How did the turtle get its shell? It sounds like the beginning of a fable, but scientists have been asking themselves for years, and new fossil research provides some clues.
The way turtles developed into their modern form, with a shell fused to their skeleton and a beak Similar face without teeth was described as "one of the most enduring puzzles of evolution".
Relatively few fossils of early turtles were found, which solved a mystery of how the creature developed its unique features and even what ancestors developed them
A new study published in the journal Nature , fills in some gaps by studying a turtle fossil found in China that is 228 million years old.
The skeleton has a beak, but also a few teeth, suggesting that it may be a "missing link" in the evolution from an earlier tooth turtle to today's shape.
"This is the first early fossil turtle with a beak," said Chun Li, a researcher at the Chinese Academy of Sciences in Beijing and co-author of the paper on the fossil that Eorhynchochelys sinensis
. "Interestingly, even though a beak had developed, the teeth were preserved it's half a beak, half-toothed jaw ̵
The fossil is also large, 2.5 feet long, with a long tail and broad and flat ribs along its back that appear to have formed a disc-like precursor to a shell.
With so little evidence to go on, one of the great debates about the evolution of turtles is what animals they have evolved from.
One theory states that they have the same common ancestor as most reptiles, but some experts argue that the shape of a modern turtle skull makes this unlikely.
Mr. Chun said that the shape of the bones in the new fossil gave weight to the idea that turtles evolved from the same ancestors as most reptiles, and named the specimen "an important missing link in the turtle's early evolution"
follows a A handful of other discoveries in recent years, including a 220 million-year-old specimen with a fully formed lower abdomen but no shell on its back and a 240 million-year-old fossil without a sheath.  Mr. Chun spent the last 20 years studying reptile fossils in Guizhou Province, China, where the 220-million-year-old turtle was found.
But by chance, he came across this latest fossil when asked by a local museum in 2015 to study their marine reptilian fossils. It was still exhibited in the rocks, in the collection. "Nobody knew what it was," he said.