November 26, 2008

Fossils Of Oldest Known Turtle Discovered In China

Researchers in China reported finding fossilized remains of the most primitive turtle to date, providing more insight into how reptiles evolved.

The fossils depict an ancient turtle with teeth, a fully formed belly shell and a partially formed back shell.

Researchers say the so-called Odontochelys (half-shelled turtle with teeth) lived some 220 million years ago.

Scientists have lacked enough evidence to prove competing evolutionary theories because turtles have looked pretty much the same as they do now sinve the age of dinosaurs.

Previous digs had uncovered the Proganochely, another class of turtle previously thought to be the oldest. Discovered in Germany, Proganochely was 10 million years younger than the Odontochely.

"What we found (Odontochely) is an intermediate missing link," said lead researcher Li Chun at the Chinese Academy of Sciences and Institute of Vertebrate Paleontology and Paleoanthropology.

Li Chin worked alongside Xiao-Chun Wu of the Canadian Museum of Nature in Ottawa and Field's Rieppel. Their report is published in the journal Nature.

"This is the first turtle with an incomplete shell," Rieppel said. "The shell is an evolutionary innovation. It's difficult to explain how it evolved without an intermediate example."

From the three Odontochely fossils discovered in China, Li said it was clear the turtle first developed the plastron, or the lower shell that encases the belly, before getting its upper shell, or the carapace.

"The plastron developed first and after it was fully formed, then the carapace developed," he said.

"They had neural plates, which is the part that develops first on the back shell," he said.

Li said the turtle dwelled mostly in the water and was a carnivore.

Some contemporary reptiles such as crocodiles have skin with bony plates and this was also seen in ancient creatures such as dinosaurs. Some researchers theorized that turtle shells started as bony skin plates, called osteoderms, which eventually fused to form a hard shell.

But Odontochelys has no osteoderms and it has a partial shell extending from its backbone, Rieppel said.

"This animal tells people to forget about turtle ancestors covered with osteoderms," he said.


Image Caption: Skeleton of Proganochelys quenstedti, American Museum of Natural History. Courtesy Claire Houck - Wikipedia


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