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 mudstones dating back 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 mouse, known as Pucadelphys.
And it lived alongside another mammal, Khasia,
an early true marsupial and a 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 -- the era 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
the 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 of 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 metatherian 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 squarish 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 small 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
flood waters just washed into the same area
This 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 sparassodonts.
It was only the size of a common rat, but
it had teeth adapted for slicing meat - and
from those beginnings, the sparassodonts 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 sparassodont 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
sparassodonts wound up with saber-teeth, the
metatherian version of the saber-toothed cats
of North America.
But this group would turn out to be the last
of the sparassodonts - 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 today 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’ve 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 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 this same kind of
mammal survive, while the other is left to
the fossil record?
Well, as the Cenozoic Era went on, the non-marsupial
relatives couldn’t take the changes in their
environment.
As large-bodied 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 changing environments, unlike groups that
got larger and more specialized.
And their modern descendents 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.
Big thanks to this month’s Eontologists:
Patrick Seifert, Jake Hart, Jon Davison Ng,
Sean Dennis, Hollis, and Steve!
Become an Eonite at patreon.com/eons and hang
out with us on Discord!
And thank you for joining me in the Konstantin
Haase Studio.
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