Biological contact between North and South America causing massive extinction?

Well, the GABI (Great American Biotic Interchange) went both ways, and several families of mammals from North America came to South America, and viceversa. However, only few native South Americans remain today in North America (NA), whereas many originally Holarctic families are thriving in South America (SA) at present.

The above answerer has outlined the three major "strata" defined by GG Simpson for the mammals of SA. The "original" South American mammals before the GABI included marsupials (shared with Australia, but forming a distinct cohort with different families) but also the exclusively South American xenarthrans (although there is a possible fossil anteater from Europe), and the Caviomorph rodents (closely related to the hystricognathous African rodents). In addition, there were some carnivores that had made it from NA.

After the Panama land bridge appeared, the GABI was intense. North and South American faunas mixed and they appear to have coexisted for a long time.

The subsequent Pleistocene extinctions involved groups of both origins, but the South American natives were more intensely affected. Among the hypotheses to explain this, interspecific competition has been mentioned, but also climatic changes.

First of all, direct competition is _very_ difficult to prove, and more so with respect to fossil remains from strata that may not be exactly coeval. A more recent and preferred explanation involves a little paleoclimatology.

** During the glaciations (and other stressful periods), South American conditions were more benign, because the continent is closer to the Equator, whereas environments in NA were much harsher, facilitating the extinction of the South American immigrants, along with many native North American giant forms.

**In addition, the Central American corridor, which had been at first a savanna-like environment that permitted passage, later in the Pleistocene became more like a tropical forest. Then many cursorial (running) animals that inhabited open areas were unable to "cross the bridge". The giant ground sloths and glyptodonts were trapped in NA, and when climate and vegetation kept changing toward colder and drier conditions, became extinct.

Animals left from this:

Among the original South American natives, only the xenarthrans (sloths, armadillos, and anteaters) and marsupials (opossums)[early inhabitants], and the caviomorph rodents (originally derived from their African relatives) [second stratum colonizers] are alive today. The immensely diverse native ungulates went extinct, along with the representatives of the megafauna (giant sloths, glyptodonts, etc). At least one species of armadillo (Dasypus novemcinctus) is expanding its range in NA today, and the opossums (I remember Didelphis virginiana, not sure if there are others) are well established there. Caviomorph rodents also occur in NA (porcupines, genus Erethizon), but are mostly confined to Central America.

These are the South American contributions to North American fauna. In contrast, about 50% of the South American mammal genera have North American origin.

With respect to birds, at least the Passeriformes Suboscines are South American in origin, but I have no idea _when_ they colonized NA. They include the Tyrannidae (tyrant flycatchers). Other birds with Neotropical origin are the hummingbirds (Trochiliformes or Apodiformes).

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Update:

Mmm... I’m not sure what you call “non-speed strategies”. Predators? Running forms such as ungulates? There were many native ungulates in South America and they went extinct; also many marsupial predators such as the marsupial sabertooth “cat” that also disappeared. Many caviomorph rodents are cursorial (capybaras, agoutis, etc).

I would relate it more to size. Size _is_ a constraint because large animals have different energetic balance and need large amounts of food and room. And those went extinct in both continents.

Aranae may have a point with respect to “relative fitness”. But it’s also true that fitness is… well, relative, which makes the analysis a bit circular. As I’ve said before, competition was probably not direct, but if environments in South America changed and became more similar to the ones in NA, then those NA-native species certainly had an immediate advantage.

With respect to human influence: I’ve heard local paleontologists (including some very knowledgeable ones) who advocate the importance of both the ecological changes and the influence of human hunter-gatherers for the demise of megafaunal species. And I understand that native mammalian populations were already declining prior to the establishment of the Central America bridge. Probably only some of them got to live along humans.

Also, I’d like to point out that Diatryma and its relatives (Gastornis) were North Hemisphere predatory birds. We had ecological equivalents here, called Phororhacos. The first are order Diatrymiformes, while the phororacids are Gruiformes. Both groups went extinct but I'm not sure what their relationship is...

http://www.fmnh.helsinki.fi/users/haaramo/Metazoa/...

http://www.fmnh.helsinki.fi/users/haaramo/metazoa/...

You are referring to the Great American Interchange. Marsupials (of whom the living opossums, shrew opossums, and the monito del monte are a part) were probably present prior to the complete breakup of Gondwana and represent that connection to Australia that you mention. The presence of Xenarthra (armadillos, anteaters and sloths), one of the earliest branches among the placentals, may have also been a result of the breakup of Gondwana.

Rafting took place across the then smaller southern Atlantic which led to the colonization of South America by New World monkeys and caviomorph rodents (guinea pigs, capybaras, porcupines, degus, hutias, nutrias, tuco-tucos, chinchillas, etc.).

The remaining mammals in South America were mostly the result of migration from North America either upon the completion of the isthmus of Panama or shortly before it when island hopping was reasonably easy. Bear in mind that it was a two way street (although rush hour traffic was definitely headed south). Opossums, armadillos, and porcupines in North America and many mammals in Central America trace their ancestors to South America.

This is the mammal story. I don't know the story of the rest of the animals very well. Rheas were probably the result of the Gondwana split.

Update after additional details posting:

Things that evolve in isolation tend to be a bit less fit on average (it's certainly not an absolute). North America experienced multiple connections to Asia through Beringia and Europe through Greenland. It was even indirectly connected a bit to Africa by way of Eurasia. South America, in contrast went its own way in "splendid isolation". Therefore if a highly successful group were to evolve in Africa, North America, or Eurasia, it would likely make its way to North America. South America had to contend with what it started with, a few rafters, and what subsequently evolved. Thus really successful groups such as lines of carnivorans, artiodactyls, and muroid rodents were able to make their way into N America and replace local fauna after evolving elsewhere. South America didn't have that advantage. The phenomenon is at work today when mainland species are introduced into island ecosystems and are much more likely to be a problem there than island critters are on mainland continents.

aranae has it exactly right.

some of the interesting critters that once lived in South America that failed to survive the interchange include:

Litopterns - big, weird critters that looked sort of like a camel/giraffe cross with a tapir like trunk.

Thylacosmilus - a marsupial predator with gigantic sabre-teeth like a Smilodon, but perpetually growing like a rodent's incisors

Terror-Birds - gigantic predatory flightless birds that were the top predators of the day. One type, Diatryma made it north, but didn't last very long.

Glyptodonts - big armadillo-like slothy Xenarthrans up to the size of a small car with a heavy, armoured shell.

Notoungulates - herbivores that were similar to rhinos, hippos, rabbits and rodents, and other forms unlike anything alive today. Seeing fossils of these critters in South America got Darwin thinking about the oddities of past life and why it was so different in some ways and so similar in others and ready to see what he say in the Galapagos.