Ascute (/skjuːt/ ⓘ) orscutum (Latin:scutum; plural:scuta "shield") is a bony external plate or scale overlaid with horn, as on the shell of aturtle, the skin ofcrocodilians, and the feet ofbirds. The term is also used to describe the anterior portion of themesothorax ininsects as well as some arachnids (e.g., the familyIxodidae, the scale ticks).
Scutes are similar toscales and serve the same function. Unlike the scales of lizards and snakes, which are formed from theepidermis, scutes are formed in the lower vascular layer of the skin and the epidermal element is only the top surface[citation needed]. Forming in the livingdermis, the scutes produce ahorny outer layer that is superficially similar to that of scales. Scutes will usually not overlap assnake scales (but see thepangolin). The outerkeratin layer is shed piecemeal, and not in one continuous layer of skin as seen in snakes or lizards. The dermal base may containbone and producedermal armour. Scutes with a bony base are properly calledosteoderms. Dermal scutes are also found in the feet ofbirds and tails of somemammals, and are believed to be the primitive form of dermal armour in reptiles.
The term is also used to describe the heavy armour of thearmadillo and the extinctGlyptodon, and is occasionally used as an alternative to scales in describingsnakes or certain fishes, such assturgeons, shad, herring, and menhaden.
Prehistoric ancestors of mammals, thesynapsids, are thought to have scutes which were later reduced and replaced by hair. Excluding the attachment surface offingernails, armoured scutes or scales are almost never seen in modernmammals. Thehorny scales ofpangolins are only rarely called scutes, but "scute" is used to describe the heavy armour of thearmadillo.
Theturtle's shell is covered by scutes formed mostly ofkeratin. They are built similarly to horn, beak, or nail in other species.
Some fish, such aspineconefish, are completely or partially covered in large armored scales commonly termed scutes.[1] ManyClupeomorpha have an abdominal row of scutes, which are scales with raised, sharp points that are used for protection, and in some cases they also possess dorsal scutes anterior to the dorsal fin, as inKnightia spp.,Diplomystus spp. and certain extant taxa.[2] Jacks of familyCarangidae often have a row of scutes following thelateral line on either side.[3]Coelorinchus spp. have scutes suborbitally from theirpreoperculum extending anteriorly onto a triangular snout.[4]Sturgeon have five rows of scutes—modifiedganoid scales—and are otherwise scaleless.[5][6]
Thetarsometatarsus and toes of most birds are covered in two types of scales. Large scutes run along the dorsal side of the tarsometatarsus and toes, whereas smaller scutellae run along the sides. Both structures share histochemical homology with reptilian scales; however, work on their evolutionary development has revealed that the scales in bird feet have secondarily evolved via suppression of the feather-building genetic program.[7][8][9] Unblocking the feather suppression program results in feathers growing in place of scales along the tarsometatarsus and toes.[7][8][9] Dinosaur species very close to the origin of birds have been shown to have had "hind wings" made of feathers growing from these areas, suggesting that the acquisition of feathers in dinosaurs was a whole-body event.[9] The bottoms of bird feet are covered in small, keeled scale-like structures known as reticulae. Evolutionary developmental studies on these scale-like structures have revealed that they are composed entirely of alpha keratin (true epidermal scales are composed of a mix of alpha and beta keratin).[9] These data have led some researchers to suggest that reticulae are in fact highly truncated feathers.[9][10]
The term "scutum" is also used ininsect anatomy, as an alternative name for the anterior portion of themesonotum (and, technically, themetanotum, though rarely applied in that context).
In thehard ticks, theIxodidae, the scutum is a rigid,sclerotised plate on theanteriordorsal surface, justposterior to the head. In species with eyes, the eyes are on the surface of the scutum. The flexibleexoskeleton posterior to the rigid scutum of the female tick, is called thealloscutum, the region that stretches to accommodate the blood with which the mature female tick becomesengorged. Males do not engorge nearly as drastically as females, so they do not need a flexible alloscutum; instead the rigid scutum covers practically the entire dorsal surface posterior to the head, and may be referred to specifically as theconscutum.[11]
In some species ofOpiliones, fused abdominal segments are referred to as a scutum.[12]