ScienceShot: How the Turtle Got Its Shell

The benefits of walking around with a built-in coat of armor are clear, but for evolutionary biologists, understanding how and when the turtle shell evolved has been a slow and steady race. Turtles are the only beasts in the animal kingdom that don hard outer shells formed from bone during embryonic development, and a scarcity of ancient turtle fossils has made it difficult to track the evolution of this unique trait. Now, researchers have taken a closer look at a previously discovered 260-million-year-old reptile fossil from South Africa (pictured) and concluded that it's a stem turtle, a sister genus to modern turtles' ancestors. Twenty million years older than the oldest known fossil of a direct turtle ancestor, Eunotosaurus africanus has broadened ribs inside its body and elongated vertebrae in its spine compared with other reptiles, but those bones aren't fully fused to form an external shell as is the case in modern turtles. The addition of the fossil to existing data on ancient turtles solidifies the theory that changes to the ribs and vertebrae were early steps in shell evolution and occurred during the Permian geological period, when early mammal, bird, and crocodile relatives were also diversifying.

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