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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. 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.
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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.
Many taxa produce exoskeletons, which may be composed of a range of materials, including bone, cartliage, or dentine, as in the Ostracoderm fish and turtles; chitin, as in arthropods (see arthropod exoskeleton) and some fungi and bacteria; calcium carbonates, as in the molluscs (see Mollusc shell), brachiopods, and some polychaete worms; or silica, as 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.[1] 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 "shells" are constructed of bone; crocodiles have bony scutes and horny scales).
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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.
Exoskeletons, as hard parts of organisms, are greatly useful in assisting preservation of organisms, whose soft parts usually rot before they can be fossilised. Mineralised exoskeletons can be preserved "as is", as shell fragments, for example. The possession of an exoskeleton also permits a couple of other routes to fossilisation. For instance, the tough layer can resist compaction, allowing a mould of the organism to be formed underneath the skeleton, which may later decay.[2] Alternatively, exceptional preservation may result in chitin being mineralised, as in the Burgess shale,[3] or transformed to the resistant polymer keratin, which can resist decay and be recovered.
Mineralised skeletons first appear in the fossil record shortly before the base of the Cambrian period, . The evolution of a mineralised 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,[2] while others, such as Cloudina, had a calcifed exoskeleton.[4] Some Cloudina shells even show evidence of predation, in the form of borings.[4]
Exoskeletons often contain "muscle scars", marks where muscles have been attached to the exoskeleton, which may allow the reconstruction of the internal parts of an organism from its exoskeleton alone.[2]
On the whole, the fossil record only contains mineralised exoskeletons. Since most lineages with exoskeletons are thought to have started out with a non-mineralised exoskeleton, which they later mineralised, this makes it impossible to comment on the very early evolution of each lineage's exoskeleton. What we do know is that skeletonisation appeared very suddenly indeed; in a very short course of time just before the Cambrian period, exoskeletons composed of silica, phosphate, calcite, aragonite, and agglutinated sedimentary particles sprung up in a range of different environments.[5]
Some Precambrian (Ediacaran) organisms produced tough outer shells,[2] while others, such as Cloudina, had a calcifed exoskeleton,[4] but mineralised skeletons did not become common until the base 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 - so suddenly that it may represent a preservational artefact.[6] Most other shell forming organisms appear during the Cambrian period, with the Bryozoans being the only calclfying 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, this is unlikely to be the case, as the main energy expenditure is involved in creating the proteins required for the shell's structure, not in the preciptiation of the calcite itself; besides, skeletonisation appeared at almost exactly the same time that animals started burrowing to avoid predation, suggesting that it was a response to an increased pressure from predators.
Ocean chemistry may also control which mineral shells are constructed of. Calcium carbonate has two forms, the stable calcite, and the metastable aragonite. When the oceans contain a relatively high proportion of magnesium (compared to calcium), aragonite is more stable, but as the magnesium concentration drops, it becomes less stable, hence harder to incorporate into an exoskeleton, as it will tend to dissolve.
With the exception of the molluscs, whose shells often comprise both forms, most lineages use just one form of the mineral. The form used appears to reflect the seawater chemistry - thus which form was more easily precipitated - at the time that the lineage first evolved a calcified skeleton, and does not change thereafter.[1] However, the relative abundance of calcite- and aragonite-using lineages does not reflect subsequent seawater chemistry - the magnesium/calcium ratio of the oceans appears to have a negligible impact on organisms' success, which is instead controlled mainly by how well they recover from mass extinctions.[7]
Humans have long used armour as an artificial exoskeleton for protection, especially in combat. Exoskeletal machines are also starting to be used for medical and industrial purposes, while powered human exoskeletons are a feature of science fiction writing, but are currently moving into prototype stage. Orthoses are a limited, medical form of exoskeleton.
An orthosis (plural orthoses) is a device which attaches to a limb, or the torso, to support the function or correct the shape of that limb or the spine. Orthotics is the field dealing with orthoses, their use, and their manufacture. An orthotist is a person who designs and fits orthoses.
A limb prosthesis (plural prostheses) is a device that substitutes for a missing part of a limb. If the prosthesis is a hollow shell and self-carrying, it is exoskeletal. If internal tubes are used in the device and the cover (cosmesis) to create the outside shape is made of a soft, non-carrying material, it is endoskeletal. Prosthetics is the field that deals with prostheses, use, and their manufacture. A prosthetist is a person who designs and fits prostheses.
Throughout the history of humanity, shells of many types and from many different kinds of animals have been popular as human adornments.
Seashells are often used whole and drilled so that they can be threaded like a bead, or cut into pieces of various shapes. Shells have been formed or incorporated into pendants, beads, buttons, brooches, rings, and hair combs, among other uses. Tortoiseshell has been used for jewelry and hair combs, and for many other items as varied as inkwells, sunglasses, guitar picks, and knitting needles.
The Moche culture of ancient Peru worshipped animals and the sea and often depicted shells in their art.[8] Some tribes of the indigenous peoples of the Americas used shells for wampum and hair pipes.[9]
Small pieces of colored and iridescent shell have been used to create mosaics and inlays, which have been used to decorate walls, furniture and boxes. Large numbers of whole seashells, arranged to form patterns, have been used to decorate mirror frames, furniture and man-made grottos.