 years ago, three dramatic developments occurred in the Americas at roughly the same time (geologically speaking). 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 Great American Interchange, 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 (cougars and jaguars may have been temporarily reduced to South American refugia also). Others, such as capybaras, survived in their original range but died out in areas they had migrated to. Notably, this extinction pulse eliminated all Neotropic migrants to North America larger than about 15 kg (the size of a big porcupine), and all native South American mammals larger than about 65 kg (the size of a big capybara or giant anteater). 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.
Baird's tapir, 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 there is no plausible general climate-based megafauna-killing mechanism that could explain the continent-wide extinctions. The climate change took place worldwide, but had little effect on the megafauna in areas like Africa and southern Asia, where megafaunal species had coevolved with humans. Numerous very similarglacial retreats had occurred previously within the ice age of the last several Ma without ever producing comparable waves of extinction in the Americas or anywhere else.
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 down to North America. The reason that a number of groups went extinct in North America but lived on in South America (while there are no examples of the opposite pattern) 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.[n 27][n 28]
South American invasions of North America exclusive of Central America
+American lion (Panthera leo atrox), reported from Peru and Argentina and Chile; however, the former set of remains has later been identified as belonging to a jaguar and the latter set of remains were initially identified as being from jaguars
^During the Eocene, astrapotheres and litopterns were also present in Antarctica.
^Sequencing of collagen from fossils of one recently extinct species each of notoungulates and litopterns has indicated that these orders comprise a sister group to the perissodactyls. Mitochondrial DNA obtained from Macrauchenia corroborates this and gives an estimated divergence date of 66 Ma ago.
^Once in Australia, facing less competition, marsupials diversified to fill a much larger array of niches than in South America, where they were largely carnivorous.
^It is the sister group to a clade containing all other extant australidelphians (roughly 238 species).
^Ziphodont (lateromedially compressed, recurved and serrated) teeth tend to arise in terrestrial crocodilians because, unlike their aquatic cousins, they are unable to dispatch their prey by simply holding them underwater and drowning them; they thus need cutting teeth with which to slice open their victims.
^It is also notable that both simian primates (ancestral to monkeys) and hystricognath rodents (ancestral to caviomorphs) are believed to have arrived in Africa by rafting from Eurasia about 40 Ma ago.
^An alternative explanation blames climatic and physiographic changes associated with the uplift of the Andes.
^The status of Amahuacatherium is controversial, and it is sometimes considered as a misinterpreted fossil of a Pleistocene gomphothere.
^Of the 6 families of North American rodents that did not originate in South America, only beavers and mountain beavers failed to migrate to South America. (However, human-introduced beavers have become serious pests in Tierra del Fuego.)
^In this connection, however, it is interesting to note that chalicotheres, clawed perissodactyl herbivores ecologically similar to ground sloths, died out in North America in the Miocene about 9 Ma ago, while they survived to the early Pleistocene in Asia and Africa.
^Of the 11 extant families of South American caviomorph rodents, 5 are present in Central America; only 2 of these, Erethizontidae and Caviidae, ever reached North America. (The nutria/coypu has been introduced to a number of North American locales.)
^The more dog-like sparassodonts, borhyaenids and similar types, were already in decline before the main pulse of the interchange, at a time when Thylacosmilus and phorusrhacids were still common. Suggested reasons for this decline include competition with phorusrhacids, carnivorous opossums, or early-arriving procyonids. However, it is clear that the remaining sparassodonts and most of the phorusrhacids (Titanis and the much smaller Psilopterus being exceptions) disappeared quickly once canids and felids reached South America.
^P. S. Martin (2005), pp. 30-37, 119. The figure of 25 South American megaherbivore species breaks down as follows: 4 gomphotheres, 2 camelids, 9 ground sloths, 5 glyptodonts, and 5 toxodontids. This can be compared to Africa's present and recent total of 6 megaherbivores: 1 giraffe, 1 hippo, 2 rhinos and 2 elephants (considering the African forest elephant as a separate species).
^Including extinct genera, South America has hosted 9 genera of cervids, 8 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. There is significant disagreement 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 (excluding Urocyon and Canis, although sometimes transferring C. gezi to the South American group) 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.
^Canis, e.g. Canis dirus, was present in South America until the end of the Pleistocene.
^According to data on the EQ (encephalization quotient, a measure of the brain to body size ratio adjusted for the expected effect of differences in body size) of fossil ungulates compiled by H. Jerison, North American ungulates showed a trend towards greater EQs going from the Paleogene to the Neogene periods (average EQs of 0.43 and 0.64, respectively), while the EQs of South American ungulates were static over the same time interval (average EQ unchanged at 0.48). This analysis was later criticized. Jerison subsequently presented data suggesting that native South American ungulates also lagged in the relative size of their neocortex (a measurement not subject to the vagaries of body mass estimation). It is interesting to note that the late survivor Toxodon had one of the highest EQ values (0.88) among native Neotropic ungulates.
Jerison also found that Neogene xenarthrans had low EQs, similar to those he obtained for South American ungulates.
^The estimated EQ of Thylacosmilus atrox, 0.41 (based on a brain mass of 43.2 g, a body mass of 26.4 kg, and an EQ of 43.2/[0.12*26400^(2/3)]), 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 (based on body mass estimates of 330 and 175 kg, respectively).
^A number of recently extinct North American (and in some cases also South American) taxa such as tapirs, equids, camelids, saiga antelope, proboscids, dholes and lions survived in the Old World, probably mostly for different reasons - tapirs being a likely exception, since their Old World representative survived only in the rainforests of Southeast Asia. (Cheetahs in the broadest sense could be added to this list, although the New and Old World forms are in different genera.) Old World herbivores may in many cases have been able to learn to be vigilant about the presence of humans during a more gradual appearance (by development or migration) of advanced human hunters in their ranges. In the cases of predators, the Old World representatives in at least some locations would thus have suffered less from extinctions of their prey species. In contrast, the musk ox represents a rare example of a megafaunal taxon that recently became extinct in Asia but survived in remote areas of arctic North America (its more southerly-distributed relatives such as Harlan's musk ox and the shrub ox were less fortunate).
^ abcThis listing currently has fairly complete coverage of mammals, but only spotty coverage of other groups. Crossings by nonflying mammals and birds occurred during the last 10 Ma. Crossings by fish, arthropods, rafting amphibians and reptiles, and flying bats and birds were made before 10 Ma ago in many cases. Taxa listed as invasive did not necessarily cross the isthmus themselves; they may have evolved in the adopted land mass from ancestral taxa that made the crossing.
^Mixotoxodon remains have been collected in Central America and Mexico as far north as Veracruz and Michoacán, with a possible find in Tamaulipas; additionally, one fossil tooth has been identified in eastern Texas, United States.
^Salamanders apparently dispersed to South America by the Early Miocene, about 23 Ma ago. Nevertheless, the salamander fauna of South America, which is restricted to the tropical region, consists of only 2 clades, and has fewer species and is far less diverse than that of much smaller Central America. Salamanders are believed to have originated in northern Pangea, perhaps not long before it separated to become Laurasia, and are not present anywhere else in the Southern Hemisphere (see the world salamander distribution map). In contrast, caecilians have a mostly Gondwanandistribution. Apart from a small region of overlap in southern China and northern Southeast Asia, Central America and northern South America are the only places in the world where both salamanders and caecilians are present.
^Hippidion, a relatively short-legged equid that developed in South America after invading from North America about 2.5 Ma ago, has traditionally been thought to have evolved from pliohippines. However, recent studies of the DNA of Hippidion and other New World Pleistocene horses indicate that Hippidion is actually a member of Equus, closely related to the extant horse, E. ferus. Another invasion of South America by Equus occurred about one Ma ago, and this lineage, traditionally viewed as the subgenus Equus (Amerhippus), appears indistinguishable from E. ferus. Both these lineages became extinct at the end of the Pleistocene, but E. ferus was reintroduced from Eurasia by Europeans in the 16th century. Note: the authors of the DNA sequence study of Equus (Amerhippus) use "E. caballus" as an alternative specific name for "E. ferus".
^Not to be confused with the American mastodon (Mammut americanum), a proboscid from a different family whose remains have been found no further south than Honduras.
^Sometimes classified as elephantids rather than as gomphotheres.
^Condors apparently reached South America by the late Miocene or early Pliocene (4.5 - 6.0 Ma ago), several million years before the formation of the isthmus. Condor-like forms in North America date back to the Barstovianstage (middle Miocene, 11.8 - 15.5 Ma ago).
^ The native South American ungulates dwindled gradually as North American ungulates invaded and diversified. The changes in number and composition of South America's ungulate genera over time are given in the table below. The Quaternary extinction event that delivered the coup de grâce to the native Neotropic ungulates also dealt a heavy blow to South America's ungulate immigrants.
Change in number of South American ungulate genera over time
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