Scales are generally classified as part of an organism'sintegumentary system. There are various types of scales according to the shape andclass of an animal.
Fish scales aredermally derived, specifically in themesoderm. This fact distinguishes them from reptile scales paleontologically.Genetically, the same genes involved in tooth and hair development in mammals are also involved in scale development.[1]
Truecosmoid scales can only be found on theSarcopterygians. The inner layer of the scale is made oflamellar bone. On top of this lies a layer of spongy orvascular bone and then a layer ofdentine-like material calledcosmine. The upper surface iskeratin. Thecoelacanth has modified cosmoid scales that lack cosmine and are thinner than true cosmoid scales.
Ganoid scales can be found ongars (familyLepisosteidae),bichirs, and reedfishes (familyPolypteridae). Ganoid scales are similar to cosmoid scales, but a layer ofganoin lies over the cosmine layer and under the enamel[clarification needed]. Ganoin scales are diamond shaped, shiny, and hard. Within the ganoin areguanine compounds, iridescent derivatives of guanine found in a DNA molecule.[2] The iridescent property of these chemicals provide the ganoin its shine.
Placoid scales are found oncartilaginous fish includingsharks andstingrays. These scales, also called denticles, are similar in structure toteeth, and have one median spine and two lateral spines. The modern jawed fish ancestors, the jawlessostracoderms and later jawedplacoderms, may have had scales with the properties of both placoid and ganoid scales.
Leptoid scales are found on higher-order bony fish. As they grow they add concentric layers. They are arranged so as to overlap in a head-to-tail direction, like roof tiles, allowing a smoother flow of water over the body and therefore reducingdrag.[3] They come in two forms:
Cycloid scales have a smooth outer edge, and are most common on fish with soft fin rays, such assalmon andcarp.
Ctenoid scales have a toothed outer edge, and are usually found on fish with spiny fin rays, such asbass andcrappie.
Reptile scale types include cycloid, granular (which appear bumpy), and keeled (which have a center ridge). Scales usually vary in size, the stouter, larger scales cover parts that are often exposed to physical stress (usually the feet, tail and head), while scales are small around the joints for flexibility. Most snakes have extra broad scales on the belly, each scale covering the belly from side to side.
The scales of all reptiles have an epidermal component (what one sees on the surface), but many reptiles, such ascrocodilians and turtles, haveosteoderms underlying the epidermal scale. Such scales are more properly termedscutes. Snakes,tuataras and many lizards lack osteoderms. All reptilian scales have a dermal papilla underlying the epidermal part, and it is there that the osteoderms, if present, would be formed.
Many reptiles possess large scales not supported by osteoderms known asfeature scales. Thegreen iguana possesses large feature scales on the ventral sides of its neck, and dorsal spines not supported by osteoderms. Many extinct non-aviandinosaurs such asCarnotaurus andBrachylophosaurus are known to possess feature scales from skin impressions.
Birds' scales are found mainly on the toes and metatarsus, but may be found further up on the ankle in some birds. The scales and scutes of birds were thought to behomologous to those of reptiles,[4] but are now agreed to have evolved independently, being degenerate feathers.[5][6]
An example of a scaled mammal is thepangolin. Its scales are made of keratin and are used for protection, similar to anarmadillo's armor. They have been convergently evolved, being unrelated to mammals' distant reptile-like ancestors (sincetherapsids lost scales), except that they use a similar gene.
On the other hand, themusky rat-kangaroo has scales on its feet and tail.[7] The precise nature of its purported scales has not been studied in detail, but they appear to be structurally different from pangolin scales.
Anomalures also have scales on their tail undersides.[8]
Foot pad epidermal tissues in most mammal species have been compared to the scales of other vertebrates. They are likely derived from cornification processes or stunted fur much like avian reticulae are derived from stunted feathers.[9]
Scales on aluna moth (Actias luna)Generalized structure of a keeled, lanceolate scale from ajumping spider
Butterflies andmoths - the orderLepidoptera (Greek "scale-winged") - have membranouswings covered in delicate, powdery scales, which are modifiedsetae. Each scale consists of a series of tiny stacked platelets of organic material, and butterflies tend to have the scales broad and flattened, while moths tend to have the scales narrower and more hair like. Scales are usuallypigmented, but some types of scales are iridescent, without pigments; because the thickness of the platelets is on the same order as thewavelength ofvisible light the plates lead to structural coloration andiridescence through the physical phenomenon described asthin-film optics. The most common color produced in this fashion isblue, such as in theMorpho butterflies.
Some types ofspiders also have scales. Spider scales are flattened setae that overlay the surface of thecuticle. They come in a wide variety of shapes, sizes, and colors. At least 13 different spider families are known to possess cuticular scales, although they have only been well described forjumping spiders (Salticidae) andlynx spiders (Oxyopidae).[10][11]
^Levy-Lior A, Pokroy B, Levavi-Sivan B, Leiserowitz L, Weiner S, Addadi L (2008). "Biogenic guanine crystals from the skin of fish may be designed to enhance light reflectance".Crystal Growth & Design.8 (2):507–511.doi:10.1021/cg0704753.
^Sawyer RH, Knapp LW (August 2003). "Avian skin development and the evolutionary origin of feathers".Journal of Experimental Zoology Part B: Molecular and Developmental Evolution.298 (1):57–72.doi:10.1002/jez.b.26.PMID12949769.
^Meyer-Rochow, V. B. (1980). "Cuticular surface structures inGlyptonotus antarcticus — a marine isopod from the Ross Sea (Antarctica)".Zoomorphologie.94 (2):209–216.doi:10.1007/BF01081935.S2CID9216365.
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