The Creature That Stumped Darwin

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,