I’m super excited to announce that we’ve got
new merch! We’ve created a set of three pins based on some of our favorite Eons episodes and
they’re on sale now! Check them out at DFTBA.com. Isolation breeds weirdness. That's something I learned the hard way during lockdown! But the same principle applies to nature, too. One place we see it in action is in the weird, giant mammals that lived during the
Pleistocene epoch in South America. Because, around 35 million years ago,
South America basically became an island. And it’s in this isolation that
unique animals originated -- like giant ground sloths and armored glyptodons,
the enormous relatives of today’s armadillos. But among them was another huge
mammal that was more ... enigmatic. It was an herbivore, weighing around 1,400
kilograms and measuring over 2.5 meters long. It looked a little like a hippo,
or maybe a rhino without a horn. Today, we call it Toxodon and it seems to
have been one of the last members of a lineage that vanished 11,000 years ago, after
thriving in isolation for millions of years. And its fossils would inspire a
revolutionary thinker to tackle a bigger mystery than Toxodon itself: evolution. We’ve known about this
animal for almost 200 years, but it’s taken us most of that time
just to figure out what it even was, and what its relatives were, and how it fit
into our understanding of the tree of life. And it turns out, it didn’t fit where we expected it to. The mystery of Toxodon begins with
none other than Charles Darwin. Perhaps you've heard of him It was 1833, and Darwin was halfway through his
famous voyage on the Beagle in what’s now Uruguay, when he heard about some interesting
fossils at a farmhouse nearby. There Darwin came face to face with
the skull of what he would describe as “perhaps one of the strangest
animals ever discovered.” The skull was almost as big as a hippo’s, but
it had teeth that looked like a ... rodent’s. And this mash-up of features from some of the biggest living mammals and
some of the smallest puzzled him. Ok and side note: When Darwin first saw the skull, it was
sitting on top of a fence post and was being used FOR TARGET PRACTICE by some kids who were trying
to knock out its teeth by chucking rocks at it. Anyway, thankfully, the skull was still in decent
shape, and Darwin could tell that he was looking at something exciting. He considered
it one of his best finds of the entire voyage. He compared the strange new
skull to that of the giant ground sloth Megatherium, because of its size… But he concluded that it probably
belonged to some kind of aquatic animal, like a manatee or a dugong, based on its
somewhat similar anatomy to those animals. Darwin turned out to be totally wrong
about Toxodon. But the time he spent staring that strange creature in
the face wasn’t a total loss... Because it got him thinking about extinction,
and about how ancient animals and modern ones with similar features and that live
in the same place might be related. So, Darwin shipped the fossil back to the UK, where it was studied by an anatomist
named Richard Owen -- and he’s the one who named it Toxodon, which means
“bow tooth” because of its curved teeth. That curved shape, combined with Owen’s suspicion
that the teeth grew constantly when the animal was alive, led Owen to think that Toxodon might
have been the largest rodent ever discovered. But he was also thrown off by its strange
combination of features. Like, really thrown off. Based on its teeth, it could be a
rodent. But the back of its skull made it look like a manatee-relative.
And its nose indicated that it wasn’t fully aquatic...so maybe it was something kinda
like an elephant, but also like a capybara...?! By 1837, Toxodon had already stumped two
of the biggest names in natural history. And Toxodon wasn’t the only weird mammal
that Darwin sent home from South America… There was also Macrauchenia, which looked a
lot like a giant llama, but it probably had a trunk — or at least a very
fleshy nose. Which is 100% relatable to me And over the last 200 years or so,
paleontologists have found even more fossils in South America that all kind of
look like they’re related to each other, but don’t look very much like any other
group of mammals found anywhere else. These five orders of mammals
have often been lumped together in one larger group called the Meridiungulata. Their fossils first appeared
maybe around 64 million years ago and all vanish from the fossil record by the
late Pleistocene...or maybe the early Holocene. And the odd combinations of features that
many of these animals had has made it nearly impossible to figure out exactly how they’re
related to other mammals using anatomy alone. This is what I meant when I said
“isolation breeds weirdness.” Think of the marsupials of Australia -
their ancient ancestors got isolated on that continent and radiated into
a bunch of different open niches. It looks like a similar thing happened in
South America. Toxodon and its relatives evolved in isolation to fill
whatever niches were open there. And that’s what makes placing them on the
tree of life so hard. Over millions of years, all of those descendants had started looking
less and less like any other group of mammals. Now, we did have some anatomical clues to go on.
Like, almost all of these extinct South American species had hooves - a feature that’s associated
with the living ungulates, like deer and horses. So some researchers thought
they were ungulates, too. But other researchers thought
more like Darwin and Owen, pointing out Toxodon’s similarities to the
group that includes elephants and manatees. So, finally, the time came when we could turn to
new technologies that Darwin and Owen didn’t have. In a study published in 2015, researchers tested
48 specimens of Toxodon and Macrauchenia to see if there was any collagen - which is a really common
kind of protein - left in their remains. These proteins break down more slowly than DNA and can stick around even in climates where
ancient DNA usually isn’t preserved. And in many cases, there was collagen in them! So
they compared the proteins from the two South American ungulates to those from a range of 76
other mammals from across the mammal family tree. And they found that Toxodon and Macrauchenia —
Darwin’s original two weirdos — looked like they belonged in the same clade, meaning
they’d come from a common ancestor. They were also most closely related to a group
called perissodactyls or odd-toed ungulates - like horses and rhinos - and not the group
that includes elephants and manatees. BUT! They weren’t part of the perissodactyl group
itself; they’re what’s called a sister group. That means they share a common ancestor with
the ancestor of all living odd-toed ungulates. And it had been over 60 million years
the two sister groups split - plenty of time for Toxodon to evolve its weird looks. This conclusion was confirmed in 2017,
when another group of researchers announced that they were able to recover
ancient mitochondrial DNA from Macrauchenia, which revealed the exact same results. So, finally, we have a much clearer
idea of where to place some of the South American native mammals in
the tree of life: they’re ungulates, but more closely related to horses and
tapirs than they are to deer and cattle. The weirdness of Toxodon and
its relatives reminds us that isolation can be a powerful force in evolution, making even the relatives of familiar animals,
like horses, seem unfamiliar, given enough time. And although these enigmatic
mammals left no descendants, they helped inspire Darwin
on the voyage of the Beagle. The extinct fossil mammals of South America
got him thinking about common descent before he ever visited the
Galapagos and saw its finches. Through Darwin’s story, the legacy of
the Toxodon and its relatives lives on, and almost 200 years later, they’re
still giving up their secrets. Before you go - if you’re looking for a fun show
for the budding young scientists in your lives, like I am we think you would love MEGAWOW on the PBS Kids
Youtube Channel! Hosted by comedian Katie Nguyen, this show is built to get kids excited about
science through fun and wacky experiments. We guarantee your curious scientist will
scream… MEGAWOW!!! Tell them Eons sent you! And thanks to this month’s Eontologists, who
really raise the hoof. Raise the hoof? Kallie, we have to talk about this: Sean
Dennis, Jake Hart, Annie & Eric Higgins, John Davison Ng, and Patrick Seifert! Raise the hoof? I don't-- Have I
been saying it wrong all this time? By becoming an Eonite at patreon.com/eons you
get fun perks, such as submitting a joke for us to read, like this one from Jared Jordan Ok here we go. I haven't seen it. How did Coelurosaurs listen to music? ThAirpods Alright, I'll allow it And as always thanks for joining me in the Konstantin Haase studio. Subscribe
at youtube.com/eons for more fantastic fossils.
YES!!! I’ve been wanting for a meridiungulata episode for YEARS!!! I kinda wish they would’ve gone more into depth about the members of the groups like the rabbit-mimic or the deer-sized burrower, but this is all still so cool! Any idea where I can get more information on these guys?