Bone fragments from a 210-million year-old, land-dwelling reptile from New Mexico suggest that the earliest turtles didn't have much of a shell at all.
Over millions of years, rows of protective armour plates gradually fused together and to the reptile's vertebrae, eventually creating a complete shell.
"Turtles ultimately originated from something that looked like an armadillo," says lead author Walter Joyce, a palaeontologist at the Peabody Museum of Natural History in New Haven, Connecticut.
Exactly why turtles evolved their shell remains a mystery, Joyce says. A full shell might offer added protection and stability. And the proof could be in the pudding – their body plan is the world's oldest, changing little over 200 million years. "For some reason just being a turtle is an idea that came along and just really works," he says.
"And the proof could be in the pudding – their body plan is the world's oldest, changing little over 200 million years." This sounds suspiciously like the Chelonic Principle to me.