Animal shell: Wikis

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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. Some animals, such as the tortoise, 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 of animals.

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Role of the exoskeleton

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

Exoskeletons contain chitin and when calcium carbonate is added, the exoskeleton grows in strength and hardness.

Diversity

Many taxa produce exoskeletons, which are composed of a range of materials. Bone, cartilage, or dentine is used in the Ostracoderm fish and turtles. Chitin forms the exoskeleton in arthropods including insects, arachnids such as spiders, crustaceans such as crabs and lobsters (see arthropod exoskeleton), and in some fungi and bacteria. Calcium carbonates constitute the shells of molluscs (see Mollusc shell), brachiopods, and some tube-building polychaete worms. Silica forms the exoskeleton in the microscopic diatoms and radiolaria.

Some organisms, such as some foraminifera, 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.[2] 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

Borings in exoskeletons can provide evidence of animal behavior. In this case, boring sponges 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.[3] Alternatively, exceptional preservation may result in chitin being mineralized, as in the Burgess Shale,[4] 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.[3] 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.[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) 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 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.[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 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.[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

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 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 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.  

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Simple English

A shell is a hard outer layer, which has evolved in a very wide variety of different animals, including molluscs, sea urchins, crustaceans, turtles and tortoises, armadillos, etc.[1]

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Seashells

Seashells[2] are perhaps the most familiar and most common shells, both in the wild and for sale as decorative objects. They are usually composed of calcium carbonate, in the form of calcite or aragonite[3]. This can have different forms, one is nacre or mother of pearl. Other kinds of animal shells are made from chitin, bone and cartilage, or silica.

Use of Shells

Shells of many types were popular as human decorations, either used whole or cut into pieces. One used them for all kinds of pendants, beads, buttons, brooches, rings, and hair combs.

Small pieces of colored shell and iridescent shell have been used to create mosaics and inlays which have been used to decorate larger items such as boxes and furniture. Large numbers of whole seashells, arranged to form patterns, have been used to decorate man-made grottos.

Several kinds of sea animals have exoskeletons which one can find after their death in beach drift. Often beachcombers pick them. But usually only those of molluscs (also spelled "mollusks") are known as seashells.

Classes

The majority of shell-forming molluscs belong to two classes: Gastropoda (univalves, or snails) and Bivalvia (bivalves or clams, oysters, scallops, etc).

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