An exoskeleton (from Greek έξω, éxō "outer" and σκελετός, skeletós "skeleton") is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton (endoskeleton) of, for example, a human. In usage, some of the larger kinds of exoskeletons are known as [[LINK|lang_"en|Armour_(anatomy)|shells]]". Examples of animals with exoskeletons include insects such as grasshoppers and cockroaches, and crustaceans such as crabs and lobsters, as well as the shells of certain sponges and the various groups of shelled molluscs, including those of snails, clams, tusk shells, chitons and nautilus. Some animals, such as the tortoise, have both an endoskeleton and an exoskeleton.
Exoskeletons contain rigid and resistant components that fulfill a set of functional roles in many animals including protection, excretion, sensing, support, feeding and acting as a barrier against desiccation in terrestrial organisms. Exoskeletons have a role in defense from pests and predators, support and in providing an attachment framework for musculature.
Exoskeletons contain chitin; the addition of calcium carbonate makes them harder and stronger. Ingrowths of the arthropod exoskeleton known as apodemes serve as attachment sites for muscles. These structures are composed of chitin and are approximately six times stronger and twice the stiffness of vertebrate tendons. Similar to tendons, apodemes can stretch to store elastic energy for jumping, notably in locusts. Calcium carbonates constitute the shells of molluscs, brachiopods, and some tube-building polychaete worms. Silica forms the exoskeleton in the microscopic diatoms and radiolaria. One species of mollusc, the scaly-foot gastropod, even makes use of the iron sulfides greigite and pyrite.
Some organisms, such as some foraminifera, agglutinate exoskeletons by sticking grains of sand and shell to their exterior. Contrary to a common misconception, echinoderms do not possess an exoskeleton, as their test is always contained within a layer of living tissue.
Exoskeletons have evolved independently many times; 18 lineages evolved calcified exoskeletons alone. Further, other lineages have produced tough outer coatings analogous to an exoskeleton, such as some mammals. This coating is constructed from bone in the armadillo, and hair in the pangolin. The armor of reptiles like turtles and dinosaurs like Ankylosaurs is constructed of bone; crocodiles have bony scutes and horny scales.
Since exoskeletons are rigid, they present some limits to growth.
Exoskeletons, as hard parts of organisms, are greatly useful in assisting preservation of organisms, whose soft parts usually rot before they can be fossilized.
However, our dependence on fossilized skeletons also significantly limits our understanding of evolution.
Mineralized skeletons first appear in the fossil record shortly before the base of the Cambrian period, 550  million years ago. The evolution of a mineralized exoskeleton is seen by some as a possible driving force of the Cambrian explosion of animal life, resulting in a diversification of predatory and defensive tactics. However, some Precambrian (Ediacaran) organisms produced tough outer shells while others, such as Cloudina, had a calcified exoskeleton. Some Cloudina shells even show evidence of predation, in the form of borings.
On the whole, the fossil record only contains mineralised exoskeletons, since these are by far the most durable.
Some Precambrian (Ediacaran) organisms produced tough but non-mineralized outer shells, while others, such as Cloudina, had a calcified exoskeleton, but mineralized skeletons did not become common until the beginning of the Cambrian period, with the rise of the "small shelly fauna". Just after the base of the Cambrian, these miniature fossils become diverse and abundant – this abruptness may be an illusion, since the chemical conditions which preserved the small shellies appeared at the same time. Most other shell-forming organisms appear during the Cambrian period, with the Bryozoans being the only calcifying phylum to appear later, in the Ordovician. The sudden appearance of shells has been linked to a change in ocean chemistry which made the calcium compounds of which the shells are constructed stable enough to be precipitated into a shell. However this is unlikely to be a sufficient cause, as the main construction cost of shells is in creating the proteins and polysaccharides required for the shell's composite structure, not in the precipitation of the mineral components. Skeletonization also appeared at almost exactly the same time that animals started burrowing to avoid predation, and one of the earliest exoskeletons was made of glued-together mineral flakes, suggesting that skeletonization was likewise a response to increased pressure from predators.
Ocean chemistry may also control which mineral shells are constructed of.
With the exception of the molluscs, whose shells often comprise both forms, most lineages use just one form of the mineral.