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 meta theory ins 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
tio Papa was home to a small burrowing
net etherion about the size of a mouse
known as Puka Delphis and it lived
alongside another mammal cassia an early
true marsupial and close relative of
most of the marsupials alive today from
these two species we can trace to
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
air that we're in now marsupials and
their meta theory and 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 meta therians 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 in 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 meta theory ins
but the earliest known fossil evidence
of meta theory uns dates back to the
early cretaceous about 125 million years
ago
- 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 meta Syrians forming
one branch and the other containing u
Syrians which includes all placental
mammals like us and our closest extinct
relatives these first meta Theory ins
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
meta theory ins are marsupials for
example of our friends from tio Papa
cassia was a marsupial while Puma
Delphis 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 men Assyrians had arrived
from Asia in North America where they
diversified into lots of new species
most of which were small and are boreal
and based on molecular dating methods
the earliest true marsupials seem to
have branched off from the other meta
theory ins 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 KPG
extinction which wiped out all of the
non-avian dinosaurs and sad to say all
of the meta syrians took a big hit -
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 meta
Syrians 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 met
etherion 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 Puka Delphis that burrower from the
site in Bolivia which had one weird
behavior unlike that of any modern South
American metatheory n' 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 meta
therians that would become South
America's top mammalian carnivores for
millions of years this relative was
called Maya les T's and it's probably
the earliest member of a group of meta
Syrians called the spa residents it was
only the size of a common rat but it had
teeth adapted for slicing meat and from
those beginnings the spa residents would
only get larger and more ferocious this
group of predators evolved into lots of
forms and thrived throughout most of the
Sina's oeq era like 27 million years ago
in Bolivia there was a spur a Sadat the
size of a black bear making it one of
the largest met etherion carnivores ever
and around 14 million years later an
ambush predator the size of a Wolverine
roamed Colombia by about seven million
years ago one group of the spa residents
wound up with saber teeth the meta
theory inversion of the saber-toothed
cats of North America
but this group would turn out to be the
last of the spa residents and the last
of the non marsupial met a theory ins
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 Mehta theory ins have no
living relatives their story ends here
now it would be up to the true
marsupials to re-establish medicines
outside of South America and one of
those early true marsupials was cassia
it lived alongside Puka Delphis at tio
papa
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 cassia 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 cassia was a very small insectivore
it also might have been a climber like
its modern-day relative and other meta
theory uns and more importantly that
tooth tells us that cassia 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 bonito del Monte
there's a second South American
marsupial group that's still around
these are the possums in 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 three
million years ago and they soon
proliferated becoming the first
successful marsupial in North America in
millions of years so those two groups of
meta theory uns 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 men Assyrians like little Puka
Delphis 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 went
on the non marsupial relative
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 cassia and the other true
marsupials survived because as smaller
generalist they were better able to
adapt to a changing environment unlike
groups that got larger and more
specialized and their modern descendants
continued 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 meta theory ins is because
they were able to thrive and diversify
in South America the survival of
unassuming little critters like cassia
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
[Music]
big thanks to this month's otologist
Patrick Seyfert Jake Hart John Davidson
Sean Dennis Hollis and Steve become a
neo night at patreon.com slash yawns and
hang out with us on discord and thank
you for joining me in the Konstantin
hosta studio go subscribe at youtube.com
slash yawns
[Music]