Fossilised remains of the most ancient turtle yet discovered are helping scientists to unravel the Kiplingesque puzzle of how the animal grew its shell.

Only the underside of the turtle is covered by a fully formed protective shell, giving researchers an invaluable glimpse into how it evolved.

The 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. Three fossilised specimens dug up near Guanling in the southern Chinese province of Guizhou have been dated at 220 million years old and the species has been identified as the ancestor of all other known turtles.

Fossils from the dig have now enabled researchers to discount the theory that the shell originally formed from bony plates, like those on a crocodile, which expanded and fused together. An international team has concluded that the rival theory that the shell was created when backbones and ribs spread out and joined up to form a hard bony cover is likely to be correct.

Xiao-chun Wu, a palaeontologist at the Canadian Museum of Nature in Ottawa and a member of the research team, said: “Since the 1800s, there have been many hypotheses about the origin of the turtle shell. 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.”

Olivier Rieppel, of the Field Museum, in Chicago, added: “This is the first turtle with an incomplete shell. It’s difficult to explain how it evolved without an intermediate example.”

Because the shell was incomplete the researchers were able to conclude that the shell on the underside of turtles, the plastron, developed before the upper section, the carapace.

All three specimens were found last year and were described as “remarkably intact”. Among the features never before seen in turtles were the rows of stumpy teeth on both jaws.

The turtle would have had a pointed snout and the researchers, who reported their findings in the journal Nature, are confident that it could swim. They said that the development of armour on the underside suggested an aquatic lifestyle because it would have offered protection from being attacked from below. The discovery of fossilised marine reptiles and invertebrates close to the three turtles also indicated that the species lived by the sea or in river deltas.

Odontochelys is ten million years older than Proganochelys, which was found in Germany and had a complete shell. 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.

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