This 228 million-year-old turtle didn’t have a shell

A newly discovered early turtle fossil from 228 million years ago is missing something crucial: its shell.

But in the long, intriguing tale of turtle evolution, turtles didn’t always have it all — shell included. This new species, found in China’s Guizhou Province, did possess a toothless beak, another key turtle feature.

A study published Wednesday in the journal Nature details the discovery.

The species has been dubbed Eorhynchochelys sinensis, which translates to “dawn beak turtle from China,” because it’s essentially the first turtle to have a beak.

A newly discovered early turtle fossil from 228 million years ago is missing something crucial: its shell.
CREDIT: Nick Fraser/National Musems Scotland

It had a Frisbee-shaped body with wide ribs, but those ribs didn’t contribute to the formation of a shell like in modern turtles.

“This creature was over six feet long, it had a strange disc-like body and a long tail, and the anterior part of its jaws developed into this strange beak,” said Olivier Rieppel, study co-author and paleontologist at Chicago’s Field Museum, in a statement. “It probably lived in shallow water and dug in the mud for food.”

Turtle evolution is complicated and difficult to piece together because their evolutionary track can’t be compared to others’.

Other early turtles had a partial shell but no beak. Odontochelys, dating to 220 million years ago, had a protective shell on its underside but no upper shell, called a carapace. Pappochelys, which lived 240 million years ago, had a bony structure over the belly. Modern turtles have both a shell and a beak, but their evolution is more a zigzag than a straight line.

Eorhynchochelys, found nearly 25 feet beneath the sediments where Odontochelys was found, adds another piece to the puzzle.

By developing a beak before other turtles, this early turtle is a prime example of mosaic evolution, in which traits evolve independently and at different times, the researchers said.

Eventually, the genetic mutations for a beak and a shell happened in the same turtle.

“This impressively large fossil is a very exciting discovery, giving us another piece in the puzzle of turtle evolution,” said Nick Fraser, a study co-author with National Museums Scotland, in a statement. “It shows that early turtle evolution was not a straightforward, step-by-step accumulation of unique traits but was a much more complex series of events that we are only just beginning to unravel.”

But this fossil discovery is filling in more than one gap. Due to their complicated evolution, it’s been difficult for researchers to pin down whether turtle ancestors could be included in the same group of reptiles as lizards and snakes, known as diapsids.

Early on, diapsids had two holes on the sides of their skulls. Without this feature, they would be anapsids.

This newly discovered skull shows that the turtle ancestor was a diapsid.

“With Eorhynchochelys’s diapsid skull, we know that turtles are not related to the early anapsid reptiles, but are instead related to evolutionarily more advanced diapsid reptiles. This is cemented, the debate is over,” Rieppel said. “Eorhynchochelys makes the turtle family tree make sense. Until I saw this fossil, I didn’t buy some of its relatives as turtles. Now, I do.”