How The Turtle Got Its Shell : The Two-Way The ribs of a 240 million-year-old fossil hold clues to how the first turtle shell evolved. And its skull shape seems closer to that of lizards and snakes than to an ancestor of dinosaurs and birds.
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How The Turtle Got Its Shell

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How The Turtle Got Its Shell

How The Turtle Got Its Shell

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RACHEL MARTIN, HOST:

Ever wonder why the turtle has a shell? Come on, I know you have. The evolutionary origin of those shells has been a mystery until now. NPR's Nell Greenfieldboyce has more.

NELL GREENFIELDBOYCE, BYLINE: Other animals have body armor too. Think of armadillos. But Hans Sues says turtles are really special.

HANS SUES: Just to be completely enclosed basically in its own little bony house is something that's unique to turtles.

GREENFIELDBOYCE: He's a paleontologist at the Smithsonian's National Museum of Natural History in Washington, D.C., and he says fully formed turtles were plodding around about the same time as early dinosaurs.

SUES: The first turtle that we have where the whole shell is developed in a manner that anyone on the street would recognize as a turtle shows up about 214 million years ago.

GREENFIELDBOYCE: But what kind of creature came before that recognizable turtle? There's been a big gap in the fossil record.

SUES: That had nothing to offer us in terms of a plausible turtle precursor.

GREENFIELDBOYCE: That is starting to change. For example, today in the journal Nature, Sues and a colleague describe fossils that were recently unearthed in a limestone quarry in Germany. They're the bones of a reptile that lived some 25 million years earlier in a large lake. This lizard had slender legs, a long tail and neck and a strangely boxy, rigid belly.

SUES: It has real beginnings of the belly shell developing, little rib-like structures beginning to fuse together into larger plates and then ultimately making up the belly shell.

GREENFIELDBOYCE: He says scientists can use this fossil along with other turtle-like fossils from China and South Africa to piece together the sequence of key events in turtle shell evolution.

SUES: You first make broad ribs. Then you build the belly shell, and then you complete the back shell. And then you have basically what's a modern turtle.

GREENFIELDBOYCE: With the when and how of turtle shell evolution pretty much solved, the next question is why. Tyler Lyson is at the Denver Museum of Nature and Science at Colorado. He says it's a puzzle.

TYLER LYSON: The classic idea has always been that the shell evolved for protection.

GREENFIELDBOYCE: But he points out that the ribs play a vital role in breathing.

LYSON: Why lock up your ribs into a shell? No other animal does that.

GREENFIELDBOYCE: And doing that meant that turtles had to come up with a new way to get air in and out. Why evolution went for this option is the latest turtle mystery to ponder. Nell Greenfieldboyce, NPR News.

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