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. 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 is considered by some to have played a role in the subsequent Cambrian explosion of animals.

Role of the exoskeleton

Exoskeletons contain rigid and resistant components that fulfil a set of functional roles including protection, excretion, sensing, support, feeding and (for terrestrial organisms) acting as a barrier against desiccation. Exoskeletons have a role in defence from predators, support, and in providing a framework which musculature can attach to.

Diversity

Many taxa produce exoskeletons, which are composed of a range of materials. Bone, cartliage, or dentine is used in the Ostracoderm fish and turtles. Chitin forms the exoskeleton in arthropods including insects, arachnids such as spiders, crustaceans such as crabs and lobsters (see arthropod exoskeleton), and in some fungi and bacteria. Calcium carbonates constitute the shells of molluscs (see Mollusc shell), brachiopods, and some tube-building polychaete worms. Silica forms the exoskeleton in the microscopic diatoms and radiolaria.

Some organisms, such as some formanifera, 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 - (constructed from bone in the armadillo, and hair in the pangolin) - and reptiles (turtle and Ankylosaur armour are constructed of bone; crocodiles have bony scutes and horny scales).

Growth in an exoskeleton

Since exoskeletons are rigid, they present some limits to growth. Some organisms grow by adding new material to the aperture of their shell, but many must moult their shell when they outgrow it, producing a replacement.

Palaeontological significance

However our dependence on fossilised skeletons also significantly limits our understanding of evolution. Only the parts of organisms that were already mineralised are usually preserved, such as the shells of molluscs. It helps that exoskeletons often contain "muscle scars", marks where muscles have been attached to the exoskeleton, which may allow the reconstruction of much of an organism's internal parts from its exoskeleton alone. The most significant limitation is that, although there are 30-plus phyla of living animals, two-thirds of these phyla have never been found as fossils, because most animal species are soft-bodied and decay before they can become fossilised.