The first fossil proto-turtles come from Upper Triassic of the Mesozoic era, about 220 million years ago. Their shell evolved from bony extensions of their backbones and broad ribs that expanded and grew together to form a complete shell. It offered protection at every stage of its evolution, even when the bony component of the shell was not complete. This proved a long-lasting adaptation, and the group as a whole has survived many changes in the seas, and several extinction events.