An exoskeweton (from Greek έξω, éxō "outer" and σκελετός, skewetós "skeweton") is de externaw skeweton dat supports and protects an animaw's body, in contrast to de internaw skeweton (endoskeweton) of, for exampwe, a human. In usage, some of de warger kinds of exoskewetons are known as "shewws". Exampwes of animaws wif exoskewetons incwude insects such as grasshoppers and cockroaches, and crustaceans such as crabs and wobsters. The shewws of certain sponges and de various groups of shewwed mowwuscs, incwuding dose of snaiws, cwams, tusk shewws, chitons and nautiwus, are awso exoskewetons. Some animaws, such as de tortoise, have bof an endoskeweton and an exoskeweton, uh-hah-hah-hah.
Exoskewetons contain rigid and resistant components dat fuwfiww a set of functionaw rowes in many animaws incwuding protection, excretion, sensing, support, feeding and acting as a barrier against desiccation in terrestriaw organisms. Exoskewetons have a rowe in defense from pests and predators, support, and in providing an attachment framework for muscuwature.
Exoskewetons contain chitin; de addition of cawcium carbonate makes dem harder and stronger. Ingrowds of de ardropod exoskeweton known as apodemes serve as attachment sites for muscwes. These structures are composed of chitin, and are approximatewy six times as strong and twice as stiff as vertebrate tendons. Simiwar to tendons, apodemes can stretch to store ewastic energy for jumping, notabwy in wocusts.
Many different species produce exoskewetons, which are composed of a range of materiaws. Bone, cartiwage, or dentine is used in de Ostracoderm fish and turtwes. Chitin forms de exoskeweton in ardropods incwuding insects, arachnids such as spiders, crustaceans such as crabs and wobsters, and in some fungi and bacteria. Cawcium carbonates constitute de shewws of mowwuscs, brachiopods, and some tube-buiwding powychaete worms. Siwica forms de exoskeweton in de microscopic diatoms and radiowaria. One species of mowwusc, de scawy-foot gastropod, even makes use of de iron suwfides greigite and pyrite.
Some organisms, such as some foraminifera, aggwutinate exoskewetons by sticking grains of sand and sheww to deir exterior. Contrary to a common misconception, echinoderms do not possess an exoskeweton, as deir test is awways contained widin a wayer of wiving tissue.
Exoskewetons have evowved independentwy many times; 18 wineages evowved cawcified exoskewetons awone. Furder, oder wineages have produced tough outer coatings anawogous to an exoskeweton, such as some mammaws. This coating is constructed from bone in de armadiwwo, and hair in de pangowin. The armor of reptiwes wike turtwes and dinosaurs wike Ankywosaurs is constructed of bone; crocodiwes have bony scutes and horny scawes.
Since exoskewetons are rigid, dey present some wimits to growf. Organisms wif open shewws can grow by adding new materiaw to de aperture of deir sheww, as is de case in snaiws, bivawves and oder mowwuscans. A true exoskeweton, wike dat found in ardropods, must be shed (mouwted) when it is outgrown, uh-hah-hah-hah. A new exoskeweton is produced beneaf de owd one. As de owd one is shed, de new skeweton is soft and pwiabwe. The animaw wiww pump itsewf up[ambiguous] to expand de new sheww to maximaw size, den wet it harden, uh-hah-hah-hah. When de sheww has set, de empty space inside de new skeweton can be fiwwed up as de animaw eats. Faiwure to shed de exoskeweton once outgrown can resuwt in de animaw being suffocated widin its own sheww, and wiww stop subaduwts from reaching maturity, dus preventing dem from reproducing. This is de mechanism behind some insect pesticides, such as Azadirachtin.
Exoskewetons, as hard parts of organisms, are greatwy usefuw in assisting preservation of organisms, whose soft parts usuawwy rot before dey can be fossiwized. Minerawized exoskewetons can be preserved "as is", as sheww fragments, for exampwe. The possession of an exoskeweton awso permits a coupwe of oder routes to fossiwization. For instance, de tough wayer can resist compaction, awwowing a mowd of de organism to be formed underneaf de skeweton, which may water decay. Awternativewy, exceptionaw preservation may resuwt in chitin being minerawized, as in de Burgess Shawe, or transformed to de resistant powymer keratin, which can resist decay and be recovered.
However, our dependence on fossiwized skewetons awso significantwy wimits our understanding of evowution, uh-hah-hah-hah. Onwy de parts of organisms dat were awready minerawized are usuawwy preserved, such as de shewws of mowwuscs. It hewps dat exoskewetons often contain "muscwe scars", marks where muscwes have been attached to de exoskeweton, which may awwow de reconstruction of much of an organism's internaw parts from its exoskeweton awone. The most significant wimitation is dat, awdough dere are 30-pwus phywa of wiving animaws, two-dirds of dese phywa have never been found as fossiws, because most animaw species are soft-bodied and decay before dey can become fossiwized.
Minerawized skewetons first appear in de fossiw record shortwy before de base of de Cambrian period, . The evowution of a minerawized exoskeweton is seen by some as a possibwe driving force of de Cambrian expwosion of animaw wife, resuwting in a diversification of predatory and defensive tactics. However, some Precambrian (Ediacaran) organisms produced tough outer shewws whiwe oders, such as Cwoudina, had a cawcified exoskeweton, uh-hah-hah-hah. Some Cwoudina shewws even show evidence of predation, in de form of borings.
On de whowe, de fossiw record onwy contains minerawised exoskewetons, since dese are by far de most durabwe. Since most wineages wif exoskewetons are dought to have started out wif a non-minerawised exoskeweton which dey water minerawised, dis makes it difficuwt to comment on de very earwy evowution of each wineage's exoskeweton, uh-hah-hah-hah. It is known, however, dat in a very short course of time, just before de Cambrian period, exoskewetons made of various materiaws – siwica, cawcium phosphate, cawcite, aragonite, and even gwued-togeder mineraw fwakes – sprang up in a range of different environments. Most wineages adopted de form of cawcium carbonate which was stabwe in de ocean at de time dey first minerawised, and did not change from dis mineraw morph - even when it became de wess favorabwe.
Some Precambrian (Ediacaran) organisms produced tough but non-minerawized outer shewws, whiwe oders, such as Cwoudina, had a cawcified exoskeweton, but minerawized skewetons did not become common untiw de beginning of de Cambrian period, wif de rise of de "smaww shewwy fauna". Just after de base of de Cambrian, dese miniature fossiws become diverse and abundant – dis abruptness may be an iwwusion, since de chemicaw conditions which preserved de smaww shewwies appeared at de same time. Most oder sheww-forming organisms appear during de Cambrian period, wif de Bryozoans being de onwy cawcifying phywum to appear water, in de Ordovician. The sudden appearance of shewws has been winked to a change in ocean chemistry which made de cawcium compounds of which de shewws are constructed stabwe enough to be precipitated into a sheww. However dis is unwikewy to be a sufficient cause, as de main construction cost of shewws is in creating de proteins and powysaccharides reqwired for de sheww's composite structure, not in de precipitation of de mineraw components. Skewetonization awso appeared at awmost exactwy de same time dat animaws started burrowing to avoid predation, and one of de earwiest exoskewetons was made of gwued-togeder mineraw fwakes, suggesting dat skewetonization was wikewise a response to increased pressure from predators.
Ocean chemistry may awso controw which mineraw shewws are constructed of. Cawcium carbonate has two forms, de stabwe cawcite, and de metastabwe aragonite, which is stabwe widin a reasonabwe range of chemicaw environments but rapidwy becomes unstabwe outside dis range. When de oceans contain a rewativewy high proportion of magnesium compared to cawcium, aragonite is more stabwe, but as de magnesium concentration drops, it becomes wess stabwe, hence harder to incorporate into an exoskeweton, as it wiww tend to dissowve.
Wif de exception of de mowwuscs, whose shewws often comprise bof forms, most wineages use just one form of de mineraw. The form used appears to refwect de seawater chemistry – dus which form was more easiwy precipitated – at de time dat de wineage first evowved a cawcified skeweton, and does not change dereafter. However, de rewative abundance of cawcite- and aragonite-using wineages does not refwect subseqwent seawater chemistry – de magnesium/cawcium ratio of de oceans appears to have a negwigibwe impact on organisms' success, which is instead controwwed mainwy by how weww dey recover from mass extinctions. A recentwy discovered modern gastropod Chrysomawwon sqwamiferum dat wives near deep-sea hydrodermaw vents iwwustrates de infwuence of bof ancient and modern wocaw chemicaw environments: its sheww is made of aragonite, which is found in some of de earwiest fossiw mowwusks; but it awso has armor pwates on de sides of its foot, and dese are minerawized wif de iron suwfides pyrite and greigite, which had never previouswy been found in any metazoan but whose ingredients are emitted in warge qwantities by de vents.
- Spiracwe – smaww openings in de exoskeweton dat awwow insects to breade
- Hydrostatic skeweton
- Powered exoskeweton
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