Why Turtles Have Shells (It’s Not What We Thought)

Artwork by Andrey Atuchin
Artwork by Andrey Atuchin / Artwork by Andrey Atuchin

Science is a discipline of questions. Many of these questions have puzzled great thinkers for a long, long time. Others have been completely overlooked, because we assume we know the answers. Case in point: Nobody wondered why turtles have shells; everybody knows they evolved to keep the turtles safe. Except maybe they didn’t. That's what some researchers are saying. They propose that turtle shells originally evolved not to protect the leathery reptiles, but to help them burrow into the ground. Their report was published in the journal Current Biology. 

Turtles have been around for a very, very long time. They were here before, during, and after the dinosaurs, quietly toughing it out through drastic changes in climate and multiple extinction events. One of the earliest turtle ancestors was a long-tailed species called Eunotosaurus africanus that lived 260 million years ago in what is today South Africa.

Based on fossilized remains recovered from the Karoo Basin, scientists believe E. africanus was an evolutionary stepping stone between lizards and turtles. It had a lizard’s long tail and clawed feet, with a modern turtle’s manhole-shaped body and broad, squat ribs curving into a shallow dome.

It’s those ribs that gave researchers pause. Ribs protect the organs in our chest, but they must be big enough to allow our lungs to expand or we’ll have trouble breathing and moving. There’s not a lot of room for variation, says lead author Tyler Lyson, a curator of vertebrate paleontology at the Denver Museum of Nature and Science.

"Ribs are generally pretty boring bones,” he said in a press statement. “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." 

But, as we said, there’s a reason most animals’ ribs look the same: It’s really hard to breathe and get around otherwise. So why would turtles evolve such a constricting body shape—and how did they survive this long with it?

Lyson and his colleagues examined 47 different E. africanus specimens, paying close attention to their skeletal structure. The best specimen in the collection is a well-preserved, amazingly complete skeleton that was found by 8-year-old Kobus Snyman on his father’s farm in South Africa.

Tyler R. Lyson

In looking at the specimens' ribs, the researchers noticed that the overall shape was similar to the ribs of other species. And those species had one thing in common: They were diggers. That wide, short ribcage that would be so restrictive to a person is actually freeing for burrowing animals, because it stabilizes the body and provides power to the limbs.

This revelation makes a lot of sense, the researchers say. There are plenty of burrowing turtles, both extinct and alive today, and other studies have concluded that heading underground may have helped some species survive the extreme climate changes that killed off so many other species.

The researchers were especially grateful to their youngest contributor, says co-author Bruce Rubidge of the University of Witwatersrand. "I want to thank Kobus Snyman and shake his hand,” Rubidge says, “because without Kobus both finding the specimen and taking it to his local museum, the Fransie Pienaar Museum in Prince Albert, this study would not have been possible.”

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