In an article published in Nature, they said the Odontochelys lived about 220 million years ago and their discovery sheds light on one of the biggest mysteries in reptile evolution -- how the shell and body of the turtle developed.

Their discovery displaces the Proganochely, another class of turtle previously thought to be the oldest. With fully formed upper and lower shells, the Proganochely was found in Germany and was 10 million years younger than the Odontochely.

"What we found (Odontochely) is an intermediate missing link," lead researcher Li Chun at the Chinese Academy of Sciences said in a telephone interview from Beijing.

"The German turtle is already very similar to the turtles we know of today. But we had never known how the turtle shell evolved. It couldn't have just appeared suddenly."

From the three Odontochely fossils discovered in China, Li said it was clear the turtle first developed the plastron, or the lower shell that encases the belly, before getting its upper shell, or the carapace.

"The plastron developed first and after it was fully formed, then the carapace developed," he said.

The fossils were uncovered in marine deposits in the Nanpanjiang Trough Basin in China's southern Guizhou province. Two of the fossils were complete.

Li said the three fossils, two between 20 and 30 cm and the third about 40 cm showed early evolvement of the carapace.

"They had neural plates, which is the part that develops first on the back shell," he said.

Li said the creatures were meat eaters.

"They were aquatic animals but we cannot be certain if they were freshwater or saltwater."