Why the turtle got its shell

Turtle shells didn’t get their start as natural armor, it seems. The reptiles’ ancestors might have evolved partial shells to help them burrow instead, new research suggests. Only later did the hard body covering become useful for protection.

The findings might also help explain how turtles’ ancestors survived a mass extinction 250 million years ago that wiped out most plants and animals on earth, scientists report online July 14 in Current Biology.

Most shelled animals, like armadillos, get their shells by adding bony scales all over their bodies. Turtles, though, form shells by gradually broadening their ribs until the bones fuse together. Fossils from ancient reptiles with partial shells made from thickened ribs suggest that turtles’ ancestors began to suit up in the same way.
It’s an unusual mechanism, says Tyler Lyson, a paleontologist at the Denver Museum of Nature and Science who led the study. Thicker ribs don’t offer much in the way of protection until they’re fully fused, as they are in modern turtles. And the modification makes critical functions like moving and breathing much harder — a steep price for an animal to pay. So Lyson suspected there was some advantage other than protection to the partial shells.

He and his colleagues examined fossils from prototurtles, focusing on an ancient South African reptile called Eunotosaurus africanus.

Eunotosaurus shared many characteristics with animals that dig and burrow, the researchers found. The reptile had huge claws and large triceps in addition to thickened ribs.
“We could tell that this animal was very powerful,” says Lyson.
Broad ribs “provide a really, really strong and stable base from which to operate this powerful digging mechanism,” he adds. Like a backhoe, Eunotosaurus could brace itself to burrow into the dirt.

Thanks to a lucky recent find of a fossil preserving the bones around the eyes, the team was even able to tell that the prototurtles’ eyes were well adapted to low light. That’s another characteristic of animals that spend time underground.

Swimming and digging use similar motions, Lyson says, so you would expect to find similar skeletal adaptations in water-dwelling animals. But large claws good for moving dirt suggest a life on land.

Fossils from other prototurtle species also have wider ribs and big claws. So the researchers think these traits may have been important for early turtle evolution in general, not just for Eunotosaurus.

Not everyone is entirely convinced. “It’s a very plausible idea, although many other animals burrow but don’t have these specializations,” says Hans Sues, a paleontologist at the Smithsonian Institution’s National Museum of Natural History. Sues says that it will be important to find and study other turtle ancestors well-adapted to digging to bolster the explanation.

Lyson thinks the prototurtles’ burrowing tendencies might have helped them survive the end-Permian mass extinction around 250 million years ago (SN: 9/19/15, p. 10).

“Lots of animals at this time period burrowed underground to avoid the very, very arid environment that was present in South Africa,” Lyson says. “The burrow provides more climate control.”