Image: Turtle shell evolution
Royal Society
This graphic suggests how the turtle got its shell, based on recent fossil finds: Over millions of years, thin bits of dermal armor ardened armor may have fused together to create the full shell.
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updated 10/8/2008 1:18:55 PM ET 2008-10-08T17:18:55

Famous for carrying its shelled "home" on its back, the humble, plodding turtle has also been toting around one of the biggest mysteries of the animal kingdom. Paleontologists have now unearthed a bizarre fossil beast in the eastern New Mexican desert that might put that mystery to rest.

A foot long and armored from head to tail, the 215-million-year-old fossil Chinlechelys tenertesta is a missing link in turtle evolution that promises to finally settle a controversy that's been raging for the past two centuries over how turtles got their shells.

There are two camps in the debate. As turtle embryos develop, their shells grow directly from the animals' ribs, and adult turtles' ribs are fused to the shell carapace. Some scientists conclude this must have been how the shells originally developed in antiquity, too — normal rib bones gradually flattened out and spread until they formed a complete shell.

But animals like armadillos have shells that aren't attached to their ribs. Instead the shell is skin that has thickened and hardened to provide protection. This so-called "dermal armor" is also prevalent among ankylosaurs, a group of stoutly built dinosaurs that lived in the Jurassic and Cretaceous eras.

Walter Joyce of Yale University was the first to identify the new fossil as a primordial turtle from just a few bits of the neck and shell. "It's a pretty ugly fossil, really," Joyce said of the jumbled pieces he examined, "almost like a shoebox full of crud."

But the key, Joyce said, was an intact series of three neck spines, a small piece of the belly shell, and a fragment of the back shell with ribs attached.

"That's what really gave it away," Joyce said of the final piece. "You can see that the ribs are not fused to the shell."

Covered in dermal armor, the ancient turtle probably looked a lot like an ankylosaur, though the two species are unrelated. It couldn't yet retract its neck or feet, and its shell was thinner than a modern turtle's, but Chinlechelys tenertesta was bristled with sharp spines along its neck and tail.

"This is very clear evidence that the shell is a composite structure," James Parham of the Field Museum in Chicago said. "It is a missing link. This is one of the most important turtle fossils ever found, I think."

© 2012 Discovery Channel

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