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It is one of evolution's hardest mysteries: How did the turtle get its shell?

Scientists in Canada, China and the United States have cracked it with the discovery of the world's oldest turtle fossil, a 220-million-year-old creature with only a partial shell.

In Thursday's edition of the British journal Nature, they describe the primitive turtle unearthed in southwestern China, a find they say offers convincing evidence of how the protective covering evolved.

“No other turtle has such intermediate patterns in the shape of the shell,” says Xiao-chun Wu, a paleontologist with the Canadian Museum of Nature in Ottawa and a co-author of the paper.

Turtles have looked the same since the time of the dinosaurs. Their shell consists of the upper covering – called a carapace – and the lower belly shield known as the plastron.

The two are joined by a bridge, and the head, tail and limbs poke out.

No other animal has a shell with the same kind of tank-like design, and, until now, all turtle fossils had a complete version of it. That made it impossible for researchers to know how it first grew.

There are two main theories about how the turtle got its shell.

One proposes that the shell developed from plates on the skin, and was similar to the body armour found on ankylosaurs and some other dinosaurs.

The other posits that the shell was an extension of the backbone and ribs. The bone theory was supported by embryonic evidence; as turtles mature in the egg, their backbone expands outward and ribs broaden to meet and form a shell, Dr. Wu says.

But that wasn't enough to prove how the shell evolved.

The newly discovered fossil clinches it in favour of the bone theory, says Olivier Rieppel, a researcher at the Field Museum in Chicago and a co-author of the paper.

The animal's ribs are broad, but have not fused. A partial shell extends from its backbone. The bottom part of the shell is complete, but the top part isn't, Dr. Rieppel says.

This makes sense because it lived in the water, and predators would have probably attacked it from below, say Dr. Wu and his colleagues. The team also included Chinese researchers Chun Li, Li-Ting Wang and Li-Jun Zhao.

The ancient turtle was long and relatively skinny, like some of the 300 species of turtles found today, but it had teeth. Modern turtles don't. They have horny beaks and swallow their food without chewing.

The discovery suggests that turtles may have first evolved in the water, not on land, as was suspected.

It is a plausible theory, says Robert Reisz, a paleontologist at the University of Toronto. But he says there is an alternative way to look at the new find.

It could be that the shell of the 220-million-year-old turtle was a specialized adaptation to an aquatic environment. Perhaps, he says, its ancestors were terrestrial. It may have been the first turtle to move into the water.

In either case, the fossil is a “really great discovery,” Dr. Reisz says, and an important chapter in turtle history. The reptiles have kept the same body type for hundreds of millions of years, and today they inhabit every continent but Antarctica.

“Dinosaurs came and went,” he says, “but turtles are still around. ''