Turtle shells evolved for burrowing, not protection

by Mary Caperton Morton
Tuesday, October 18, 2016

How the turtle got its shell is one of the long-standing conundrums of paleontology. Paleontologists know that one of the first steps toward a shell was a broadening of the ribs, which occurred about 50 million years before full shells evolved. But why the broadening, which conferred some disadvantages to movement and breathing, began, has been a mystery. Now, the discovery of a proto-turtle with a partial shell in South Africa is shedding some light on the early stages of shell development.

The new Eunotosaurus africanus specimen, dated to 260 million years ago, is the oldest turtle found to date and one of the only specimens that shows the shell in a transitional form. The 15-centimeter-long fossil, found by an 8-year-old boy in the Karoo Basin of South Africa, displays broadened ribs and the beginnings of a shell that seem to have aided the animal in burrowing — along with its large forelimbs and sharp claws — by providing “an intrinsically stable base on which to operate a powerful forelimb digging mechanism,” researchers wrote in a new study in Current Biology.

“Why the turtle shell evolved is a very Dr. Seuss-like question, and the answer seems pretty obvious — it was for protection. But just like the bird feather did not initially evolve for flight — we now have early relatives of birds such as tyrannosaur dinosaurs with feathers that definitely were not flying — the earliest beginnings of the turtle shell was not for protection but rather for digging underground to escape the harsh South African environment where these early proto-turtles lived,” said lead author Tyler Lyson, of the Denver Museum of Nature and Science, in a statement.

The fossil’s well-preserved broadened ribs were integral to the breakthrough, Lyson said. Why the turtle’s ribs began broadening, eventually forming into a shell, has long been a mystery, as the modifications could have affected its ability to breathe and move efficiently, the team reported.

“The integral role of ribs in both locomotion and breathing is likely why we don’t see much variation in the shape of ribs,” Lyson said. “Ribs are generally pretty boring bones. The ribs of whales, snakes, dinosaurs, humans, and pretty much all other animals look the same. Turtles are the one exception, where they are highly modified to form the majority of the shell.”

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