What Supports And Protects An Animal's Tissue And Allows It To Move

External skeleton of an organism

An exoskeleton (from Greek έξω, éxō "outer" and σκελετός, skeletós "skeleton"^[1]) is the external skeleton that supports and protects an animal'southward trunk, in dissimilarity to the internal skeleton (endoskeleton) of, for example, a human. In usage, some of the larger kinds of exoskeletons are known equally "shells". Examples of exoskeletons within animals include the arthropod exoskeleton shared by chelicerates, myriapods, crustaceans, and insects, as well equally the shell of certain sponges and the mollusc shell shared by snails, clams, tusk shells, chitons and nautilus. Some animals, such equally the turtle (shell page), have both an endoskeleton and an exoskeleton.

Role [edit]

Exoskeletons contain rigid and resistant components that fulfill a set of functional roles in many animals including protection, excretion, sensing, support, feeding and acting as a barrier confronting desiccation in terrestrial organisms. Exoskeletons take a part in defense force from pests and predators, back up and in providing an attachment framework for musculature.^[2]

Arthropod exoskeletons incorporate chitin; the add-on of calcium carbonate makes them harder and stronger, at the price of increased weight.^[3] Ingrowths of the arthropod exoskeleton known every bit apodemes serve every bit attachment sites for muscles. These structures are composed of chitin and are approximately six times stronger and twice the stiffness of vertebrate tendons. Similar to tendons, apodemes can stretch to store rubberband energy for jumping, notably in locusts.^[4] Calcium carbonates constitute the shells of molluscs, brachiopods, and some tube-building polychaete worms. Silica forms the exoskeleton in the microscopic diatoms and radiolaria. One species of mollusc, the scaly-human foot gastropod, even makes use of the iron sulfides greigite and pyrite.

Some organisms, such every bit some foraminifera, agglutinate exoskeletons by sticking grains of sand and beat to their outside. Contrary to a common misconception, echinoderms practice not possess an exoskeleton, as their test is always contained within a layer of living tissue.

Exoskeletons have evolved independently many times; eighteen lineages evolved calcified exoskeletons lonely.^[5] Farther, other lineages accept produced tough outer coatings analogous to an exoskeleton, such as some mammals. This coating is constructed from bone in the armadillo, and hair in the pangolin. The armor of reptiles similar turtles and dinosaurs like Ankylosaurs is synthetic of bone; crocodiles have bony scutes and horny scales.

Growth [edit]

Since exoskeletons are rigid, they nowadays some limits to growth. Organisms with open shells tin grow by adding new material to the aperture of their beat, as is the case in snails, bivalves and other molluscans. A true exoskeleton, like that constitute in arthropods, must be shed (moulted) when it is outgrown.^[6] A new exoskeleton is produced beneath the old ane. As the old 1 is shed, the new skeleton is soft and pliable. The animal will typically stay in a den or burrow for this fourth dimension,^{[ commendation needed ]} as it is quite vulnerable during this period. Once at to the lowest degree partially set up, the organism will plump itself upwards to try to expand the exoskeleton.^{[ ambiguous ]} The new exoskeleton is withal capable of growing to some degree, notwithstanding.^{[ citation needed ]} Animals of the order arthropoda, like lizards, amphibians, and many other animals that shed their peel, are indeterminate growers.[1] Animals that are indeterminate growers abound in size continually throughout their life considering, in this case, their exoskeleton is always existence replaced. Failure to shed the exoskeleton once outgrown can effect in the brute being suffocated within its own vanquish, and will stop subadults from reaching maturity, thus preventing them from reproducing. This is the mechanism behind some insect pesticides, such as Azadirachtin.^[7]

Paleontological significance [edit]

Borings in exoskeletons can provide evidence of beast beliefs. In this case, boring sponges attacked this hard clam beat out later on 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 tin be fossilized. Mineralized exoskeletons can be preserved "as is", as shell fragments, for instance. The possession of an exoskeleton permits a couple of other routes to fossilization. For example, the tough layer tin can resist compaction, allowing a mold of the organism to exist formed underneath the skeleton, which may after decay.^[8] Alternatively, infrequent preservation may result in chitin being mineralized, as in the Burgess Shale,^[9] or transformed to the resistant polymer keratin, which tin resist disuse and exist recovered.

However, our dependence on fossilized skeletons besides significantly limits our understanding of development. Only the parts of organisms that were already mineralized are normally preserved, such every bit the shells of molluscs. It helps that exoskeletons often contain "muscle scars", marks where muscles take been attached to the exoskeleton, which may let the reconstruction of much of an organism's internal parts from its exoskeleton lone.^[8] The most significant limitation is that, although there are 30-plus phyla of living animals, 2-thirds of these phyla take never been found equally fossils, considering near animal species are soft-bodied and decay before they can go fossilized.^[10]

Mineralized skeletons outset appear in the fossil tape shortly earlier the base of the Cambrian period, 550 million years agone. The evolution of a mineralized exoskeleton is seen by some every bit a possible driving force of the Cambrian explosion of brute life, resulting in a diversification of predatory and defensive tactics. However, some Precambrian (Ediacaran) organisms produced tough outer shells^[eight] while others, such as Cloudina, had a calcified exoskeleton.^[11] Some Cloudina shells fifty-fifty testify evidence of predation, in the form of borings.^[11]

Evolution [edit]

On the whole, the fossil record just contains mineralised exoskeletons, since these are by far the most durable. Since most lineages with exoskeletons are thought to accept started out with a non-mineralised exoskeleton which they afterwards mineralised, this makes information technology hard to annotate on the very early evolution of each lineage's exoskeleton. It is known, however, that in a very brusk class 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.^[12] Most lineages adopted the class of calcium carbonate which was stable in the body of water at the time they start mineralised, and did not change from this mineral morph - even when it became the less favorable.^[5]

Some Precambrian (Ediacaran) organisms produced tough but non-mineralized outer shells,^[8] while others, such every bit Cloudina, had a calcified exoskeleton,^[11] but mineralized skeletons did not go common until the beginning of the Cambrian period, with the rise of the "small shelly beast". Just after the base of the Cambrian, these miniature fossils become diverse and abundant – this abruptness may exist an illusion, since the chemical weather which preserved the pocket-sized shellies appeared at the aforementioned fourth dimension.^[13] Well-nigh other shell-forming organisms announced during the Cambrian menses, with the Bryozoans being the merely 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 trounce. All the same this is unlikely to be a sufficient cause, equally the master construction cost of shells is in creating the proteins and polysaccharides required for the shell'due south composite structure, not in the precipitation of the mineral components.^[2] Skeletonization also appeared at most exactly the same fourth dimension that animals started burrowing to avoid predation, and i of the earliest exoskeletons was made of glued-together mineral flakes, suggesting that skeletonization was likewise a response to increased force per unit area from predators.^[12]

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 exterior this range. When the oceans contain a relatively loftier proportion of magnesium compared to calcium, aragonite is more stable, simply as the magnesium concentration drops, it becomes less stable, hence harder to comprise into an exoskeleton, as it will tend to dissolve.

With the exception of the molluscs, whose shells often comprise both forms, most lineages use merely 1 form of the mineral. The form used appears to reflect the seawater chemistry – thus which class was more than easily precipitated – at the time that the lineage starting time evolved a calcified skeleton, and does non change thereafter.^[5] However, the relative abundance of calcite- and aragonite-using lineages does not reverberate subsequent seawater chemistry – the magnesium/calcium ratio of the oceans appears to have a negligible bear on on organisms' success, which is instead controlled mainly by how well they recover from mass extinctions.^[14] A recently discovered^[15] modern gastropod Chrysomallon squamiferum that lives near abyssal hydrothermal vents illustrates the influence of both ancient and mod local chemical environments: its shell is made of aragonite, which is institute in some of the earliest fossil mollusks; simply information technology also has armor plates on the sides of its human foot, and these are mineralized with the iron sulfides pyrite and greigite, which had never previously been found in any metazoan simply whose ingredients are emitted in large quantities by the vents.^[ii]

Meet also [edit]

Spiracle – minor openings in the exoskeleton that allow insects to breathe
Hydrostatic skeleton
Endoskeleton
Powered exoskeleton
Osteoderm

References [edit]

^ "exoskeleton". Online Etymology Dictionary. Archived from the original on 2013-04-20.
^ ^a ^b ^c S. Bengtson (2004). "Early skeletal fossils" (PDF). In J. H. Lipps; B. One thousand. Waggoner (eds.). Neoproterozoic–Cambrian Biological Revolutions. Paleontological Society Papers. Vol. 10. pp. 67–78. Archived from the original (PDF) on 2008-10-03.
^ Nedin, C. (1999). "Anomalocaris predation on nonmineralized and mineralized trilobites". Geology. 27 (11): 987–990. Bibcode:1999Geo....27..987N. doi:10.1130/0091-7613(1999)027<0987:APONAM>2.three.CO;2.
^ H. C. Bennet-Clark (1975). "The energetics of the jump of the locust, Schistocerca gregaria" (PDF). Journal of Experimental Biology. 63 (i): 53–83. doi:10.1242/jeb.63.ane.53. PMID 1159370.
^ ^a ^b ^c Susannah M. Porter (2007). "Seawater chemistry and early carbonate biomineralization". Science. 316 (5829): 1302. Bibcode:2007Sci...316.1302P. doi:x.1126/scientific discipline.1137284. PMID 17540895. S2CID 27418253.
^ John Ewer (2005-10-11). "How the Ecdysozoan Changed Its Coat". PLOS Biology. 3 (10): e349. doi:10.1371/periodical.pbio.0030349. PMC1250302. PMID 16207077.
^ Gemma Due east. Veitch; Edith Beckmann; Brenda J. Burke; Alistair Boyer; Sarah L. Maslen; Steven V. Ley (2007). "Synthesis of Azadirachtin: A Long but Successful Journeying". Angewandte Chemie International Edition. 46 (40): 7629–32. doi:ten.1002/anie.200703027. PMID 17665403.
^ ^a ^b ^c ^d M. A. Fedonkin; A. Simonetta; A. Y. Ivantsov (2007). "New data on Kimberella, the Vendian mollusk-similar organism (White ocean region, Russian federation): palaeoecological and evolutionary implications". In Patricia Vickers-Rich & Patricia (ed.). The Rise and Fall of the Ediacaran Biota. Geological Guild of London Special Publications. Vol. 286. London: Geological Society. pp. 157–179. Bibcode:2007GSLSP.286..157F. doi:10.1144/SP286.12. ISBN978-1-86239-233-five. OCLC 191881597. S2CID 331187.
^ Nicholas J. Butterfield (2003). "Exceptional fossil preservation and the Cambrian Explosion". Integrative and Comparative Biology. 43 (1): 166–177. doi:10.1093/icb/43.1.166. PMID 21680421.
^ Richard Cowen (2004). History of Life (4th ed.). Wiley-Blackwell. ISBN978-1-4051-1756-2.
^ ^a ^b ^c Hong Hua; Brian R. Pratt; Lu-yi Zhang (2003). "Borings in Cloudina shells: complex predator-casualty dynamics in the terminal Neoproterozoic". PALAIOS. 18 (iv–v): 454–459. Bibcode:2003Palai..18..454H. doi:10.1669/0883-1351(2003)018<0454:BICSCP>2.0.CO;2. S2CID 131590949.
^ ^a ^b J. Dzik (2007). "The Verdun Syndrome: simultaneous origin of protective armor and infaunal shelters at the Precambrian–Cambrian transition" (PDF). In Patricia Vickers-Rich & Patricia (ed.). The Rise and Fall of the Ediacaran Biota. Geological Guild, London, Special Publications. Vol. 286. London: Geological Society. pp. 405–414. Bibcode:2007GSLSP.286..405D. CiteSeerXten.one.1.693.9187. doi:10.1144/SP286.thirty. ISBN978-1-86239-233-5. OCLC 191881597. S2CID 33112819. Archived (PDF) from the original on 2008-10-03.
^ J. Dzik (1994). "Evolution of 'small shelly fossils' assemblages of the early Paleozoic". Acta Palaeontologica Polonica. 39 (3): 27–313. Archived from the original on 2008-12-05.
^ Wolfgang Kiessling; Martin Aberhan; Loïc Villier (2008). "Phanerozoic trends in skeletal mineralogy driven by mass extinctions". Nature Geoscience. 1 (8): 527–530. Bibcode:2008NatGe...ane..527K. doi:10.1038/ngeo251.
^ Anders Warén; Stefan Bengtson; Shana Thou. Goffredi; Cindy L. Van Dover (2003). "A hot-vent gastropod with iron sulfide dermal sclerites". Scientific discipline. 302 (5647): 1007. doi:10.1126/science.1087696. PMID 14605361. S2CID 38386600.