An exoskeleton is 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". Mineralized exoskeletons first appeared in the fossil record about 550 million years ago, and their evolution is considered by some to have played a role in the subsequent Cambrian expl… (More on Exoskeleton)