How the Turtle Got Its Shell, With Apologies to Aesop
By Hans-Dieter Sues Smithsonian.com
Turtles stand alone among living and extinct backboned animals because of the unique hard shell that encases their bodies. Other animals, like the armadillo, evolved body armor, but the turtle shell fully integrates the backbone and trunk ribs.
The turtle shell is made up of rows of firmly connected bony plates, which are covered by horny plates, or scutes. The domed carapace covering the back of the animal is connected to the flat plastron on the underside of the animal by a bridge of bone. The resulting box encloses the shoulder and hip girdles, but is open at the front for the head, neck and forelegs, and at the back for the tail and hind legs.
How did this unusual structure evolve? One of Aesop’s fables tells of the wedding of Zeus and Hera. Hermes had invited all the animals, but the turtle did not show up. When an angry Hermes demanded to know why it had not come to the feast, the turtle responded that it preferred its own home. Enraged, Hermes made the animal carry its house forever after.