Name: How the Turtle Got Its Shell
Uploaded: 2021-05-04T07:11:00.000Z
Duration: 9 min 12 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

In 1887, a scientist in Germany announced the discovery of a fossil animal that was

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I mean, it had a shell, with a flat plastron on the bottom that was fused to a carapace

on top — so it had all the requirements for what makes an official turtle.

But this was not only a whole new genus and species of turtle, it was also the oldest

turtle that had ever been found at the time.

And this discovery ended up spawning a debate over turtles that would last for more than

What lineage gave rise to these weird reptiles with beaks for mouths and retractable necks?

And … how did the turtle get its shell?

Well, I don't want to spoil it for ya, but the answers would eventually cause scientists

to rethink the entire history of reptile evolution.

The German turtle that started all this was called Proganochelys, which means “early

shell,” and it lived about 210 million years ago, in the late Triassic Period.

At about a meter long, it was pretty big — about half the length of the largest species alive

But unlike modern turtles, Proganochelys hadn't yet developed the ability to retract its head

The fact that its shell was found intact was really helpful, because that made it easy

to identify this animal as a true turtle.

But at the same time, it left a lot of open questions about how that shell evolved, and

why it resembled modern turtles in some ways, but not in others.

So the discovery of this creature kicked off a debate about which major taxonomic group,

or clade, of reptiles that turtles belonged to.

Now, most reptiles fall under the clade known as eureptilia, or “true reptiles.”

This includes stuff like lizards, snakes, dinosaurs, and birds.

But there's also, parareptilia, or “side reptiles.”

These are some of the earliest reptiles, all of which are now extinct — like the spiky-cheeked

procolophonids and the mesosaurs, which were probably the first aquatic reptiles.

Now, by and large, which clade you put turtles in, depends on where you think its shell came

Starting in the late 19th century, most paleontologists thought that turtles belonged in the clade

we now call parareptilia, because they shared the same basic skull structure with other

And by the late 1940s, this theory became even more specific, suggesting that turtles

were related to a particular group of extinct parareptiles called pareiasaurs.

These are sometimes called the “ugliest reptiles,” but we don't judge here

Bradysaurus, for example, lived around 260 million years ago, in the Permian Period,

and it was covered in a layer of hardened scutes.

Likewise, a later pareiasaur called Anthodon had more developed bony plates that formed

a layer of armor a lot like a turtle's shell.

So it's not too hard to see why a lot of paleontologists thought that pareiasaurs were

closely related to turtles — and were maybe even their direct ancestors.

The idea was that over time, the scutes found in pareiasaurs could have fused into a solid

protective layer, eventually combining with the ribs to form a shell.

No one likes a good, rigorous, centuries-long argument more than they do!

Enter the developmental biologists -- they study how living things grow throughout the

Starting in the late 1920s, some of these biologists studied the embryonic growth of

They found that, as turtles develop, bones that basically function as a set of lower

ribs widen and fuse, forming the plastron.

Then another set of bones up top, which are the “normal” ribs, do the same, widening

So these scientists proposed that turtle evolution took a similar path, with widened ribs forming

By the 1990s, the idea had really taken off.

Keep in mind, this was basically the opposite of what the paleontologists were proposing

— that the shell on top evolved from scutes that fused together before combining with

And if the developmental biologists were right, turtles wouldn't be parareptiles; they'd

So, both sides had some evidence to support their cases.

But it was hard to resolve the debate without more, and older, fossils.

Then, in 2008, researchers found one in China.

It was shaped a lot like a turtle, but it only had the bottom part of the shell —

They called it Odontochelys, and it dated to about 220 million years ago — around

10 million years before Proganochelys from Germany.

The fact that it had a plastron without a carapace was pretty strong evidence for the

developmental biologists' hypothesis, that the plastron evolved first.

Unlike all other known turtles, Odontochelys had … teeth.

Its name actually means “toothed turtle.”

And those teeth looked nothing like pareiasaur teeth.

Pareiasaurs had teeth with lots of little cusps on them, like human molars.

But this turtle's teeth were more like pegs.

So it looked like this ancient turtle didn't belong in the parareptile group.

Eventually, this would be supported by several other genetic studies done in the last few

decades, that compared the turtle genome with that of other reptiles, and placed turtles

But while all that was going on, more species got added to the turtle family tree.

In 2010, the discovery of Odontochelys led researchers to reexamine another ancient species,

It lived about 260 million years ago in Africa -- long before Proganochelys, and the previously

The first fossil of this species was originally found back in 1892, but most experts at the

time didn't think it was a turtle ancestor, because it didn't have a shell.

And modern paleontologists noticed that it bore more than a passing resemblance to Odontochelys.

Finally, in 2015, researchers discovered yet another early turtle in Germany, which they

called Pappochelys, or “grandfather turtle.”

It lived about 240 million years ago, and it had wide ribs, with a set of bones below

them that were partially fused — but not to the point that they formed a plastron.

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How the Turtle Got Its Shell

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