Mammals Max's Random Ramblings

The Crossroads of History: America’s Great Biotic Interchange

Millions of years ago, Central America became an almost unparalleled highway for organism migration.

Did you know that some of South America’s most famous animals are recent arrivals?

The Jaguar, a fixture of many ancient Mesoamerican cultures and South America’s top predator, is the most notable example. The spectacled bear, a vulnerable species now restricted to the Andes Mountains, is another relatively new fixture. Some other new arrivals include Tapirs, Peccaries, River Otters, Llamas and their wild cousins, the Guanaco.

The Jaguar

These organisms (and many more) have clearly found success in the lost world. But how did they get there? What was going on before they arrived? And why did they succeed? These are all questions explored in today’s article.

The story of their migration begins with one of isolation. By the time the dinosaurs went extinct 66 million years ago, South America was already separated from the rest of the world’s continents. Their extinction left massive ecological gaps across the planet, enabling new lifeforms to recolonize what had long been niches claimed solely by the dinosaurs.

Familiar mammalian orders soon colonized the globe. Some include the proboscideans (elephants), ungulates (hooved mammals), and Carnivora (dogs and cats). These lineages would become dominant in Africa, Asia, Europe, and North America over the course of the Cenozoic era (66 million years-present). While their presence on these continents became firmly established, South America remained inaccessible to them. This isolation would foster the evolution of many unique lineages of wildlife not found anywhere else on earth.

Toxodon, one of South America’s (extinct) native herbivores

While elephants occupied were top herbivores throughout the world, South America had giant ground sloths (and, of course, regular sloths). Strange lineages of hooved mammals were present on the continent, such as the Litopterns and the Notoungulates. Both orders were extremely diverse; Notoungulates ranged from the rhino-sized Toxodon to the rabbit imitator Propachyrucos, while the Litopterns included the horse-like Macrauchenia and more.

The predators were strange too. Instead of cats and dogs, South America had the Sparassodonts, predatory mammals closely related to marsupials. Most infamous of the Sparassodonts was Thylacosmilus, a saber toothed animal that resembled the saber tooth cat Smilodon. Also in this family were the Proborhyaenidsanimals that developed traits like the modern hyaena. Mammalian carnivores weren’t the only dangers; giant Caiman relatives such as Purussaurus could reach over thirteen meters long, while the infamous Phorusrhachids (or Terror Birds) ran amok.

Thylacosmilus, the most famous Sparassodont

Modern lineages were also present. Anteaters, Capybaras, and New World Monkeys are some of South America’s strange animals. Armadillos were around too, though some of their relatives were the size of cars and had spiked clubs at the end of their tails. South America’s isolation fostered these unique families for millions of years, allowing them to evolve free of competition from the animals found elsewhere.

Until about ten million years ago, that is. The gap between North and South America, which was never pronounced throughout the Cenozoic to begin with, had been closing in. During the Miocene epoch, some ten million years ago, what is now Central America was made up a series of islands like that of modern-day Oceania and Indonesia. Though traversing these islands would have been difficult, sporadic tectonic activity and the resulting movement of continental plates could have made it possible.

When the first migrations occurred is debated. Older research pointed towards the crossing beginning sometime during the Pliocene epoch, approximately three million years ago. However, fossil evidence shows a different reality. The skull of a mastodon (an ancient elephant relative) from Peru dates to over nine-and-a-half million years old. While some paleontologists debate the validity of this specimen, we know with certainty that two genera of ground sloths were in Florida by nine million years ago. Given what we know about the sloths ability to swim, it makes sense that they were among the first animals to make the journey.

The next group of animals to migrate were members of the Procyonids, mammalian carnivores that include raccoons and coatis. These animals ventured south approximately seven million years ago and diversified rapidly upon their arrival. Next were the Terror Birds, represented by the genus Titanis walleri in Florida and Texas starting five million years ago. Titanis appeared to have been quite successful, as fossils of the species have a range of over three million years on the continent.

Titanis walleri

Around four million years ago, two lineages of giant armadillo relatives – the Glyptodonts and the Pampatheriids – travelled north. Accompanying them were capybaras, giant rodents that still do well in South America, despite being a little smaller than they used to be. On the flip side, camel relatives made their way South around the same time.

Some animals clearly migrated across much earlier than previously considered. So, why did paleontologists estimate three million years? Well, the Isthmus of Panama – the connective land bridge between North and South America – formed around this time. Geological and fossil evidence, in the form of diverging lineages of marine organisms on either side of Central America, support this date as the approximate time of formation. 

For terrestrial life, the Isthmus of Panama opened the proverbial floodgates. Its formation enabled the Great American Biotic Interchange, or “GABI” for short, to begin at an unparalleled speed. GABI refers to the exchange of life between the Americas throughout the Pliocene and Pleistocene Epochs, which occurred in multiple waves for a few million years.

A scene from Pleistocene South America, featuring Smilodon (bottom left), Macrauchenia (bottom right), horses (centre) a giant ground sloth (centre-left), a Glyptodont armadillo (left), and Gomphotheres (background)

The first wave of GABI saw weasels, dogs, and horses proliferate into South America about 2.8 million years ago. More species of armadillos and ground sloths went to North America and New World Porcupines took their first voyage alongside them. 

The second wave of GABI was perhaps the most dramatic, especially for North American animals heading south. The saber toothed cat, Smilodon, travelled south and was successful, with South American populations becoming larger than their North American equivalents. Alongside them came bears, deer, peccaries, more camels, and tapirs, most of which still live in South America. Some South American species that came north include anteaters and some species of the notoungulates.

Smilodon (left) battling a Jaguar (right) over a kill

In subsequent years, GABI would have two more waves of dispersion which primarily saw North American species travelling south. The gradual trend of GABI is that North American species fared better in South America than South Americans in the north. While animals like mastodons, saber toothed cats, short-faced bears and many more succeeded in South America, the only large animals whose ranges expanded throughout North America were the ground sloths. The Notoungulates, Terror Birds, and giant armadillos were all confined to southern North America, with few genera making it farther north than Texas.

This trend persists today. While much of South America’s mammalian fauna comprises animals descended from North America, only three South Americans have found success throughout the north: New World Porcupines, the Virginia Opossum, and Nine-Banded Armadillos (which are apparently expanding to northern states in the US). 

A graph of GABI’s species dispersal. Note the number of species on the Y axis

Why the discrepancy? Some paleontologists believe the North American species outcompeted their South American counterparts. This theory isn’t exactly conclusive, however. South America’s mammalian predators like the Sparassodonts were already extinct before their competition arrived. While many herbivorous lineages experienced a massive dip in diversity, a few members stuck around to the end of the Pleistocene with the North Americans. If the northerners were evolutionary superior, we would expect an overwhelming extinction event to occur directly upon their arrival, which isn’t conclusive with the fossil record.

A more logical theory is that South American ecosystems had plenty of ecological gaps for North American species to fill. Plenty of evolutionary niches were left open, including large omnivores, megaton grazers, and wolf-like animals, to name a few. If no local animals occupied these positions, newcomers could fill the gaps without displacing existing populations. Utilizing the example above, Bears, proboscideans (such as gomphotheres and mastodons), and canids were able to move in without competition. Instead of a brutal battle for survival, the North Americans may have slotted themselves into vacant roles. It almost seems too easy…

Pleistocene North America, featuring six native species and a ground sloth.

If this is the case, why did the South Americans fail in the north? The answer may lie in the environments of the Americas. South American geography consists of tropical rainforests to the north with grasslands and arid deserts in the south. Central America’s climate is tropical, with rainforests and evergreen forests peppering the landscape. These conditions extend into Mexico but soon transform into arid conditions that stretch throughout the southernmost region of the United States.

To paraphrase, the Americas essential create a desert-rainforest sandwich. For South American animals adapted to arid climates, travelling through an extensive rainforest was the only way to North America. So, not exactly a task they were equipped to make. For animals adapted to tropical conditions, going through the Chihuahuan Desert in North America was virtually impossible and pointless, given that North America lacks the tropical rainforests necessary to support them. 

Another theory contends that the extinction of South American lineages prevented species from going north. Recent studies have shown that while animals from both continents diversified at similar rates in their newfound habitats, the extinction rate of native South American species was high. The number of species that can travel north is sensibly limited by how many species there are. If there were fewer native species in the Pliocene, it makes sense that fewer of them would travel north. This theory is still new but may explain why some lineages – like the terror birds and notoungulates – are only represented by a few successful species instead of whole families.

I should note that South American species have found success in Central America and Southern Mexico. New World Monkeys, Macaws, Toucans, poison dart frogs, and vampire bats are just a few of the animals that have found success in the rainforests of Central America. Ultimately, their dispersal was limited by the environmental conditions of the Americas, meaning that I will never see a Macaw in Canada. Shame…

Millions of years before The Avengers, The Great American Biotic Interchange became one of history’s largest crossover events. As Central America formed and animals crossed between North and South America, the ecological composition of both continents changed forever. A species that I haven’t mentioned yet was Homo sapiens, one of the last migrants who took advantage of the same land bridge that enabled other wildlife to cross into South America millions of years prior.

A Pampathere meets a new arrival in South America…

While most of the animals that were part of GABI are now extinct, plenty are still around. For those living in North America, every time you see a possum remember to give it a friendly welcome; they are (relative) newcomers, after all.

For those in South America, if you – God forbid – run into a Jaguar, don’t do the same. In the words of Nigel Marven: just run, mate.

Thanks for reading today’s article! If you want to know about the giant ground sloths and their connections to an American president, click here!

If you’re interested in South American fauna pre-GABI, I would recommend the book “Giants of the Lost World: Dinosaurs and Other Extinct Monsters of South America” by Donald Prothero. The book discusses many of the extinct lineages featured today and is an excellent read.

I do not take credit for any images found in this article.

Header image courtesy of Peter Schouten, found here

Jaguar image found here

Toxodon courtesy of Gabriel Ugueto, found at his twitter here

Thylacosmilus courtesy of unknown, found here

North American scene courtesy of George Teichmann, found here

Titanis courtesy of Jonathan Kuo, found here

Pleistocene Pampas courtesy of Mauricio Antón, found at his twitter here

Smilodon vs Jaguar courtesy of Júlia d’Oliveira, found here

GABI graphic courtesy of the journal Science, found here

Final images – A North American scene and a Pampathere meeting a Jaguar – courtesy of Peter Schouten, found here and here

Works Cited:

Black, Riley. “Toxodonts Traveled North.” Science, National Geographic, 3 May 2021,

Carrillo, Juan D., et al. “Disproportionate Extinction of South American Mammals Drove the Asymmetry of the Great American Biotic Interchange.” Proceedings of the National Academy of Sciences, vol. 117, no. 42, 2020, pp. 26281–26287.,

MacFadden, Bruce J., et al. “Revised Age of the Late Neogene Terror Bird (Titanis) in North America during the Great American Interchange.” Geology, vol. 35, no. 2, 1 Feb. 2007, p. 123.,

Prothero, Donald R. Giants of the Lost World: Dinosaurs and Other Extinct Monsters of South America. Smithsonian Books, 2022.

Woodburne, Michael O. “The Great American Biotic Interchange: Dispersals, Tectonics, Climate, Sea Level and Holding Pens.” Journal of Mammalian Evolution, vol. 17, no. 4, 2010, pp. 245–264.,

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