How did the turtle get its shell? It sounds like the beginning of a fable, but scientists have wondered about that for years and new fossil research gives some clues.
The way turtles evolved to their modern form, with a shell fused to their skeleton and a pelting face without teeth, has been described as "one of the most enduring puzzles of evolution."
Relatively few fossils of early turtles have been found, making it a mystery how the creature developed its unique properties, and even of which ancestors they evolved.
A new study published in the journal Nature fills in a number of gaps by examining a turtle fossil discovered in China that is 228 million years old.
The skeleton has a mouth, but also some teeth, suggesting that it may be a "missing link" in the evolution of a previous toothy tortoise to the form of today.
"This is the first early fossil turtle with a beak", says Chun Li, a researcher at the Chinese Academy of Sciences in Beijing and co-author of the paper on the fossil called Eorhynchochelys sinensis.
"The interesting thing is that, although a beak was developed, the teeth were preserved, so it is a half-basin, half-toothed jaw – an excellent transition property," he said.
The fossil is also large, with a length of 2.5 meters, with a long tail and broad and flat ribs along the back that resemble a disc-like precursor of a scale.
With so little evidence to go further, one of the big debates about the evolution of the turtle is precisely which animals they have evolved from.
One theory states that they share the same common ancestor as most reptiles, but some experts claim that the shape of the skull of a modern tortoise makes this unlikely.
Chun said that the shape of the bones in the new fossil gives weight to the idea that turtles have evolved from the same ancestors as most reptiles, and called the specimen an important missing link in the early evolution of the turtle. ;
It follows a handful of other discoveries in recent years, including a 220 million-year-old copy with a fully formed lower abdominal cover, but no scale on its back and a 240 million-year-old fossil without any scale.
Chun has spent the last 20 years researching reptile fossils in the Chinese province of Guizhou, where the 220 million year old turtle was found.
But he accidentally stumbled upon this latest fossil, when a local museum asked him in 2015 to investigate their marine reptile fossils. It was still in the rock, shown in the collection. "No one knew what it was," he said.