The unearthing of a 220m-year-old fossil in China has solved the enduring mystery of how the turtle got its shell.

The ancient remains are the only evidence fossil hunters have of a turtle that is midway through evolving a shell, revealing for the first time how the process happened.

Fossil hunters uncovered the remains of three remarkably intact adult turtles 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 a report in the journal Nature.

Turtles have had full shells since the time of the dinosaurs. Before the latest find, the oldest known turtle fossil was a sample unearthed in Germany and dated to 210m years ago. That creature, named Proganochelys, did not shed light on the evolution of shells because its own was already fully formed.

Scientists have been divided on how the shell originally evolved, with some arguing that the shell grows from bony plates on the skin that broaden to form a kind of armour before fusing to the underlying ribs and backbone.

Modern reptiles such as crocodiles have bony plates, called osteoderms, a feature also seen in some dinosaurs, including the ankylosaurs.

The latest fossil, named Odontochelys semistestacea, meaning “toothed, half-shelled turtle”, shows that shells formed in two stages. First, the underside of the shell, called the plastron, developed, then the ribs and backbone grew out to form the upper shell or carapace.

Odontochelys has a fully formed plastron but only a partial upper shell extending from its widened ribs and backbone.

“Now we have these fossils of the earliest known turtle. They support the theory that the shell would have formed from below as extensions of the backbone and ribs, rather than as bony plates from the skin as others have theorised,” said Dr Xiao-chun Wu at the Canadian Museum of Nature in Ottawa.

Wu’s student and lead author of the report, Chun Li, of the Institute of Vertebrate Palaeontology at the Chinese Academy of Sciences in Beijing, discovered the fossils with the help of farmers in Guizhou province.

Detailed inspection of the fossils suggests Odontochelys was an aquatic animal and not land-based. The lower shell is one clue, as it would have protected the turtle from predators below as it swam. The remains of other marine reptiles and invertebrates were found in the same rock formations.

“Reptiles living on the land have their bellies close to the ground with little exposure to danger,” said Olivier Rieppel, head of geology at the Field Museum in Chicago. “This animal tells people to forget about turtle ancestors covered with osteoderms.”—Dawn/Guardian News Service