Exoskeleton
An exoskeleton (from Greek έξω éxō "outer"
Examples of exoskeletons in animals include the
Role
Exoskeletons contain rigid and resistant components that fulfil a set of functional roles in many animals including protection, excretion, sensing, support, feeding, and acting as a barrier against desiccation in terrestrial organisms. Exoskeletons have roles in defence from pests and predators and in providing an attachment framework for musculature.[4]
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 and their test is always contained within a layer of living tissue.[citation needed]
Exoskeletons have evolved independently many times; 18 lineages evolved calcified exoskeletons alone.[7] Further, other lineages have produced tough outer coatings, such as some mammals, that are analogous to an exoskeleton. This coating is constructed from bone in the armadillo, and hair in the pangolin. The armour of reptiles like turtles and dinosaurs like Ankylosaurs is constructed of bone; crocodiles have bony scutes and horny scales.
Growth
Since exoskeletons are rigid, they present some limits to growth. Organisms with open shells can grow by adding new material to the aperture of their shell, as is the case in snails,
Paleontological significance
Exoskeletons, as hard parts of organisms, are greatly useful in assisting the preservation of organisms, whose soft parts usually rot before they can be fossilized. Mineralized exoskeletons can be preserved as shell fragments. The possession of an exoskeleton permits a couple of other routes to fossilization. For instance, the strong layer can resist compaction, allowing a mould of the organism to be formed underneath the skeleton, which may later decay.[11] Alternatively, exceptional preservation may result in chitin being mineralised, as in the Burgess Shale,[12] or transformed to the resistant polymer keratin, which can resist decay and be recovered.
However, our dependence on fossilised skeletons also significantly limits our understanding of evolution. Only the parts of organisms that were already mineralised are usually preserved, such as the shells of molluscs. 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.[11] 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 fossilised.[13]
Mineralized skeletons first appear in the fossil record shortly before the base of the
Evolution
Part of a series related to |
Biomineralization |
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The fossil record primarily contains mineralized exoskeletons, since these are by far the most durable. Since most lineages with exoskeletons are thought to have started with a non-mineralized exoskeleton which they later mineralized, it is difficult to comment on the very early evolution of each lineage's exoskeleton. It is known, however, 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.[15] Most lineages adopted the form of calcium carbonate which was stable in the ocean at the time they first mineralized, and did not change from this mineral morph - even when it became less favourable.[7]
Some Precambrian (Ediacaran) organisms produced tough but non-mineralized outer shells,
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.[citation needed]
Except for the molluscs, 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.
See also
- Spiracle – small openings in the exoskeleton that allow insects to breathe
- Hydrostatic skeleton
- Endoskeleton
- Powered exoskeleton
- Osteoderm
- Scaly-foot gastropod – Deep-sea gastropod known to incorporate iron into its exoskeleton
References
- ^ Liddell, Henry George; Scott, Robert (1940). "ἔξω". A Greek-English Lexicon. Perseus Digital Library.
- ^ Liddell, Henry George; Scott, Robert (1940). "σκελετός". A Greek-English Lexicon. Perseus Digital Library.
- ^ Douglas, Harper (2001). "exoskeleton". Online Etymology Dictionary. Archived from the original on 20 April 2013.
- ^ a b c S. Bengtson (2004). "Early skeletal fossils" (PDF). In J. H. Lipps; B. M. Waggoner (eds.). Neoproterozoic–Cambrian Biological Revolutions. Paleontological Society Papers. Vol. 10. pp. 67–78. Archived from the original (PDF) on 2008-10-03.
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- ^ 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.
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