PARIS (AFP) -– A stunningly intact 220-million-year-old fossil found in southwestern China appears to have settled a long-simmering debate over reptile evolution: how did turtles get their shell?

In a study to be published Thursday, scientists report on the discovery of a missing-link species -- Odontochelys semitestacea, for ""toothed, half-shell turtle"" -- whose outer shell emerged directly from the ribs and backbone and not from the skin, as some have argued.
The find also suggests that turtles originated in water rather than on land, and pushes back the group's first known appearance on Earth by some 10 million years.
Since the era of dinosaurs, which roamed the planet until 65 million years ago, turtles have looked pretty much the way they do today.
They sport an armor-like upper shell, known as a carapace, connected to a softer lower part, called a plastron.
But in the absence of hard evidence, scientists argued since the early 1800s over exactly how this reptilian mobile home came into being.
One school of thought said the shell evolved from skin. According to this theory, small bony plates called osteoderms -- like those found on crocodiles -- broadened in size to form a kind of dermal plating that fused over time with the ribs.
The competing theory said that the plastron formed first, followed by an outgrowth and widening of the ribs and backbone to form the hard-shell carapace into which turtles withdraw to escape predators.
A similar process unfolds in the transformation of modern-day turtle embryos into hatchlings.
""With Odontochelys, we now have clear fossil evidence of this process emerging in an adult,"" said Xiao-chun Wu, a palaeontologist at the Canadian Museum of Nature in Ottawa and co-author of the study.
The team of scientists, led by Lau Li-Jun of the Zhejiang Museum of Natural History in Hangzhou, China also suggest that the new find points to an aquatic origin for turtles.
The fact that O. semitestacea has only a half-shell on top but a fully-formed plastron -- like turtles today -- is evidence that its underside was exposed to predators in the water.
""Reptiles living on the land have their bellies close to the ground with little exposure to danger,"" said co-author Olivier Rieppel of The Field Museum in Chicago.
The scientists also found other marine reptiles and invertebrates embedded in the same rocks in Guizhou Province that yielded the new turtle species.
The study was published in Nature, the flagship journal of the London-based Nature Publishing Group.