Turtle Shell Evolved From Ribs: Does New Fossil Research Settle Dispute?

By iScienceTimes Staff on May 31, 2013 1:24 PM EDT

sea turtle
A new paper claims that a 250-million-year old reptile, Eunotosaurus, was an ancestor to the modern turtle, and shows how turtles got their hard shells. (Photo: Reuters / Tim Wimborne)

A new paper says that an ancient reptile fossil tells us how turtles got their hard shell, something that has divided the scientific community for years.

While scientists have long known that animals with body armor, like armadillos or lizards, form their armor from the bony scales of their skin, how turtles developed their outer shell has always eluded scientists.

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The scientific community has divided into two camps. In the first camp are scientists who believe that turtles' body armor came about when bony scales of skin fused with the rib cage to make a hard outer shell. The second camp believes that the turtle's carapace came from the rib broadening into a shell.

Because of a 55-million-year gap in the fossil record of turtles, scientists have never known what took place to lead to a turtle's shell. Until, perhaps, now.

In the new paper, published yesterday in the journal Current Biology, the authors claim that the latter is how turtles formed their shells. As evidence, they point to the 250-million-year-old fossil of the Eunotosaurus, which they say is an ancestor to the modern turtle.

The Eunotosaurus was discovered in the 19th century and proposed as a possible turtle ancestor. Given the huge gap in fossil records, the idea of Eunotosaurus as a turtle ancestor never gained currency.

Then in 2008 a new early turtle fossil was discovered in China. Odontochelys semitestacea had a belly shell like a modern turtle, but no carapace. It had no external scales on its body, but did have distinctively broadened ribs, leading credence to the second camp's theory.

The discovery in China "released us from this self-imposed constraint" about not looking to species like the Eunotosaurus as possible modern turtle ancestors, says Tyler Lyson, lead author of the paper.

Lyson began studying Eunotosaurus fossils and started to think that it might be a missing turtle link, due to features like its backbone and broadened ribs.

"The reason, I think, that more animals don't form a shell via the broadening and eventually suturing together of the ribs is that the ribs of mammals and lizards are used to help ventilate the lungs," said Lyson.

"If you incorporate your ribs into a protective shell, then you have to find a new way to breathe," he added, noting that that is what turtles have done

The paper won't settle the issue of how the turtle got its shell, but it's a "useful working hypothesis," Kenneth Angielczyk, a paleobiologist from the Field Museum in Chicago, told the Boston Globe in an email.

"I think his results are pretty convincing; previously I was skeptical as to whether Eunotosaurus was a likely relative of turtles," Angielczyk said. "But [Lyson's] results make me think it is a plausible idea."

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