Great American Interchange

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The Great American Interchange was an important paleozoogeographic event in which land and freshwater fauna migrated from North America via Central America to South America and vice versa, as the volcanic Isthmus of Panama rose up from the sea floor and bridged the continents. The migration peaked dramatically around 3 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.

South America's endemic mammals

South America during most of the Cenozoic was characterised by endemic mammals found nowhere else on earth, consisting of marsupials, xenarthrans (i.e., armadillos, anteaters and sloths, like the giant ground sloth Megatherium), 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 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.) The marsupials present in South America included didelphimorphs (opossums and relatives), but many larger predatory forms also existed, like the borhyaenids and the saber-toothed marsupial Thylacosmilus. The marsupials shared the ecological niches for large predators with the fearsome flightless "terror birds" (phorusrhacids). The notoungulates and litopterns had many strange forms, like Macrauchenia, a litoptern with a small proboscis. Both groups started evolving in the Lower Paleocene, possibly from condylarth stock, diversified, dwindled before the great interchange, and went extinct in the Pleistocene. The pyrotheres and astrapotheres were also strange but had fewer forms and disappeared earlier, well before the interchange.

The North American fauna was a pretty typical boreoeutherian one.

Island-hopping ‘waif dispersers’

The invasions of South America started at least 31.5 Ma ago (late Eocene/early Oligocene), when cavimorph rodents arrived. This gave rise to – among others – capybaras, chinchillas, viscachas, and New World porcupines. This invasion most likely came from Africa.[1] 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). 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).

A little later (at least 25 Ma ago) primates followed. The ancestor of the South American monkeys is believed to have arrived from Africa in a fashion similar to the rodents. The primates capable of migrating had to be small. These gave rise to the New World monkeys (Platyrrhini). (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).

The earliest arrivals from North America were carnivorous procyonids that island-hopped from Central America prior to the formation of a land bridge, around 7 Ma ago. Some 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.[2] Similarly, ground sloths had island-hopped to North America by 10 Ma ago, perhaps via the Antilles.[3]

The Great American Biotic Interchange

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 llamas (also ungulates), mastodons, tapirs, felids (like cougars and saber-toothed cats), canids, bears and horses 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 "bad luck" happened both ways. Northwardly migrating animals often were not able to compete for resources as well as the Nearctic species already occupying the same ecological niches. Many Neotropic species were specialized for tropical rainforest environments, limiting their ability to penetrate beyond Central America. The southwardly migrating boreoeutherians are thought to have caused the extinction of some of the South American mammals. All of the large Neotropic marsupial and avian predators ultimately disappeared. South America's native ungulates also fared very poorly, with only Macrauchenia and a few toxodonts withstanding the northern onslaught. Its small marsupials fared better, while the primitive-looking xenarthrans proved to be surprisingly competitive. The latter had evolved effective defensive measures: an armored shell in the case of armadillos and glyptodonts, and long claws in the case of sloths and anteaters. The African immigrants, the cavimorph rodents and platyrrhine monkeys, generally held their own during the interchange, although the largest rodents (e.g. Phoberomys) disappeared. With the exception of the North American porcupine and several extinct porcupines and capybaras, however, they did not migrate past Central America.

The presence of armadillos, opossums, and porcupines in the United States is explained by the Great American Interchange. Opossums and porcupines were among most successful northward migrants, reaching as far as Canada. While few xenarthrans, marsupials or cavimorph rodents currently live in North America, considerably more are present in tropical Central America. Prior to the end-Pleistocene extinctions, most major groups of xenarthrans were established in Central or North America. Among the megafauna, ground sloths were notably successful invaders of North America; Megalonyx spread as far north as Alaska, and Nothrotheriops reached the Yukon. Generally speaking, however, dispersal and consequent adaptive radiation of rodents through South America was much more successful (both spatially and by number of species).

South American species that migrated to North America

South American species that migrated only as far as Central America

North American species that migrated to South America

References

  1. ^ J. J. Flynn, A. R. Wyss (1998). "Recent advances in South American mammalian paleontology". Trends in Ecology and Evolution. 13 (11): 449–454. doi:10.1016/S0169-5347(98)01457-8.
  2. ^ K.-P. Koepfli, M. E. Gompper, E. Eizirik, C.-C. Ho, L. Linden, J. E. Maldonado, R. K. Wayne (2007). "Phylogeny of the Procyonidae (Mammalia: Carvnivora): Molecules, morphology and the Great American Interchange". Molecular Phylogenetics and Evolution. 43 (3): 1076–1095. doi:10.1016/j.ympev.2006.10.003.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  3. ^ Savage, R. J. G. (1986). Mammal Evolution: An Illustrated Guide. Facts On File. ISBN 0-8160-1194-X. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  4. ^ M. Culver, W. E. Johnson, J. Pecon-Slattery, S. J. O'Brien (2000). "Genomic Ancestry of the American Puma (Puma concolor)" (PDF). Journal of Heredity. 91 (3): 186–197. doi:10.1093/jhered/91.3.186.{{cite journal}}: CS1 maint: multiple names: authors list (link)

External links

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