FEMALE NARRATOR: Along the eastern edge of the Andes Mountains in central Bolivia, you'll find the remains of an ancient calamity.

The Badlands here are formed by sandstones and mud stones dating back to around 64 million years, the remnants of a time when this area was a warm damp forest filled with palms and ferns.

And it was home to a group of survivors that made it through the mass extinction event that wiped out the non-avian dinosaurs.

These survivors were the mammals known as metatherians, which included some of the very first marsupials.

But these animals were about to face a catastrophe of their own.

One day, or week, or season back in the very early Paleocene epoch, a river overflowed its banks and flooded the burrows where these mammals lived.

Anything unlucky enough to get caught up in the water quickly drowned, preserving them, this event, and their ecosystem.

Today, we think of Australia as the continent of the marsupials, the one place where they reign supreme in a world dominated by placental mammals, like us.

But this place in South America was their original stronghold.

This site in Bolivia, called Tiupampa, was home to a small burrowing metatherian about the size of a mouth, known as pucadelphys.

And it lived alongside another mammal, khasia, an early true marsupial and close relative of most of the marsupials alive today.

From these two species, we can trace two parallel stories about these strange and remarkable animals-- one that ends in extinction and one that ends in survival.

Throughout the Cenozoic Era, which is the era that we're in now, marsupials and their metatherian relatives flourished all over South America, filling all kinds of ecological niches and radiating into forms that still thrive on other continents.

So where did these survivors of the dinosaur extinction, the metatherians and marsupials, come from?

And if they were so abundant, how did their reign end?

Why did some persist into the present day, while others just vanished over time?

Even though we think of Australia as the home of the vast majority of the world's marsupials, the cradle for their diversification and the bridge between their origins and their modern home would be in South America.

To understand where South America's marsupials came from, we have to go back millions of years and to another continent, where we find their earliest ancestors.

Today's marsupials are the last surviving group of metatherians, but the earliest known fossil evidence of metatherians dates back to the early Cretaceous, about 125 million years ago, to fossils found in China.

But based on genetic evidence, as well as fossils, it seems that this group first appeared way back in the Jurassic period, between 178 million and 168 million years ago.

Around this time, a split occurred in the mammal family tree, with metatherians forming one branch and the other containing eutherians, which includes all placental mammals like us and our closest extinct relatives.

These first metatherians were probably similar to the marsupials we know today, giving live birth shortly after conception and nurturing their tiny young outside the body.

But the important thing to know is that not all metatherians are marsupials.

For example, of our friends from Tiupampa, khasia was a marsupial while pucadelphys was not.

And it's really hard to pinpoint what traits make a fossil a member of one group rather than the other.

Paleontologists have to use complex statistics programs that analyze things like skulls, teeth, and other bones.

Now, by the late Cretaceous period, some of these early metatherians had arrived from Asia in North America where they diversified into lots of new species, most of which were small and arboreal.

And based on molecular dating methods, the earliest true marsupials seem to have branched off from the other metatherians at some point around this time, between about 85 million and 66 million years ago.

But then something happened: the Cretaceous-Paleogene extinction event, also known as the K-PG extinction, which wiped out all of the non-avian dinosaurs.

And sad to say, all of the metatherians took a big hit too.

About 90% of them went extinct.

This was probably because many of them were too specialized.

They had plant or meat-based diets, while their placental cousins were more flexible omnivores or insect eaters.

After the extinction, they soon became minor players in most ecosystems.

But there was one place where they maintained their dominance.

Some metatherians had arrived in South America around the end of the Cretaceous, and there, they quickly became the most diverse group of mammals, claiming the niches that placentals filled on other continents.

For example, one fossil metatherians from the early Eocene of Brazil glided from tree to tree, like a modern-day flying squirrel or sugar glider.

Another from the Middle Eocene of Argentina had square-ish molars like a primate, and they likely lived like one, eating fruit in the trees.

And then there was pucadelphys, that burrower from the site in Bolivia, which had one weird behavior unlike that of any modern South American metatherian.

It appears to have lived in groups, or at least tolerated frequent social interaction, which is what I try to do.

This is based on the discovery of 35 individuals in the same area, including both males and females, adults and juveniles.

Their bones were found together, but the skeletons weren't broken apart.

So it's not like floodwaters had just washed them into the same area.

The discovery has been interpreted by paleontologists as some of the earliest evidence of social behavior among mammals, but it was another group of non-marsupial metatherians that would become South America's top mammalian carnivores for millions of years.

This relative was called mayulestes, and it's probably the earliest member of a group of metatherians called the Sparassodons.

It was only the size of a common rat, but it had teeth adapted for slicing meat.

And from those beginnings, the Sparassodons would only get larger and more ferocious.

This group of predators evolved into lots of forms and thrived throughout most of the Cenozoic era.

Like 27 million years ago in Bolivia, there was a Sparassodon the size of a black bear, making it one of the largest metatherian carnivores ever.

And around 14 million years later, an ambush predator the size of a wolverine roamed Colombia.

By about 7 million years ago, one group of the Sparassodons wound up with saber teeth, the metatherians version of the saber tooth cats of North America.

But this group would turn out to be the last of the Sparassodons and the last of the non-marsupial metatherians.

They stuck around until about 3 million years ago before dying out, probably due to a cooling climate that limited the availability of their prey.

Today, the non-marsupial metatherians have no living relatives.

Their story ends here.

Now, it would be up to the true marsupials to re-establish metatherians outside of South America, and one of those early true marsupials was khasia.

It lived alongside pucadelphys at Tiupampa, but its closest living relative is the delightfully tiny marsupial known as the monito del monte, found in Chile.

We don't know that much about khasia because it's only represented by one fossil, a single tooth.

But one tooth can tell you a lot about a mammal.

For one thing, it tells us that khasia was a very small insectivore.

It also might have been a climber, like its modern-day relative and other metatherians.

And more importantly, that tooth tells us that khasia occupies a very interesting place in the marsupial family tree.

It's part of the group of marsupials that today live mostly in Australia, and the monito del monte is the only member of this group that's not found in Australia.

The marsupials got there by way of Antarctica when it formed a lush green connection between South America and Australia.

You might remember this part of the story from our episode about when Antarctica was green.

But they weren't the only true marsupials to expand beyond South America.

Along with the monito del monte, there's a second South American marsupial group that's still around.

These are the opossums and the shrew opossums, the only other marsupials found in the Western hemisphere today.

At least one species, the Virginia opossum, returned to North America after it connected to South America about 3 million years ago.

And they soon proliferated, becoming the first successful marsupial in North America in millions of years.

So those two groups of metatherians both found at the site of that ancient flood in Bolivia met very different ends.

The true marsupials live on, mostly in Australia, but also in pockets-- pouches, you might say-- of the Americas.

But the story of the non-marsupial metatherians, like little pucadelphys, ended in extinction.

So why did one group of the same kind of mammal survive, while the other is left to the fossil record?

Well, as the Cenozoic went on, the non-marsupial relatives couldn't take the changes in their environment.

As large-body predators, they needed prey, and a changing climate meant a changing ecosystem, one that their prey couldn't adapt to, either.

But the descendants of khasia and the other true marsupials survived because as smaller generalists, they were better able to adapt to a changing environment, unlike groups that got larger and more specialized.

And their modern descendants continue to thrive in North America and Australia, thanks to their relative isolation and their ability to adapt to fluctuating resources.

While we spend a lot of time thinking about our own branch of the mammal family tree, the placental mammals, the reason there are any members left of the other major branch, the metatherians, is because they were able to thrive and diversify in South America.

The survival of unassuming little critters like khasia all the way back in the Paleocene left a mark on the fauna of the planet, a reminder of the time when marsupials ruled South America.

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