In Biology, What is an Exoskeleton?
In biology, an exoskeleton refers to the chitinous or calcified external skeleton used by numerous animal taxa for structural support and defense against predators. Exoskeletons can be contrasted with endoskeletons (internal skeletons) that humans and other vertebrates have. In the animal world, exoskeletons are much more common than endoskeletons — millions of species have exoskeletons, while only a few thousand have endoskeletons. It is believed that 18 lineages have evolved calcified exoskeletons alone, with others evolving chitinous and other types of exoskeletons. Exoskeletons are especially popular among arthropods and mollusks, two of the largest animal phyla in existence. The exoskeleton first appears in the fossil record very early, about 550 million years ago, when small tube-shaped animals called Cloudina appear in the fossil record. Paleontologists have not entirely agreed on what Cloudina actually was, but the current popular guess is that it was a polychaete — a marine
