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

  • totally new to science.

  • The thing was clearly a turtle.

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

  • on topso 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

  • a century.

  • Like, where did turtles come from?

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

  • Andhow 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 meansearly

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

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

  • today.

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

  • under its shell.

  • 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, ortrue reptiles.”

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

  • But there’s also, parareptilia, orside reptiles.”

  • These are some of the earliest reptiles, all of which are now extinctlike 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

  • from.

  • 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

  • parareptiles.

  • 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 theugliest 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 turtlesand 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.

  • But, you know how scientists are!

  • 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

  • course of their lives.

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

  • modern turtles.

  • 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 thenormalribs, do the same, widening

  • and fusing to form the carapace.

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

  • first, and the shell later.

  • 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

  • the ribs below.

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

  • be much more similar to eureptiles.

  • 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

  • the plastron.

  • And it didn’t have a carapace

  • It was a shell-less turtle!

  • They called it Odontochelys, and it dated to about 220 million years agoaround

  • 10 million years before Proganochelys from Germany.

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

  • developmental biologistshypothesis, that the plastron evolved first.

  • But there was more.

  • Unlike all other known turtles, Odontochelys hadteeth.

  • Its name actually meanstoothed 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.

  • It was a eureptile.

  • 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

  • in a subgroup of eureptilia.

  • 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,

  • called Eunotosaurus.

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

  • found Odontochelys

  • 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.

  • But it did have wide, flat ribs.

  • 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, orgrandfather turtle.”

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

  • them that were partially fusedbut not to the point that they formed a plastron.

  • So old grandpa turtle seemed to mark a kind of transitional stage between Eunotosaurus,

  • with its wider ribs, and Odontochelys, with its full plastron.

  • Together, these discoveries helped fill out the timeline of turtle evolution, and it became

  • clear that the first step in the evolution of turtle shells was the formation of wider

  • ribs.

  • But there was still the question of why.

  • Why did turtles acquire these weirdly wide ribs in the first place?

  • What purpose did they serve?

  • And why did they eventually develop into shells?

  • Well, in 2016, paleontologists again took a closer look at Eunotosaurus -- which is

  • now considered the oldest of the turtle ancestors -- and noticed something funny about it.

  • Most of us think of turtles as being adapted for life in the water, with webbed feet or

  • flippers.

  • But Eunotosaurus seemed to have a lot of adaptations for burrowing through dirt.

  • Its head was shaped basically like a shovel.

  • Its front legs were stronger than its rear legs, and it had giant claws that would have

  • been great for digging.

  • So, the evidence pointed to life as a burrower.

  • And this could help explain why it had those wide ribs ... and where turtle

  • shells came from.

  • Researchers proposed that wider ribs wouldve been useful as an anchor when Eunotosaurus

  • Eunotosaurus was digging with its front legs

  • Wider ribs provide a more stabilized trunk, which would have made it easier for the turtle

  • to keep its body in one place while it was digging.

  • Other burrowing animals, like anteaters, have similar adaptations.

  • Problem is, having wide ribs with such short legs makes it awfully hard to walk.

  • Or at least, walk quickly.

  • In fact, the reason that turtles are so famously slow is that their giant ribs make it hard

  • for them to swing their little legs forward.

  • So, ancestral turtles needed extra-wide ribs for digging, but that also slowed them down.

  • Now they needed more protection.

  • And that’s when the ribs started to fuse into a plastron, which eventually became a

  • full, protective shell.

  • Over time, the evolutionary purpose of the turtle’s shell changed, from digging to

  • protection, and turtles as we know them became a thing.

  • Of course, none of this is fully resolved.

  • In theory, it makes sense for wider ribs to have evolved for digging, but right now that’s

  • just based on what weve seen in one species.

  • We still need a lot more evidence.

  • Turtlesexact place among the eureptiles isn’t settled either.

  • A lot of researchers think theyre more closely related to a clade that includes animals

  • like crocodiles and birds.

  • But others argue that theyre closer to a different group that includes lizards and

  • snakes.

  • Hopefully it won’t take another 130 years, for an answer to that debate.

  • But either way, we now know that, on the tree of life, the turtlesbranch isn’t turtles

  • all the way down.

  • It’s stacked with a diverse array of reptilian characters, some of which have no shells,

  • others of which have partial shells, and others still who sport the full, beautiful shells

  • and famously slow gaits that we know today.

  • Thanks for joining me!

  • And as always, I want to know what you want to learn about!

  • So leave me a note in the comments below!

  • And be sure to go to youtube.com/eons and subscribe.

  • And if youre interested in where things came from, then you should definitely check

  • out The Origin of Everything, it's a show that explores the social origins of our everyday

  • lives, like why we get letter grades in school, and why the heart is a symbol of love.

  • Think of it as the Eons for understanding modern society.

  • And you don’t have to go through millions of years of history to learn the answers!

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

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B1 中級 美國腔

龟是怎么得到它的壳的(How the Turtle Got Its Shell)

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    joey joey 發佈於 2021 年 05 月 04 日
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