Exoskeleton

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An exoskeleton is an external skeleton that supports and protects an animal's body, in contrast to the internal skeleton (endoskeleton) of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Some example of exoskeleton animals include insects such as grasshoppers and cockroaches, and crustaceans such as crabs and lobsters. The shells of the various groups of shelled mollusks, including those of snails, clams, tusk shells, chitons and nautilus are also exoskeletons.

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 explosion of animals.

Some animals, such as the tortoise, have both an endoskeleton and an exoskeleton.

Role of the exoskeleton

Exoskeletons contain rigid and resistant components that fulfil a set of functional roles including protection, excretion, sensing, support, feeding and acting as a barrier against desiccation in terrestrial organisms. Exoskeletons have a role in defence from pests and predators, support, and in providing an attachment framework for musculature. 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 armor 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

s attacked this hard clam shell after the death of the clam, producing the trace fossil Entobia.]] Exoskeletons, as hard parts of organisms, are greatly useful in assisting preservation of organisms, whose soft parts usually rot before they can be fossilized. Mineralized exoskeletons can be preserved "as is", as shell fragments, for example. The possession of an exoskeleton also permits a couple of other routes to fossilization. For instance, the tough layer can resist compaction, allowing a mold of the organism to be formed underneath the skeleton, which may later decay. or transformed to the resistant polymer keratin, which can resist decay and be recovered.

However our dependence on fossilized skeletons also significantly limits our understanding of evolution. Only the parts of organisms that were already mineralized are usually preserved, such as the shells of mollusks. 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 fossilized.

Mineralized skeletons first appear in the fossil record shortly before the base of the Cambrian period, . 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,

Evolution

On the whole, the fossil record only contains mineralised exoskeletons, since these are by far the most durable. Since most lineages with exoskeletons are thought to have started out with a non-mineralised exoskeleton which they later mineralised, this makes it difficult to comment on the very early evolution of each lineage's exoskeleton. We do know that in a very short course of time just before the Cambrian period exoskeletons made of various materials – silica, calcium phosphate, calcite, aragonite, and even glued-together mineral flakes – sprang up in a range of different environments. Most lineages adopted the form of calcium carbonate which was stable in the ocean at the time they first mineralissd, and did not change from this mineral morph - even when it became the less favorable. 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. A recently-discovered modern gastropod that lives near deep-sea hydrothermal vents illustrates the influence of both ancient and modern local chemical environments: its shell is made of aragonite, which is found in some of the earliest fossil mollusks; but it also has armor plates on the sides of its foot, and these are mineralized with the iron sulfides pyrite and greigite, which had never previously been found in any metazoan but whose ingredients are emitted in large quantities by the vents.

Artificial "exoskeletons"

Humans have long used armor as an artificial exoskeleton for protection, especially in combat. Exoskeletal machines (also called powered exoskeletons) 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 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.

Parenthetically, the exoskeleton has been used as an architectural model. See the lighthouse at St. Martin Island.

Perhaps the first animals to use a naturally-occurring "artificial exoskeleton" were the hermit crabs, the majority of which are obliged constantly to "wear" an empty gastropod shell, in order to protect their soft abdomens.

See also

Mechatronics

Powered exoskeleton

Spiracle – small openings in the exoskeleton that allow insects to breathe

Hydrostatic skeleton

Endoskeleton

References

External links

Biology

Geometry and Pattern in Nature 3: The holes in radiolarian and diatom tests

Modern technology

Bionic Boots

PowerSkip

Berkeley Lower Extremity Exoskeleton (BLEEX)

University of California - Santa Cruz - Exo Arm Project

Cyberdyne's HAL power suit

Category:Animal anatomy Category:Biomechanics Category:Skeletal system