It didn’t help that for the longest time, the oldest known turtle was a creature called Proganochelys, which already had a fully developed (and very spiky) shell, meaning it couldn’t tell us anything about how that structure first arose.

Everything changed in 2008, when Chinese researchers discovered a 220-million-year-old turtle with a shell that covered just its belly and not its back. They called it Odontochelys semitestacea—literally, the “toothed turtle in a half-shell.” It was as beautiful an intermediate fossil as they could have hoped for. And strikingly, it had no osteoderms at all. It did, however, have very broad ribs. The developmental biologists were right!

Once paleontologists knew what one intermediate turtle might look like, they found new ones like Pappochelys. They also added existing fossils like Eunotosaurus to the family; ironically, it had been previously ruled out as an early turtle because it lacked osteoderms. With these fossils, scientists could reconstruct the evolution of the shell.

First, the lower ribs became wider and fused with each other to give half a shell—the plastron. Then, the upper ribs followed suit and merged with the spine, creating the carapace. (This means that, contrary to cartoons, you can’t pull a turtle out of its shell.) Eventually, through an intricate bit of evolutionary origami, the ribs started growing over the shoulder blades, rather than sitting below them as in you, me, and most other land-living vertebrates.

That takes care of how the shell evolved. “For me, the next question was: Why?” says Lyson. “And there are two huge reasons why not.”

First, the ribs and their associated muscles help to inflate and deflate the lungs. If you broaden and fuse your ribs, you compromise your ability to breathe. Second, if you’re a reptile, you also become slower. Reptiles have a sprawling gait, so they bend their bodies sideways to increase the length of their stride. They effectively walk with their entire trunks, which you can see in this video of a walking Komodo dragon. But wide fused ribs prevent trunks from bending, so turtles are powered only by their limbs. That’s partly why they’re famously slow. What benefit could a shell possibly provide that compensates for being worse at both breathing and walking?

“When I went to these turtle-specific conferences and I talked to people, they automatically said it was for protection,” Lyson recalls. “But that never made any sense.” Modern turtle shells certainly make for potent defenses, but the wide ribs of Eunotosaurus and Pappochelys do not. They wouldn’t even have covered the creatures’ heads, necks, or backs. If protection was important, it would have made far more sense to follow crocodiles and armadillos down the osteoderm route, which wouldn’t have hampered breathing or walking.