Friday, November 28, 2008

Scientists discover fossil turtle with half a shell

How did the turtle get its shell? The answer is no longer a Just So story. Scientists have unearthed the first fossil found of a turtle at an intermediate stage of evolution, with only half a shell. The ancient creature's belly is fully covered but its back is not. Olivier Rieppel, of the Field Museum in Chicago, said the 220 million-year-old remains - the oldest turtle bones discovered so far - helped solve one of the great mysteries of reptile evolution - the origin of the turtle's protective armour. "The new species shows that the plastron (the lower part of the shell) evolved before the carapace (the upper part)," he said. It also overturned a theory that the turtle's shell had grown out from bony plates on top of the skin, called osteoderms, like those found on crocodiles and some dinosaurs. "This animal tells people to forget about turtle ancestors covered with osteoderms," said Dr Rieppel, a member of the Chinese, American and Canadian team that made the find. Turtles hit on a winning body plan early on. They have looked much as they do today since the time of dinosaurs. And scientists have been debating the origin of their hard, bony shells, which have provided them with shelter and protection, since the 1800s. The new species, Odontochelys semitestacea, has a small, partial shell on its back, extending from its backbone. The well-preserved specimens of three of the ancient turtles also had ribs that had begun to widen, the scientists report in the journal Nature. This supported the theory that the upper shell was an outgrowth of the backbone and ribs.

Dr Rieppel said the fact that ancient creature had a fully formed shell on its underbelly also suggested that the earliest turtles were aquatic animals, rather than land dwellers. This covering would have provided protection from the dangers of predators swimming below it. An Australian expert in ancient reptiles, Benjamin Kear, of La Trobe University in Victoria, said the lower shell might also have acted like a diving belt.


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