“How the turtle got its shell” sounds like a fable or a nursery rhyme, but a group of Chinese and Chicago scientists think they have figured out the story for real and are telling it in this week’s edition of the research journal Nature.

“The origin of the turtle shell has been a big debate in paleontology for a long time,” said Olivier Rieppel, curator of fossil reptiles at the Field Museum in Chicago. “The turtle shell is such a specialized, unique feature. No other vertebrate animal group has this kind of body plan that has always been a big mystery.”

The Nature paper, which Rieppel co-wrote, introduces the oldest turtle fossil ever found: the remains of a toothy, aquatic creature about 16 inches long. The turtle lived 220 million years ago – a time when dinosaurs first began to appear – in an area of shallow seas that is now an inland mountainous area southwest China.

The authors say the fossil skeleton shows how turtle shells evolved. Dubbed Odontochelys semitestacea, Latin for “half-shelled turtle with teeth,” the animal had a hard shell covering its belly, known scientifically as the plastron, but lacked a shell protecting its back, or the carapace.

In primitive turtles like this one, they write, the species’ ribs and backbones were beginning to expand and grow together, putting turtles on an evolutionary track that a few million years later resulted in a protective carapace.

Rieppel said the findings refute an alternative theory that turtle shells evolved from osteoderms, the tough, bony skin plates seen in crocodiles and other contemporary animals. The idea was that in the ancestors of turtles the osteoderms eventually fused to become a hard shell.

“It is a really great discovery. I am very happy to see this,” said Robert Reisz, an evolutionary biologist at the University of Toronto-Mississauga. The fossil, he added, “will change ideas about turtle origins. This pushes the history of turtles back several million years.”

Still, Reisz takes issue with the conclusions of Rieppel and his co-authors – a measure of how deeply the debate about turtle origins reaches. Reisz wrote a dissenting story in the same issue of Nature arguing that Odontochelys probably isn’t as primitive as the others think.

“I agree with much of what they say,” said Reisz, a friend of Rieppel’s who has co-written earlier major articles on turtle evolution with him.

Reisz said he thinks Odontochelys once had a hard-shell carapace but lost it as an adaptation to new environmental conditions. Such an evolutionary reduction also is evident in many of today’s sea turtles, which lack the carapace possessed by their ancestors.

There is no argument, however, that Odontochelys is the most primitive, least evolved turtle fossil ever found. Most previous finds were like modern turtles, with full shells, beaked mouths and no teeth.

The oldest previously known turtle fossil, found in Germany and dating to about 206 million years ago, had a beak as well as rudimentary teeth in the roof of its mouth. Odontochelys had teeth on its upper and lower jaws, and no beak.

The lead author of the Nature article, Beijing paleontologist Chun Li, obtained three nearly complete Odontochelys fossils in 2007 from limestone formations at three sites near the town of Guanling, which in the last decade has become a rich source of ancient marine fossils.

A building boom in the town in the 1990s prompted the opening of new stone quarries. There were so many fossils in the quarry rock that the area has attracted major scientific interest and turned local farmers into avid prospectors.

Chun and his colleagues asked Rieppel to collaborate with them in interpreting the fossils and publishing the article.

Rieppel said he thinks the fact that Odontochelys had only belly armor supports the argument that turtles first evolved in the water – an idea he and Riesz proposed in 1999, stirring hot controversy in paleontological circles.

“Their ventral body armor speaks to how turtles originated in a marine environment, protecting them from predators that could attack them from below,” he said.

Reisz calls turtles “one of history’s great success stories.”

“A lot of animals have armor, but none did it as elegantly as the turtle,” he said. “They appeared on Earth about the time of dinosaurs, and they are still part of our biota, 65 million years after the extinction of dinosaurs.”

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AP-NY-11-26-08 2221EST