Great American Interchange

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True-color NASA mosaic of satellite images.

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 (in the Piacenzian, the first half of the Upper Pliocene).

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 birds, arthropods, reptiles, amphibians and even freshwater fish to migrate.

Similar interchanges occurred earlier in the Cenozoic when the formerly isolated land masses of India^[1] and Africa^[2] made contact with Eurasia c. 50 and 30 Ma ago, respectively.

[edit] South America's endemic fauna

The sabertooth "marsupial" †Thylacosmilus

The litoptern †Macrauchenia patagonica

†Astrapotherium magnum

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.^[3] Its endemic mammals initially consisted of marsupials, the closely related metatherian sparassodonts, xenarthrans, and a diverse group of native ungulates: notoungulates (the "southern ungulates"), litopterns, astrapotheres (e.g. Trigonostylops, Astrapotherium), and pyrotheres (e.g. Pyrotherium).

Marsupials may have traveled (via Gondwanan land connections) from South America through Antarctica to Australia and/or vice versa in the late Cretaceous or early Tertiary. One living S. American marsupial, the Monito del Monte, is believed to be more closely related to Australian marsupials than to other South American marsupials. A 61-Ma-old platypus-like monotreme fossil from Patagonia may represent another Australian immigrant. It appears that ratites (relatives of S. American tinamous) migrated by this route around the same time, more likely in the direction from S. America towards Australia/New Zealand.^[4] 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 (which are now considered to be non-marsupial metatherians).^[5] The relative inefficiency of the metatherians 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.^[6][7] Terrestrial ziphodont^{[note 1]} sebecid crocodilians were also present at least through the middle Miocene.^{[8][9][10][11]} 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.^[12]

Xenarthrans are a curious group of mammals that developed morphological adaptations for specialized diets very early in their history.^[13] In addition to those extant today (armadillos, anteaters and tree sloths), a great diversity of of other kinds were present, including pampatheres, the ankylosaur-like glyptodonts, various ground sloths, some of which reached the size of elephants (e.g. Megatherium), and even aquatic 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).

[edit] Island-hopping ‘waif dispersers’

Hydrochoerus hydrochaeris

Saguinus imperator

The invasions of South America started at least 31.5 Ma ago (late Eocene/early Oligocene), when cavimorph rodents arrived. Their subsequent vigorous diversification displaced some of S. 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.^[14] 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.^[15] 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. (Island-hopping cavimorphs would subsequently colonize the West Indies as far as the Bahamas). Over time, some cavimorph 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.^[3]

A little later (at least 25 Ma ago) primates followed, probably from Africa in a fashion similar to that of the rodents. Primates capable of migrating had to be small. With little effective competition they also diversified widely, giving rise to the New World monkeys. (Not long after arriving, monkeys apparently most closely related to titis island-hopped to Cuba, Hispaniola and Jamaica.) The South American cavimorph rodents and monkeys are both believed to be clades (i.e., monophyletic).

Tortoises also arrived in South America in the Oligocene. It was long thought that they had come from N. America, but a recent comparative genetic analysis concludes that S. American members of Geochelone are actually most closely related to African hingeback tortoises.^{[note 2][16]} 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.^[16] S. American tortoises then went on to colonize the West Indies and Galápagos Islands. Skinks of genus Mabuya apparently floated across the Atlantic from Africa during the last 9 Ma.^[17]

The earliest mammalian arrival from North America was a carnivorous procyonid that island-hopped from Central America prior to the formation of a land bridge, around 7 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.^[18] It has been suggested that the first S. 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.^[a][11] The procyonids were followed to S. America by island-hopping sigmodontine rodents^[19], peccaries and hog-nosed skunks.^[20]

Similarly, megalonychid and mylodontid ground sloths island-hopped to North America by 9 Ma ago.^[19] Megalonychids had colonized the Antilles previously, by the early Miocene.^[21] (Megatheriid ground sloths had to wait for the formation of the isthmus, but then sent several lineages north.)

[edit] The Great American Biotic Interchange

†Titanis walleri, the only known N. American terror bird.

The formation of the Isthmus of Panama led to the last and most conspicuous wave, the great interchange, around 3 Ma ago. This included the immigration of North American ungulates (including llamas, tapirs, deer and horses), proboscids (gomphotheres), carnivorans (including felids like cougars and saber-toothed cats, canids, mustelids, procyonids and bears) and a number of types of rodents^{[note 3]} into South America.

In general, the initial net migration was symmetrical. Later on, however, the Neotropic species proved far less successful than the Nearctic. This misfortune happened both 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 succeeded in becoming established did not diversify much. 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 N. America obviously attributable to S. American immigrants.) Although terror birds were initially able to invade N. America (they may have island-hopped over as early as 5 Ma ago),^[22] this success was temporary; all of the large Neotropic avian and metatherian predators ultimately disappeared. South America's native ungulates also fared very poorly, with only several of the largest forms, Macrauchenia and a few toxodontids, withstanding the northern onslaught. (Among the notoungulates, the mesotheriids and hegetotheriids did manage to survive into the Pleistocene.) Its small marsupials fared better, while the primitive-looking xenarthrans proved to be surprisingly competitive. The African immigrants, the cavimorph rodents and platyrrhine monkeys, generally held their own during the interchange, although the largest rodents (e.g. the dinomyids) seem to have disappeared. With the exception of the North American porcupine and several extinct porcupines and capybaras, however, they did not migrate past Central America.^{[note 4]}

The presence of armadillos, opossums, and porcupines in North America today is explained by the Great American Interchange. Opossums and porcupines were among most successful northward migrants, reaching as far as Canada. While only one example each of xenarthrans, marsupials and cavimorph rodents currently lives in the United States, 32 species from these taxa are present in tropical Central America. Prior to the end-Pleistocene extinctions, most major groups of xenarthrans were established in North America (as a result of at least seven 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; Megalonyx spread as far north as the Yukon^[23] and Alaska, and might have eventually reached Eurasia if the Quaternary extinction event had not intervened.

Generally speaking, however, the dispersal and subsequent explosive adaptive radiation of sigmodontine rodents throughout South America was much more successful (both spatially and by number of species) than any northward migration of S. American mammals. Other examples of N. American mammal groups that diversified conspicuously in S. America include canids and cervids, both of which currently have 4 genera in N. America, 2 or 3 in Central America, and 6 in S. America. Although Canis currently ranges only as far south as Panama, S. America still has more extant canid genera than any other continent.

[edit] 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. Obviously, any species that reached Panama from either direction 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.

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. The smaller area available for Neotropic species to evolve in tended to put them at a competitive disadvantage during the interchange.

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 multiple migrations back and forth to unite the faunas of the two continents. Eurasia was connected in turn to Africa, which contributed further to the species that made their way to North America. South America, on the other hand, was connected to Antarctica and Australia, two much smaller continents, only in the earliest part of the Cenozoic, and 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). Effectively, this means that northern hemisphere species arose over a land area roughly six times larger than was available to S. American species. This calculation may not be entirely fair, in that migrations between continents would have been more difficult and less frequent than migrations within S. America. Nevertheless, it is clear that N. American species were products of a larger and more competitive arena, where evolution would have proceeded more rapidly. They tended to be more efficient and brainier,^[b] generally able to outrun and outwit their S. American counterparts. These advantages can be clearly seen in the cases of ungulates and their predators, where S. American forms were replaced wholesale by the invaders.

Against this backdrop, the ability of S. 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).^[24][25] Their low metabolic rate may in turn have been advantageous in allowing them to specialize on less abundant and/or less nutritious food sources.

[edit] End-Pleistocene extinctions

Homo sapiens and †Glyptodon

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,^[26][27] dire wolves, lions and Smilodon 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, dholes, native cheetahs, scimitar cats, 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 S. American ranges also). Others, such as capybaras, survived in their original range but died out in areas they had migrated to.

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 South Asia, where megafaunal species had coevolved with humans. Numerous very similar glacial 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. Similar megafaunal extinctions have occurred on other recently populated land masses (e.g. Australia, Madagascar, New Zealand, and many smaller islands around the world, such as Cyprus, Crete, Tilos and New Caledonia) at different times that correspond closely to the first arrival of humans at each location. Additionally, on sizable islands far enough offshore from newly occupied territory to escape immediate human colonization, megafaunal species sometimes survived for thousands of years after they became extinct on the mainland; examples include meiolaniid turtles on Lord Howe Island and New Caledonia, ground sloths on the Antilles,^[28] Steller's sea cows off the Commander Islands^[29] and woolly mammoths on Wrangel Island^[30] and Saint Paul Island.^[31] The glacial retreat may have played a primarily indirect role in the extinctions by simply facilitating the movement of humans southeastward from Beringia down to N. America. The reason that a number of groups went extinct in N. America but lived on in S. America (while there are no notable examples of the opposite pattern) appears to be that the dense rain forests of the Amazon basin and the high peaks of the Andes provided environments that afforded a degree of protection from human predation.^{[note 5]}

[edit] South American invasions of North America exclusive of Central America

†Glyptotherium

The ground sloth †Megatherium.

Extant or extinct (†) North American taxa whose ancestors migrated out of South America during the last 10 Ma:^{[note 6]}

• Cichlids (Cichlidae: e.g. Herichthys cyanoguttatus) – freshwater fish that often tolerate brackish conditions
• Virginia Opossum (Didelphis virginiana)
• Armadillos (Dasypus novemcinctus, †Dasypus bellus, †Pachyarmatherium leiseyi)
• Pampatheres (†Plaina,^[32] †Holmesina) – large armadillo-like animals
• Glyptodonts (†Glyptotherium)
• Megalonychid Ground Sloths^{[note 7]} (†Pliometanastes, †Megalonyx)
• Mylodontid Ground Sloths (†Thinobadistes, †Glossotherium,^[32] †Paramylodon)
• Megatheriid Ground Sloths (†Eremotherium, †Nothrotheriops)
• New World porcupines (Erethizon dorsatum, †Erethizon poyeri, †E. kleini)
• Capybaras (†Neochoerus pinckneyi, †Hydrochaeris holmesi)
• Vampire Bats (†Desmodus stocki, †D. archaeodaptes)
• Dire Wolf (†Canis dirus) – which evolved from earlier N. American migrants
• Cougar (Puma concolor) – returning from a S. American refugium after N. American cougars were extirpated in the Pleistocene extinctions^[33]
• Terror Birds (†Phorusrhacidae: †Titanis walleri)
• Hummingbirds (Trochilidae)
• Tanagers (Thraupidae) – descended from earlier (perhaps Miocene) N. American migrants^[34]
• Tyrant Flycatchers (Tyrannidae)
• Parrots (Arini: Rhynchopsitta pachyrhyncha, †Conuropsis carolinensis)

[edit] South American invasions that failed to penetrate beyond Central America

Oophaga pumilio

The notoungulate †Toxodon.

Tinamus major

Extant or extinct (†) Central American taxa whose ancestors migrated out of South America during the last 10 Ma:^{[note 6]}

• Gonyleptid Harvestmen (Opiliones: Gonyleptidae)
• Electric Knifefishes (Gymnotiformes)
• Caeciliid Caecilians (Caecilia, Dermophis, Gymnopis, Oscaecilia) – snake-like amphibians^[35]
• Poison Dart Frogs (Dendrobatidae)
• Boine Boas (Boidae: Boinae)
• Spectacled Caiman (Caiman crocodilus)^[36]
• other Opossums (Didelphidae) – 12 additional extant species, listed on discussion page
• Northern Naked-tailed Armadillo (Cabassous centralis)
• Hoffmann's Two-toed Sloth (Megalonychidae: Choloepus hoffmanni)
• Three-toed Sloths (Bradypodidae: Bradypus variegatus, B. pygmaeus)
• Silky Anteater (Cyclopedidae: Cyclopes didactylus)
• other Anteaters (Myrmecophagidae: Myrmecophaga tridactyla, Tamandua mexicana)
• Toxodontids (†Mixotoxodon) – a rhino-sized notoungulate
• Rothschild's and Mexican Hairy Dwarf Porcupines (Coendou rothschildi, Sphiggurus mexicanus)
• other Caviomorph Rodents (Caviomorpha) – 9 additional extant species, listed on discussion page
• Platyrrhine Monkeys (Platyrrhini) – at least 8 extant species, listed on discussion page^[c]
• other Vampire Bats (Desmodontinae) – all 3 extant species
• Toucans (Ramphastidae)
• Tinamous (Tinamidae)
• Great Curassow (Crax rubra)

[edit] North American invasions of South America

Skull of †Cuvieronius hyodon, a gomphothere.

The coati Nasua nasua.

The saber-toothed cat †Smilodon.

Extant or extinct (†) South American taxa whose ancestors migrated out of North or Central America during the last 10 Ma:^{[note 6]}

• Lungless Salamanders^[d][35] (Bolitoglossa,^[37] Oedipina)
• Coral Snakes (Leptomicrurus, Micrurus)^[38][39]
• South American Rattlesnake (Crotalus durissus)^[40]
• Lanceheads (Bothrops)
• Bushmasters (Lachesis)
• other Pit Vipers (Bothriechis schlegelii, Bothriopsis, Porthidium)^[41]
• Small-eared Shrews (Cryptotis) – only present in NW S. America: Colombia, Venezuela, Ecuador, Peru
• Geomyid Pocket Gophers (Orthogeomys thaeleri) – one species, in Colombia
• Heteromyid Mice (Heteromys) – only present in NW S. America: Colombia, Venezuela, Ecuador
• Cricetid – primarily Sigmodontine – Rats and Mice (Cricetidae: Sigmodontinae)
• Tree Squirrels (Sciurus, Microsciurus, Sciurillus)
• Cottontail Rabbits (Sylvilagus brasiliensis, S. floridanus, S. varynaensis)
• Tapirs (Tapirus bairdii, T. pinchaque, T. terrestris)
• Equids (†Hippidion, Equus caballus)^[e]
• Peccaries (Tayassu pecari, Catagonus wagneri, Pecari tajacu, P. maximus)
• Deer (Odocoileus, Blastocerus, Ozotoceros, Mazama, Pudu, Hippocamelus)
• Camelids (Lama glama, L. guanicoe, Vicugna pacos, V. vicugna)
• Gomphotheres (†Cuvieronius hyodon, †Stegomastodon^[f] waringi, †S. platensis)^[27] – elephant relatives
• Otters (Lontra, Pteronura)
• other Mustelids (Mustelinae: Eira, Galictis, Lyncodon, Mustela)
• Hog-Nosed Skunks (Conepatus chinga, C. humboldtii, C. semistriatus)
• Procyonids (Procyon, Nasua, Nasuella, Potos, Bassaricyon)
• Short-Faced Bears (Tremarctinae: Tremarctos ornatus, †Arctotherium brasilense, †A. latidens)
• Wolves (†Canis ambrusteri, †C. dirus)
• Gray Fox^{[note 8]} (Urocyon cinereoargenteus) – only present in NW S. America: Colombia, Venezuela
• other Canids (Atelocynus, Cerdocyon, Lycalopex, Chrysocyon, Speothos)
• small Felids (Leopardus) – all 9 extant species (e.g. L. pardalis, L. wiedii)
• Cougar (Puma concolor) and Jaguarundi (P. yagouaroundi)
• Jaguar (Panthera onca)
• American Lion (†Panthera leo atrox)
• Scimitar Cats (†Homotherium) – known so far only from Venezuela^[42][43]
• Saber-Toothed Cats (†Smilodon gracilis, †S. fatalis, †S. populator)
• Condors (†Dryornis, †Geronogyps, †Wingegyps, †Perugyps, Vultur gryphus)^{[44][45][46][g]}
• Trogons (Trogon)^[47]

[edit] See also

• List of North American mammals
• List of Central American mammals
• List of South American mammals
• Lists of birds by region
• Lists of reptiles by region
• Lists of amphibians by region
• List of prehistoric mammals
• List of extinct animals of North America

[edit] Notes

1. ^ 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.
2. ^ North American gopher tortoises are most closely related to the Asian genus Manouria.
3. ^ Of the 6 families of North American rodents that did not originate in South America, only beavers and mountain beavers failed to migrate to S. America. (However, introduced beavers have become serious pests in Tierra del Fuego.)
4. ^ Of the 11 extant families of South American cavimorph 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 N. American locales.)
5. ^ 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 went extinct in Asia but survived in remote areas of arctic N. America (its more southerly-distributed relatives such as Harlan's musk ox and the shrub ox were less fortunate).
6. ^ ^{a b c} This listing currently has fairly complete coverage of nonflying mammals. but only spotty coverage of other groups. Crossings may have been made before 10 Ma ago by some fish, arthropods, waif-dispersing amphibians and reptiles, and flying bats and birds. 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.
7. ^ While all megalonychid ground sloths are extinct, extant two-toed tree sloths are from the same family. Three-toed tree sloths, in contrast, are not closely related to any of the groups of extinct ground sloths.
8. ^ Not to be confused with the South American gray fox.

[edit] References

[edit] External links

• History of the Fauna of Latin America by George Gaylord Simpson (1950)