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.

An international team of paleontologists led by Chun Li of the Chinese Academy of Sciences in Beijing analyzed the 16-inch- (40 centimeter-) long fossil, which was discovered in Guizhou Province in southwestern China. They conclude that the bones belonged to a Triassic turtle ancestor, which they named Odontochelys semitestacea, meaning "toothed turtle with a half shell."

The newly discovered species sported a shelled belly and a little extra bone on its spine, supporting the theory that turtles' shells formed over eons as their backbones and ribs grew. The main competing idea is that hard, bony plates in their skin (such as those in the skin of modern-day crocodiles) fused together.

The authors say that this turtle species probably lived in water and that their stomach shell kept them safe from predators below while they were swimming.

(Turtle fossil photo above courtesy of Institute of Vertebrate Palaeontology and Palaeanthropology, Beijing; Illustration of Odontochelys semitestacea below courtesy of Marlene Donnelly)