The discovery in China of the oldest known turtle fossil, estimated at 220- million-years-old, gives scientists a clearer picture of how the turtle got its shell.From the press release:
Evidence supports the notion that turtle shells are bony extensions of their backbones and ribs that expanded and grew together to form a hard protective covering.
The fossilized turtle ancestor, dubbed Odontochelys semitestacea (translation: half-shelled turtle with teeth), likely lived in the water rather than on land.
Prior to discovery of Odontochelys, the oldest known turtle specimen was Proganochelys, which was found in Germany. Because Proganochelys has a fully-formed shell, it provides little information about how shells were formed. Odontochelys is older than Proganochelys and is helpful because it has only a partial shell, Rieppel said.
Odontochelys has no osteoderms and it has a partial shell extending from its backbone. It also shows a widening of ribs. Although Odontochelys has only a partial shell protecting its back, it does have a fully formed plastron – complete protection of its underside – just as turtles do today.
Odontochelys was a water dweller whose swimming exposed its underside to predators, Rieppel said. "Reptiles living on the land have their bellies close to the ground with little exposure to danger," he said.
"This animal tells people to forget about turtle ancestors covered with osteoderms," he said.
Note: I did try to get a photo of the turtle for the blog. The press release says to contact the Field Museum's press person whose automatic e-mail reply states that they're out of the office until next Monday. Brilliant.
Our old friend Xiao-chun Wu from the Canadian Museum of Nature is a co-author on the paper. They'll have their press release posted tomorrow.
Update: Thanks to Dan at the CMN for the photos!