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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.