ExtinctZoo - What Happened When Two Continents Collided

Episode Date: December 25, 2025

You ever play one of those games, where you can pit animals you shouldn't in a battle royale to see what happens? Well turns out that mother nature once played that game...with interesting results.

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on April 30th, the powerful vocals of Demi Lovato on May 17th, and the signature Southern Country Rock of Eric Church on July 19th. Tickets on sale now at Yamava Theater.com, only at Yamava Resort and Casino, celebrating its 40th anniversary. You in? Must be 21 to enter. Since life has existed, so has extinction. The complete and utter eradication of a species, genus, and so forth. And just as diverse as life has become, so have the causes behind extinctions. With some of the more common tropes being space rocks not following the speed limits, volcanoes getting a little too hot, and natural climate

change. In other words, a lot of the time, animals facing extinction are really just duking it out with nature, not with their fellow living beings. However, given how long life's been around, it has indeed occurred on occasion of animals killing animals to such an extent that it results in extinction. But usually it's quite local and rather overwhelming. But once upon a time, and actually not so long ago, there was a bloody battle royal which took place that led to not one but multiple different extinctions and permanently changed history forever, with its rampant. ramifications still even being seen today. And this ancient war, you could say, was all started by a girl, a girl named Gabby.

Just kidding. Gabby is not a real person, but rather an acronym for the Great American Biotic Interchange. Might not sound like the most exciting thing ever, but trust me, it was a chaotic, bloody event. Today, if you take a walk throughout the majority of North America, especially the U.S., there is a chance that at some point you may encounter an opossum, a staple of urban American wildlife, that is nicely adjusted to the continent. And key word, they're being adjusted, is it actually doesn't have North American origins, rather South American. Kind of odd, right?

And what you might also observe walking around the U.S. is that there aren't any camels, llamas, or any kind of camel it, for that matter, walking about, which is what you'd expect, not very odd. Whereas in South America, you'll find various kinds like the iconic alpaca. And that's understating it, with these animals being near synonymous with certain countries in South America. And yet, in reality, camelids are not from South America. They're from North America. Kind of shocking, right?

Well, it turns out that this example, and the opossums, are just a couple of the clear signs of this great interchange really happened. And just telling you about these animals without any more context is a bit confusing. So, let's rewind the clock a bit, see just how we wound up here. And to achieve this, we need to go back to the moment right before Gabby began, which brings us to 3 million years ago. In other words, the late Pleiocene. At this point, both North and South America were still isolated from one another,

and largely had been for about 100 million years, allowing each to evolve their very own unique life. And because of this, taking a trip to either place, despite their relative close proximity, would end up in you having very different experiences. South America in particular would have been quite the mind trip, as it primarily was an isolated island that had been so for quite a while, leading to many kind of animals you simply could not find elsewhere, including endemic kind of marsupials, the sparasidants, xenarthens, the top turns, astrapathiers, gondwanaeers, and the list goes on.

Some of the more famous individual animals include the Enkilosaur-like Leptidants, giant ground sloths, and numerous terabirds, that could be found elsewhere but held unrivaled diversity here. Now, if you were to visit North America, you would have found a whole lot less of these things. Instead finding camelids, deer, horses, proboscideans, and carnivorans, that came in the form of cougars, jaguars, canids, bears, lions, and of course, saber-toothed cats. So really, they couldn't have been more different or separate. Yet, spoiler alert, things were about to change. And actually, small signs of the upcoming chaos were already visible. For skirmishes, as I like to call them, had already taken place for a very long time, tracing as far back as 40 million years ago.

And what I mean by this is that some animals had already invaded through something called oceanic dispersal, and most of it seems to have been aimed at South America. But just like a true surprise attack that you would expect from a real war, it wasn't from the north that these skirmishers came, rather they got flanked and they came from the east. Because, as it turns out, most of the pre-interchange invaders weren't actually North American, rather African fauna, and they mainly included rodents, bats, and tortoises. And this may seem rather dubious, considering the giant distance between the two continents,

yet long ago the two were much closer than now, with a distance likely having been smaller and the Mediterranean Sea is wide, allowing these animals to raft or island to hop their way over. And even though this was just the rising action, not the climax, so to speak, the arrival of these fauna had their own ramifications, as they out-competed many of the South American animals, namely small marsupials, and underwent immense diversification of their own, leading to things like the Capibara, New World Porcupines, New World Monkeys, Chinchillas, and more. Now, this continued for millions of years, but then about 7.3 million years ago, North America made its likely first attack, and it came in the form of an unnamed procyonid, the group to which

animals like raccoons belong to. And similar to the earlier African arrivals, it was done through oceanic dispersal or island hopping and represent perhaps the first carnivorous placental mammal to ever step foot on the continent. In seeing that predatory placental mammals are quite successful and pretty much everywhere today, it perhaps comes as no surprise that this procyonid's arrival sent shockwaves across the continent, especially because a part of their diets would have consisted of the eggs of endemic animals, which is even why a handful of scientists believe that the sepacids, i.e., the continent's largest predator for millions of years, might have been indirectly erased by these new faces, with them eating all their unhatched children. Not very nice of them.

Shortly after that, hog-nosed skunks and sigma-dontine rodents grab the next raft over, causing further change and damage. Yet, this was not an attack without a response, for there is partial evidence that both continents traded warning shots, I guess you could say, as terror birds and groundsloss, in at least small numbers, might have briefly inhabited the southernmost parts in North America before the true interchange. But whatever the case, all of these skirmishes were really just an appetizer, a taste of the true war that was to come. Because somewhere along the way, the two continents were called in for support and actually started budding heads, literally and figuratively, as their tectonic plates had collided.

And this collision started a long period of uplift, while also setting off numerous chains of underwater volcanoes that continuously pumped new land and islands to the surface, which culminated in the formation of the Isthmus of Panama about 2.7 million years ago. You know, that narrow ship of land that connects the continents. And this really cranked up the craziness all the way to 100. As with its creation, for the first time and a long time, North America and South America were connected again by a physical land bridge, allowing for the unparalleled movements of animals, and thus the floodgates of chaos were opened. At first, animals from both sides rushed across at a near equal rate,

with camelids, deer, horses, and proboscideans moving into South America, while South America sent glyptodons, ground sloths, and various ungulates to represent their herbivores. While in the predator department, it was mainly the terabirds, the form of the form of the world. the front lines, whilst cougars, jaguars, saber-tooth cats, canines, and bears marched south. Now, setting up a little watching post in the isthmus of Panama would have been absolutely insane in these first days, given just how narrow the trip of land was. And you could have seen some of the most iconic animals of the Cenozoic meet for the very first time, which for them was probably like seeing aliens.

I mean, just imagine how confused these animals must have been when a giant harebird met a Smilodon, or when a Smilodon encountered a car-sized armadillo. I know I sure as heck would have been very confused. And this confusion seemed to do the trick, as for a while things remained quite neutral, with various groups streaming in and out in both directions. Yet, it wouldn't take too long for the tides to shift in favor of one continent. You see, for the South American animals that reached the higher latitudes of the North, they started to encounter a major obstacle,

which was that all the niches they had occupied in South America were already dominated by animals in the North. And worse yet, they found themselves unable to out-specialize the North American competition, leading to many of the ways of South American creatures to die off, while those who were able to somewhat establish themselves found it very difficult to diversify. Take the Tererbirds, for example, flightless apex predators that essentially made the rules in South America. It could reach gigantic sizes.

And yet, only one genus, the Titanus, is known from the north, and its range is restricted to the most southern U.S. states, with bones only being recovered from Florida, Texas, and California. And this sort of restriction is also seen in the glyphidons, those giant armadillos, who in Southwerexia, who in South America had become widespread and highly diversified, but in the north, only the genus glyphtherium managed to secure roots. A nearly half a ton member whose range was even more limited

than its South American commanders, the terabirds, with fossils only being known from Mexico and Texas. But with that said, there was actually a third South American animal that did do relatively well, and that was Sid the Sloth. In other words, massive ground sloths. Which, of the three animals I've mentioned so far, was largely the most successful invader, as four different lineages are known to successfully occupy the north,

with even one genus, a three-meter or ten-foot-long megalonics, having successfully made their way all the way up to Alaska, where it might have even briefly crossed into Asia via the Bering Strait. And this surely would have been a sight for sore eyes, as atop of being tall, it was a thick boy too, weighing well over one ton. And across the three other lineages, they could be even bigger. And the reason they're able to be so successful, relatively speaking, is because it's thought that these guys' ability to rear up in their hind limbs,

thus reaching great heights, gave them an advantage over both other South American migrants and also the native large megafauna like the Elephantaday. However, while South America did struggle there, for the most part, there was a certain area where their soldiers excelled, so to speak, and that was in the small animal department. As mentioned to the start, opossums are found all over in North America, and they were one of the many to come on over during the interchange, largely owing their success to their extremely adaptive diets, ability to scavenge, and of course, play dead. That definitely helped confuse a few hungry mouths along the way. And two other little guys that did nearly just as well,

included the smaller armadillos and porcupines, whose unique defenses and size proved enough for them to carve out their own niches in the north. And then on top of this, a few amphibians, reptiles, and birds, including hummingbirds and parents, found decent success as well. But past these animals, there really wasn't that much gain for South America. Lots of places can expose you to identity theft. Oh no. That's why LifeLock monitors hundreds of millions of data points a second for threats to your identity, which is way more than anyone can do on their own. If we find anything suspicious, like new loans or changes to your financial accounts, we alert you right away, all through text, phone, email, or the LifeLock app.

Get the alerts that could make all the difference. Save up to 40% your first year at at lifelock.com slash special offer. Terms apply. I don't think it's too much of a stretch to say that in this interchange, North America definitely came out on top, as its invading animals proved far more successful, and not only that, but ravaged much of South America's ecosystems. And out of all the areas, it was the South American Predators that likely suffered the most. Prior to the interchange, the South American Predator Guild was largely made up of non-placental mammals, large predatory birds, and a myriad of nasty reptiles. And each of these groups was successful and lethal carnivores within their own right,

and yet they all met their match when the North arrived. Short-faced bears, dire wolves, and saber-tooth cats were all part of the exchange, and each brought their own problems. Big ones at that. The bears had great size, hibernation, and general as teeth. The direwolves brought their complex social packs, and the mychirondes had their size and giant saber-like teeth, allowing each of them to carve huge chunks out of different niches. And of course, you also can't forget that otters, raccoons, and skunks entered as well, causing their own mischief and upending things that much more. And it's not over yet, as on top of these animals, it's ultimately the same.

the Great American Interchange that resulted in not only every canine, but every feline we know from South America today, including animals regarded as Amazonian icons, like jaguars. And thus, with all this influx of carnivores, it's quite easy to see how the continent's predatory guild was completely flipped on its head. And originally it was even thought that North American carnivores made many of their South American counterparts go fully extinct, just by existing, namely a varying amount of native reptiles and then also the legendary terabirds.

But with that said, in recent years, new debate has sprung up, pushing back on this idea a little bit. But it does seem that for the very least, the Great American Interchange re-shuffled the Predator Guild and led to a drastic decrease in numbers, with Terrebird fossils, for example, becoming distinctively more rare only after the land bridge formed, while the diversity greatly slowed as well. And the ways of North American predators didn't just flip the script for the Predator Guild, but also the herbivorous one, too, with the influx of killers taking a toll on certain groups. to mention that the arrival of North American herbivores also influenced things as well.

And of all those impacted here, it seems like the South American native ungulates, or Sanu for short, drew the shortest dick. This collection of mammals included five orders of now-exeemed herbivores that were placental, typically being a bit larger, with some even being multi-toned. And in many cases, through convergent evolution, looked similar to herbivores you would find elsewhere on the planet. And for almost 60 million years, these guys have been living it up in South America, but now the party was coming crashing down,

as they were unable to compete with the invaders, and huge portions of them were swiftly replaced. In fact, it was only a handful of families within these orders, like the McCroconids and Toxodontids, that held on to the very end. And of the families that did survive, to the end, that is, they tended to be even bigger than the norm, thus leading to the hypothesis that perhaps their size helped them put up a better defense. Furthermore, it seemed that in general, many South American megaherbivores fared quite well against the invading northerners, and this particularly extended to the giant Zanarthans, like ground slots and armadillos. And in fact, these guys managed to hold their ground so well that the megaherbivore space was just about the only one North Americans could not crack, with only a handful of gonfethears and camelids being successful enough to survive their migration south.

And even after 2 million years of the land bridge being around, more than 75% of the herbivores in South America, who weighed over one ton, were native. And when it comes to the reason why Zanarthens were so successful beyond their size, one explanation, is often their seemingly overkill level of defense, with Glyptodons having their thick armor and in some cases literal spiked clubs, whereas ground sloths had not only their stature, with one member in fact being the largest mammal to ever walk on two legs, but also had deceptively sharp claws that were pretty massive, especially when you consider the fact that they were attached to quite powerful arms.

If you get hit by this, game over. But with that all said, while the megaherbivores did do well, it was really an unlikely group that survived the best, and that is marsupials, which were explicitly small. These guys did fantastic when it came to withstanding the onslaught, as besides a few groups dying out, they continue to persist, and importantly did so in large abundance. And opposed to the South American megaherbivores, they are of a select handful of animals, which existed not only before the interchange, but still exist today,

whereas on the flip side, not a single one-ton mammal remains in the continent today, well, unless you're counting domesticated animals. But overall, despite the small winds for South America here and there, North America, as mentioned, was definitely the winner of the Great American Interchange, as not much of its native wildlife went extinct, whereas South America was fundamentally changed by the event. And according to some research, the continent was actually doomed before Gabby even occurred, and it all comes down to just one thing, geography. Just like a real war, geography played a huge part in this exchange. It might have been what gave the North Americans such a massive

advantage, doing so in two distinct different ways. The first reason was the climate of each continent. Now, you'd intuitively think this would be a 50-50 situation, whereas South America equals warm and North America equals colder. But it's not as simple as that, as a climate asymmetry exists between the two. You see, if a North American animal travel down to South America, they would hit tropical warm conditions before they even stepped foot on the continent. As in the lower parts of Central America, things are already starting to heat up and humidify, meaning by the time they make it over, you already know this animal can withstand tropical

moist conditions. Otherwise, they would have died off already, someplace in the central regions. However, on the flip side, if a South American animal travels north, they will still experience a preferred climate once entering North America, as the nearby Panama is already considered North America and retains a similar climate to many of the southern countries. And you think this would be good news, and it is in the short term. Yet, the moment an animal hits the trans-Mexican volcanic belt, things quickly change, and the weather gets much drier and colder, leading to a very rude wake-up call and making their chances

of getting to, say, the U.S., extremely hard. And another way to say this is that before a North American animal reached its final destination, you already knew it could handle the new climate, whereas for animals traveling upwards, you didn't. As upon entering North America, they still would not experience the shift in climate, which is not true the other way around. And this plays a big part in explaining why the Gabby ended up the way it did. Yet, the study suggested one more way in which geography altered the outcomes, and this reason apparently had an even larger impact. So what is this factor, you ask? isolation, the isolation levels of each continent.

As stated before, South America for a very large chunk of its history was socially distancing itself. And the last time it was connected to one of its brethren, it was to places like Antarctica and Australia, two relatively inhospitable and smaller continents, relatively speaking. On the other hand, North America, while not being what I'd say as social, did have various points connected directly to Eurasia via Landbridge, which in turn connects Africa. And what this did was that it resulted in a lot of migration back and forth, between North America and the rest of the world, welcoming in new faces and exposing its animals to a lot more competition and threats. And then you also add in the fact that North America is

larger than South America too, by almost 40%, that is. So in other words, North America tougher competition, a tougher climate, and a larger map, resulting in a brutal battle real over time. And thus, by the time the interchange occurred, the animals in North America were arguably much more meaner and tougher than the somewhat chiller animals down south, who evolved with less threats, a smaller playing area, and we're unaware of the carnage awaiting for them. And obviously this is kind of sad because we're never able to see the megafauna of South America. But selfishly, I guess we should kind of be happy for this event because its effects ultimately played a big part in how nature was when humans first arrived,

with the date being about 23,000 years ago for North America and 20,000 for South America, give or take. And something you may have noticed throughout this video is that many of the animals I talked about which found success or survived, like the Macyridons or certain Xenarthens are no longer around. So, why is that? Well, it probably has nothing to do with the actual great interchange, but instead a series of events sometimes referred to as the late Pleistocene extinctions. Basically, starting the late Pleistocene and lasting to the Holocene,

the world saw a huge eradication of megafauna. And no, it wasn't just in these two continents, but worldwide. And in the Nearctic, Neotropic, and Australasia, the total extinction rate for megafauna was above 50%. And for a large, two giant-sized megafauna, their rate was even abysimely higher. And for years and years, researchers have sought to explain the causes behind these mysterious vanishings. And while no conclusive cause has ever been agreed upon, the main idea does usually relate to us, mainly due to our overhunting and landscape alteration.

While then climate change is also cited as contributing cause, as around that time things started to heat up, and lots of the ice sheets were treated. But like many paleo events, you also have the smaller, less supported hypotheses, which are admittedly a bit more interesting. as some of the alternatives suggest that it could have been a global disease that could transmit through many kinds of animals, an extraterrestrial impact, a pesky space rock, which is known as the Younger Dryass Hypothesis, and a weakening of Earth's magnetic fields that could have caused a huge flux in UVB radiation. But I digress.

These alternatives are, to be honest, not very supported. But regardless, I think the most fascinating takeaway here is the fact that, unlike most extinction events, this was not causing Mother Nature, but rather by the simple presence of other animals, showing you just how devastating competition can be. And on a flip side, why it's very, very important not to introduce non-native animals. Thanks for watching, and until next time, on Extinct Zoo.