An exoskeleton is an external skeleton In biology, a skeleton is a rigid framework that provides structure as well as protection in humans and many types of animals, particularly those of the phylum Chordata and of the superphylum Ecdysozoa. Exoskeletons are external, as is typical of many invertebrates; they enclose the soft tissues and organs of the body. Exoskeletons may undergo that supports and protects an animal's body, in contrast to the internal skeleton (endoskeleton An endoskeleton is an internal support structure of an animal, composed of mineralized tissue. In three phyla and one subclass of animals, endoskeletons of various complexity are found: Chordata, Echinodermata, Porifera, and Coleoidea. An endoskeleton may function purely for support , but often serves as an attachment site for muscle and a) of, for example, a human The human skeleton consists of both fused and individual bones supported and supplemented by ligaments, tendons, muscles and cartilage. It serves as a scaffold which supports organs, anchors muscles, and protects organs such as the brain, lungs and heart. The biggest bone in the body is the femur in the upper leg, and the smallest is the stapes. Some animals, such as the tortoise Tortoises or land turtles are land-dwelling reptiles of the family of Testudinidae, order Testudines. Like their marine cousins, the sea turtles, tortoises are shielded from predators by a shell. The top part of the shell is the carapace, the underside is the plastron, and the two are connected by the bridge. The tortoise has both an endoskeleton, have both an endoskeleton and an exoskeleton. In popular usage, many of the larger kinds of exoskeletons are known as "shells".

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 The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, over a period of many million years, of most major groups of complex animals around 530 million years ago, as found in the fossil record. This was accompanied by a major diversification of other organisms, including animals, phytoplankton, and calcimicrobes. Before of animals.^{[citation needed]}

Role of the exoskeleton

Exoskeletons contain rigid and resistant components that fulfill 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 defense from pests and predators, support, and in providing an attachment framework for musculature.^[1]

Exoskeletons contain chitin Chitin n (pronounced /ˈkaɪtɨn/) is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world. It is the main component of the cell walls of fungi, the exoskeletons of arthropods such as crustaceans (e.g. crabs, lobsters and shrimps) and insects, the radulas of mollusks and and when calcium carbonate is added, the exoskeleton grows in strength and hardness.^{[citation needed]}

Diversity

Many taxa produce exoskeletons, which are composed of a range of materials. Bone, cartilage, or dentine is used in the Ostracoderm Ostracoderms are any of several groups of extinct, primitive, jawless fishes that were covered in an armor of bony plates. They belong to the taxon Ostracodermi, and their fossils are found in the Ordovician and Devonian Period strata of North America and Europe. They were often less than 30 cm (1 ft) long and were probably slow, bottom-dwelling fish and turtles Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield. "Turtle" may either refer to the Testudines as a whole, or to particular Testudines which make up a form taxon that is not monophyletic—see also sea turtle, terrapin, tortoise, and the. Chitin Chitin n (pronounced /ˈkaɪtɨn/) is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world. It is the main component of the cell walls of fungi, the exoskeletons of arthropods such as crustaceans (e.g. crabs, lobsters and shrimps) and insects, the radulas of mollusks and forms the exoskeleton in arthropods An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the Phylum Arthropoda (from Greek ἄρθρον arthron, "joint", and ποδός podos "foot", which together mean "jointed feet"), and include the insects, arachnids, crustaceans, and including insects Insects are a class within the arthropods that have a chitinous exoskeleton, a three-part body (head, thorax, and abdomen), three pairs of jointed legs, compound eyes, and two antennae. They are among the most diverse group of animals on the planet and include more than a million described species and represent more than half of all known living, arachnids Arachnids are a class of joint-legged invertebrate animals in the subphylum Chelicerata. All arachnids have eight legs, although in some species the front pair may convert to a sensory function. The term is derived from the Greek word ἀράχνη (aráchnē), meaning "spider" such as spiders Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms. Spiders are found worldwide on every continent except for Antarctica, and have become established in nearly every habitat, crustaceans Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at 0.1 mm (0.004 in), to the Japanese spider crab with a leg span of up to 14 ft (4.3 m) and a mass such as crabs True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax. Other animals, such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs and crab lice, are not true crabs and lobsters Clawed lobsters comprise a family of large marine crustaceans. Lobsters are economically important as seafood, forming the basis of a global industry that nets more than US$1 billion annually (see arthropod exoskeleton Arthropods are covered with a tough or resistant exoskeleton, which may be mineralised or constructed of a tough polymer such as chitin. This external skeleton is moulted as the organism grows), and in some fungi A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. The Fungi (pronounced /ˈfʌndʒaɪ/ or /ˈfʌŋɡaɪ/) are classified as a kingdom that is separate from plants, animals and bacteria. One major difference is that fungal cells have cell and bacteria The bacteria ( [bækˈtɪəriə] ; singular: bacterium)[α] are a large group of single-celled, prokaryote microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste,. Calcium carbonates constitute the shells of molluscs The Mollusca, common name molluscs or mollusks,[note 1] is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. This is the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. Molluscs are highly diverse, (see Mollusc shell), brachiopods Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs (such as scallops, clams, oysters and mussels). Most species of brachiopod went extinct during the P-Tr Extinction 250 million years ago, but many survive today, and some tube-building polychaete The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000 species are described in this class. Common worms. Silica The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with a chemical formula of Si forms the exoskeleton in the microscopic diatoms Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans (e.g. Meridion), zigzags (e.g. Tabellaria), or stellate colonies (e.g. Asterionella). Diatoms are producers within the food chain. A characteristic and radiolaria Radiolarians are amoeboid protozoa that produce intricate mineral skeletons, typically with a central capsule dividing the cell into inner and outer portions, called endoplasm and ectoplasm. They are found as zooplankton throughout the ocean, and their skeletal remains cover large portions of the ocean bottom as radiolarian ooze. Due to their.

Some organisms, such as some foraminifera The Foraminifera, or forams for short, are a large group of amoeboid protists with reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net. They typically produce a test, or shell, which can have either one or multiple chambers, some becoming quite elaborate in structure. These shells are made of calcium, agglutinate exoskeletons by sticking grains of sand and shell to their exterior. Contrary to a common misconception, echinoderms Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. Aside from the problematic Arkarua, the first definitive members of the phylum appeared near the start of the Cambrian period do not possess an exoskeleton, as their test A test is a term used to refer to the shell of sea urchins, and also the shell of certain microorganisms, such as testate foraminifera and testate amoebae. The foraminifera test is of silicaeous origin is always contained within a layer of living tissue.

Exoskeletons have evolved independently many times; 18 lineages evolved calcified Calcification is the process in which calcium salts build up in soft tissue, causing it to harden. Calcifications may be classified on whether there is mineral balance or not, and the location of the calcification exoskeletons alone.^[2] Further, other lineages have produced tough outer coatings analogous to an exoskeleton, such as some mammals – (constructed from bone in the armadillo Armadillos are small placental mammals, known for having a leathery armor shell. The Dasypodidae are the only surviving family in the order Cingulata, part of the superorder Xenarthra along with the anteaters and sloths. The word armadillo is Spanish for "little armored one", and hair in the pangolin A pangolin , also scaly anteater or tenggiling, is a mammal of the order Pholidota. There is only one extant family (Manidae) and one genus (Manis) of pangolins, comprising eight species. There are also a number of extinct taxa. Pangolins have large keratin scales covering their skin and are the only mammals with this adaptation. They are found in) – and reptiles (turtle and Ankylosaur Ankylosauria is a group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. They are first known to have appeared in the early Jurassic Period of China, and persisted until the end of the Cretaceous armor are constructed of bone; crocodiles have bony scutes A scute or scutum is a bony external plate or scale, as on the shell of a turtle, the skin of crocodilians, or the feet of some birds and horny A horn is a pointed projection of the skin on the head of various animals, consisting of a covering of horn surrounding a core of living bone. True horns are found mainly among the ruminant artiodactyls,[citation needed] in the families Antilocapridae (pronghorn) and Bovidae (cattle, goats, antelope etc.). One pair of horns is usual, but two pairs scales).

Growth in an exoskeleton

^{[verification needed]}

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 Ecdysis is the moulting of the cuticula in arthropods and related groups . Since the cuticula of these animals is also the skeletal support (the exoskeleton) of the body and is inelastic, it is shed during growth and a new, larger covering is formed. The old, empty exoskeleton is called an exuvia (or "exuvium") their shell when they outgrow it, producing a replacement.

Palaeontological significance

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.^[3] Alternatively, exceptional preservation A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness. Palaeontologists distinguish two kinds may result in chitin being mineralized, as in the Burgess Shale The Burgess Shale Formation — located in the Canadian Rockies of British Columbia — is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils. It is 505 million years old, one of the earliest soft-parts fossil beds,^[4] or transformed to the resistant polymer keratin Keratin refers to a family of fibrous structural proteins. Keratin is the key structural material making up the outer layer of human skin. It is also the key structural component of hair and nails. Keratin monomers assemble into bundles to form intermediate filaments, which are tough and insoluble and form strong unmineralized tissues found in, 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 This term may also be used to indicate the digestion process in which bacteria utilize the organic part of the matter, leaving behind the minerals, see also Fossil. This can also take place as the organic material decays and water percolating through the soil dissolves mineral salts that precipitate in place of the tissue 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.^[3] The most significant limitation is that, although there are 30-plus phyla In biology, a phylum [note 1] is a taxonomic rank below Kingdom and above Class. "Phylum" is equivalent to the botanical term division. Phylum is one of the major biological divisions called Taxa. Although "phylum" is often used as if it were a clearly defined term, no satisfactory definition of it exists. In fact, "phylum& 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.^[5]

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 The Ediacaran Period is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon. Its status as an official geological period was ratified in 2004 by the International Union of Geological Sciences (IUGS), making it) organisms produced tough outer shells,^[3] while others, such as Cloudina, had a calcified exoskeleton.^[6] Some Cloudina shells even show evidence of predation, in the form of borings.^[6]

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 The Cambrian is the first geological period of the Paleozoic Era, lasting from 542 ± 0.3 million years ago to 488.3 ± 1.7 million years ago(ICS, 2004,; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the classical name for period exoskeletons made of various materials – silica, calcium phosphate Calcium phosphate, with the empirical formula of Ca32, is the name given to a family of minerals containing calcium ions (Ca2+) together with orthophosphates (PO43-), metaphosphates or pyrophosphates (P2O74-) and occasionally hydrogen or hydroxide ions [citation needed], calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (Ca , aragonite Aragonite is a carbonate mineral, one of the two common, naturally occurring crystal forms of calcium carbonate, Ca , and even glued-together mineral flakes – sprang up in a range of different environments.^[7] 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.^[2]

Some Precambrian (Ediacaran) organisms produced tough but non-mineralized outer shells,^[3] while others, such as Cloudina, had a calcified exoskeleton,^[6] 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.^[8] Most other shell forming organisms appear during the Cambrian period, with the Bryozoans The Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about 0.5 millimetres long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia. Most marine species live in tropical waters, but being the only calcifying phylum to appear later, in the Ordovician The Ordovician [/ɔɹdəˈvɪʃən/] is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago (ICS, 2004,. It follows the Cambrian Period and is followed by the Silurian Period. The Ordovician, named after the Welsh tribe of the Ordovices, was defined by Charles. 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 Proteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in a protein is defined by the sequence of a gene, which is encoded and polysaccharides required for the shell's composite structure, not in the precipitation of the mineral components.^[1] Skeletonisation 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 skeletonisation was likewise a response to increased pressure from predators.^[7]

Ocean chemistry may also control which mineral shells are constructed of. Calcium carbonate has two forms, the stable calcite, and the metastable aragonite, which is stable within a reasonable range of chemical environments but rapidly becomes unstable outside this range. 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 mollusks, 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.^[2] 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.^[9] 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.^[1]

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

1. ^ ^{a b c} Bengtson, S. (2004). Early skeletal fossils. in Lipps, J.H., and Waggoner, B.M.. "Neoproterozoic- Cambrian Biological Revolutions" (PDF). Palentological Society Papers 10: 67–78. http://www.cosmonova.org/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf. Retrieved 2008-07-18.
2. ^ ^{a b c} Porter, Susannah M. (2007). "Seawater Chemistry and Early Carbonate Biomineralization". Science 316 (5829): 1302. doi:10.1126/science.1137284. PMID 17540895.
3. ^ ^{a b c d} New data on Kimberella, the Vendian mollusk-like organism (White sea region, Russia): palaeoecological and evolutionary implications (2007), "Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y.", in Vickers-Rich, Patricia; Komarower, Patricia, The Rise and Fall of the Ediacaran Biota, Special publications, 286, London: Geological Society, pp. 157–179, doi:10.1144/SP286.12, ISBN 9781862392335, OCLC 191881597 156823511 191881597
4. ^ Butterfield, Nicholas J. (2003). "Exceptional Fossil Preservation and the Cambrian Explosion". Integrative and Comparative Biology 43 (1): 166–177. doi:10.1093/icb/43.1.166.
5. ^ Cowen, R.. History of Life. Blackwell Science.
6. ^ ^{a b c} Hua, H.; Pratt, B.R., Zhang, Luyi (2003). "Borings in Cloudina Shells: Complex Predator-Prey Dynamics in the Terminal Neoproterozoic". PALAIOS 18: 454. doi:10.1669/0883-1351(2003)018<0454:BICSCP>2.0.CO;2.
7. ^ ^{a b} Dzik, J (2007), "The Verdun Syndrome: simultaneous origin of protective armor and infaunal shelters at the Precambrian–Cambrian transition", in Vickers-Rich, Patricia; Komarower, Patricia, The Rise and Fall of the Ediacaran Biota, Special publications, 286, London: Geological Society, pp. 405–414, doi:10.1144/SP286.30, ISBN 9781862392335, OCLC 191881597 156823511 191881597, http://www.paleo.pan.pl/people/Dzik/Publications/Verdun.pdf
8. ^ Dzik, J. (1994). "Evolution of 'small shelly fossils' assemblages of the early Paleozoic". Acta Palaeontologica Polonica 39 (3): 27–313. http://www.paleo.pan.pl/people/Dzik/Dzik1994d.htm.
9. ^ Kiessling, Wolfgang; Aberhan, Martin; Villier, Loïc (2008). "Phanerozoic trends in skeletal mineralogy driven by mass extinctions". Nature Geoscience 1: 527. doi:10.1038/ngeo251.

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

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