A turtle’s shell is its ribs

The carapace is not a house the animal sits in. It is the backbone and the ribs, grown outward and fused into bone the shoulder blades end up inside.

In other vertebrates with a shell or a shield, the armour is skin that has hardened — scales, plates, a job done by the dermis. The turtle did something rarer. During development the ribs, instead of curving down around the lungs, grow out into the skin of the back and broaden until they meet. The vertebrae lock into the same plate. The plastron, underneath, is built from other bones of the shoulder girdle. The result is a box of skeleton. The shoulder blade, which in you sits outside the ribs, ends up inside the box. Breathing has to be reinvented, because the ribs can no longer swing.

The animal is not ‘in’ the shell the way a hermit crab is in a borrowed one. Cut the shell and you have cut the skeleton and the fused skin over it. Fossils such as Odontochelys show the underside hardening before the back, so the box was not invented in one step. What is not in dispute is the identity of the box. Those are ribs. They have been ribs the whole time.