An exoskeleton's an external skeleton that supports and protects an animal's body, in contrast to the internal endoskeleton of, for example, a human. Some animals, such as the tortoise, have both an endoskeleton and an exoskeleton. In popular usage, many of the larger kinds of exoskeletons are known as "shells". Mineralised exoskeletons first appeared in the fossil record about 550 million years ago, and their evolution's considered by some to have played a role in the subsequent Cambrian explos… (More on Exoskeleton)