How did turtles get their shells?

Ever wonder how a turtle got its shell? You're not the only one. Evolutionary biologists and paleontologists have long been stumped by the question. But a recently unearthed turtle fossil, the oldest on record, may hold the answer. Researchers report in Nature today that the fossil indicates shells evolved as an extension of turtles' backbones and ribs.

"Its discovery opens a new chapter in the study of the origins and early history of these fascinating reptiles," says vertebrate paleontologists Robert Reisz and Jason Head of the University of Toronto, in a commentary accompanying the article. 

Scientist have been in the dark until now because all fossilized turtles previously discovered had complete shells. But this 220 million-year-old fossil is an ancestor of the modern turtle at a stage when its shell was still evolving.
...