The Great American Faunal Interchange

Back in the Age of Dinosaurs (by which time the bird and mammal lineages had originated), the land that was to become North America split off from the great southern continent of Gondwanaland, and then 90 million years ago (well before the extinction of the dinosaurs), the island continent of South America separated from Antarctica. South America retained this isolated island status for about 85 million years, during the time when birds and mammals were undergoing great diversification, until the Panama Land Bridge was formed.

The processes which led to a dry-land connection between the two continents of North and South America are complex and only partly-understood.  However, one popular hypothetical explanation, consistent with all the data available so far, is as follows:

• the direct connection between North and South America was severed about 160 mya, in the late Jurassic, as the global equatorial Tethyan seaway opened;  independent evolution in North and South America begins

 
• then, about 140-120mya, during the early Cretaceous (age of dinosaurs), chains of volcanic islands began to emerge on the eastern and western edges of the Caribbean plate as it began to move to the east.  At this time, this plate was further west than it is now.  These eastern edge islands became the Antilles (Cuba, Jamaica, Puerto Rico and Hispaniola); the western edge islands became, as they eventually coalesced, the Central American isthmus;

 
• by the late Cretaceaous (about 80mya)the Caribbean plate was positioned so that the "proto-Antilles' formed a series of island "stepping stones";  limited biotic exchange between the continents via proto-Antilles

 
• by mid-Miocene times (about 20mya) the Caribbean plate had more or less reached its present position as Central American peninsula develops; limited biotic exchange between the continents via C.Am. peninsula

 
• but it was not until about 3.5mya that the land connection was complete;  direct dispersal route.

 
• by this time, the Pleistocene glaciations were soon to begin;  at these latitudes the effects amounted to a lowering of sea-level, broadening the land connection, and a cooling and drying of the climate, turning much forest into open savanna-type habitats.

How distinct are these biotas?

The great bulk of the information about GAFI comes from the study of mammals, since they have an excellent fossil record.  Birds are also important, but their study is largely restricted to modern forms, since their fossil record is so poor.

Mammals originated from their reptilian ancestors in the earliest Mesozoic Era, about 200mya, well before the dinosaurs had come to dominate the globe.  At these times, the world's land-masses formed a super-continent called Pangea.  Thus, the very earliest lineages of mammals had access to all of the earth's habitats, where they began their own diversification.

The marsupial (pouched) mammals seem to have originated somewhere in the South America - Antarctica - Australia chain of continents in the mid-to-late Cretaceous, while the placental mammals probably originated in Asia around the same time and thereafter spread throughout Eurasia and North America.  Before the full separation of North and South America, it is evident that some placentals got into South America, while some marsupials got into North America (though they went extinct soon afterwards).

By the time of the extinction of the dinosaurs (about 65-70mya), South America had already become an island, and its mammals could evolve in splendid isolation from the rest of the world.  On the other hand, North America was in extensive, though intermittent, contact with Eurasia throughout much of the Cretaceous and most of the Cenozoic (70-15mya).

The mammals of present-day South America can be separated into three major assemblages which relate to the sequence of tectonic events shown above in the previous section:

• Group 1.  The Old-timers. All of these lineages have no close relatives anywhere else in the world.  These groups descended directly from those mammal lineages present at the time of the break-up of Gondwanaland.  They comprise three kinds of mammal -

 
• the marsupials (different from the marsupials found in Australasia).  These mammals had evolved a great diversity of omnivorous, insectivorous and carnivorous forms, including the opossums;
• the edentates (placental mammals - anteaters, armadillos, sloths);
• a diversity of often rather strange placental herbivorous hoofed forms.

• Group 2. The Early Immigrants.  These forms first appear in the fossil record about 30mya, and are supposed to have colonised via one or both of the island chains referred to above (though some opon suggests that they colonised from Africa).  They have left no close relatives in the present-day fauna of North America.  They comprise two types-

• the primates (marmosets, howlers, squirrel monkeys & others - very different from those known now from Africa and Asia)
• the caviomorph rodents (porcupines, capybaras, pacas, agoutis, guinea pigs, chinchillas, vizcachas and others)
• Group 3.  The Newcomers.  These very diverse modern forms derive from the Great Exchange permitted by the very recent emergence of the Panama Land Bridge.  Though most of these lineages still have close relatives in North America (e.g. rabbits, deer, pigs, squirrels, bears, raccoons, cats and dogs), some do not, since they have gone extinct in their northern continent of origin (e.g. camels, tapirs)

What were the effects of the exchange?

Mammals

Rather few kinds of South American mammal colonised the North American continent:  porcupines (rodent), armadillos (edentate) & opossums (marsupials) have contributed living species, though only porcupines have expanded extensively into the north;  ground sloths and glyptodonts (both edentates) did colonise as far north as the USA and survived until the last few thousand years,but they are now extinct everywhere.

On the other hand, many lineages of North American mammals colonised South America, 17 families in all:  rabbits, mice, foxes, bears, raccoons, weasels, cats, peccaries & deer still have close living relatives in N. America;  tapirs & camels are still alive in South America but they went extinct in their original home;  mastodons & horses went extinct on both continents (of course horses were re-introduced from the Old World in historical times).

About half of all species of today's South American mammal fauna derives from this influx of northerners.

Having colonised South America, many of these northern groups diversified greatly, so that nowadays, most of the families have more species in the south than remain in the north.  For example, there are more species of wild dog  and cat species in South America than on any other continent, even though they got there less than 5 million years ago.

In aggregate, as a result of the faunal exchange, about 50% of all South American mammal species derive from immigrant lineages from the north, while the corresponding figure for south to north migration is 10%, and the bulk of this effect is due to higher rates of post-migration diversification on the part of the northern migrants, which in turn may have led to some higher rates of extinction of native South American species..

Other animal groups

Among reptiles and amphibians, the pattern is rather the reverse of that shown by mammals:  that is, the majority of colonization was south to north.  Toads and treefrogs, for example, are now widely distributed in North America yet derive from South America.

South America is "the bird continent" with more species than any other continental region, and over a third of all the world's species.  This bird fauna, as well as being extremely rich, is highly distinctive, with 22 endemic families (not known anywhere else in the world).  Since the fossil record of birds is so poor in the main, we cannot be sure that all of these endemic forms evolved where they are found now, however, evidence from DNA sequences makes it almost certain that a very large part of the South American perching-bird fauna is authochthonous - that is, it originated and diversified in place.

Given that birds have considerable dispersal powers through flight, one would not expect to find the clear picture described above for mammals.  Nevertheless, it seems clear that a substantial fraction of the modern bird fauna of South America came down from the north.  When they arrived is unclear, but it seems evident that it antedated the complete dry-land connection by some long time in many cases.  Following their arrival, they diversified greatly within South America, and then many of these derived lineages re-invaded back into North America.  Thus, for example, most of the summer breeding migrant birds of North America have their immediate evolutionary origins in South America.

Altogether, it is abundantly clear that the biota of South America would have remained as highly-distinctive as that of Australia is today if it had not been for the elimination of the deep ocean barrier provided by the Tethys Seaway when it opened about 150mya.

This invasion and diversification experiment has had diverse results:  most mammals moved and diversified from a north-to-south movement, while in the other vertebrates it seems that most of the effect has been in the reverse direction.