In an article published in Nature, researchers in China reported that today's turtles must have originated from an aquatic-turtle ancestor. The results also provide support for a theory of how turtle shells evolved. Chun Li and his team looked at remains that included two skulls and other skeletal bones unearthed in China's Guizhou province in 2007.

Scientists uncovered the remains of three remarkably intact adults in Guizhou province last year. Each has characteristics that have never been seen in turtles before, including teeth and an incomplete upper shell, according to the report in the journal Nature.

The fact that the turtle had a partial shell (only covering its belly) sheds light on an intermediate stage of shell evolution that scientists hadn't seen. Moreover the recent discovery of Odontochelys semi-testacea – “half-shelled turtle with teeth” – is being hailed as the long-sought missing link between turtles that have full shells and their shell-less ancestors.

Odontochelys is ten million years older than Proganochelys, which was found in Germany and had a complete shell. But because it has a fully formed shell, scientists cannot conclusively state how it developed. It is 55 million years older than another primitive turtle, Eileanchelys waldmani, which was discovered on the Isle of Skye and was announced only a week ago in a scientific journal as likely to have been the earliest aquatic turtle.

By working with colleagues in China and Canada, Olivier Rieppel, PhD, chairman of The Field Museum's department of geology, has analyzed the Chinese turtle fossil, finding evidence to support the notion that turtle shells are bony extensions of their backbones and ribs that expanded and grew together to form a hard protective covering. Some researchers theorized that turtle shells started as bony skin plates, called osteoderms, which eventually fused to form a hard shell. But this theory was infirmed in this recent discovery. These newly found features point to another mode of shell evolution in which the belly shell evolved first. Then, the ribs and backbone broadened to form the upper shell, the researchers said.

"It is a really great discovery. I am very happy to see this," Robert Reisz, an evolutionary biologist at the University of Toronto-Mississauga, said, adding that the fossil "will change ideas about turtle origins. This pushes the history of turtles back several million years."

The team also concludes that this earliest known of all turtles was an aquatic animal and not land-based as suggested by other turtle fossils. The presence of the lower plastron is one clue: it would protect the animal from predators below while swimming. The aquatic way of life employed by this earliest turtle implies that turtles, as a group, may have originated from water.

The report appears in the November 27 issue of the scientific journal Nature. Dr. Wu is co-author of the study along with
Chun Li of the Institute of Vertebrate Palaeontology, Chinese Academy of Sciences in Beijing (where the fossils reside), Dr. Olivier Rieppel of the Field Museum in Chicago, Li-Ting Wang of the Geological Survey of Guizhou Province and Li-Jun Zhao of the Zhejiang Museum of Nature History. Chun Li (Dr. Wu's student) discovered the fossils with local farmers.