'Missing link' sheds light on turtle evolution

How did the turtle get its shell? It sounds like the start of a fable, but it’s something scientists have wondered for years, and new research by a Chinese fossilized reptiles study team is providing some clues.

The way that turtles evolved into their modern form, with a shell fused to their skeleton and a beak-like face without teeth, has been described as “one of evolution’s most enduring puzzles.”

Relatively few fossils of early turtles have been found, leaving it a mystery how the creature developed its unique features, and even which ancestors it evolved from. But new research published yesterday in the journal Nature fills in some gaps by examining a turtle fossil discovered in China that dates back 228 million years.

The skeleton has a beak, but also some teeth, suggesting it may be a “missing link” in the evolution from an earlier toothy turtle to today’s form.

“This is the first early fossil turtle with a beak,” said Li Chun, a researcher at the at the Institute of Vertebrate Paleontology and Paleoanthropology under the Chinese Academy of Sciences in Beijing and co-author of the paper on the fossil dubbed Eorhynchochelys, which means “first turtle with a beak.”

“The interesting thing is that although a beak had developed, the teeth were preserved, so it is a half-beak, half-toothed jaw — an excellent transitional characteristic,” he said.

The fossil is also large, at 2.5 meters long, with a lengthy 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 over turtle evolution is just which animals they evolved from.

One theory holds that they share the same common ancestor as most reptiles, but some experts argue the shape of a modern turtle’s skull means this is unlikely.

Chun said the shape of the bones in the new fossil lends weight to the idea that turtles evolved from the same ancestors as most reptiles, calling 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 specimen with a fully-formed underbelly covering but no shell on its back and a 240-million-year-old fossil with no shell.

Chun has spent the last 20 years studying reptile fossils in southwest China’s Guizhou Province, where the 220-million-year-old turtle was also discovered.

But he stumbled upon this latest fossil by chance, when a local museum in Sanya, south China’s Hainan Province, asked him in 2015 to examine their marine reptile fossils.

It was still in the rock, displayed in the collection.

 “It looked like a primitive turtle. I guessed some people dug it out from Guizhou’s Guanling County, but obviously nobody knew what it was,” Li said.

Li and his team spent a year repairing the fossil and investigating its origin.

“This skeleton suggested the turtle might have lived an amphibious life near an estuary, and had a habit of digging holes,” Li said.