Scutes on an alligator foot

A scute or scutum (Latin: scutum; plural: scuta "shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of birds. The term is also used to describe the anterior portion of the mesonotum in insects as well as some arachnids (e.g., the family Ixodidae, the scale ticks).

Properties

This detail of a Glyptodon displays its scutes. From the collection of the Children's Museum of Indianapolis.
Detail of scutes of a leopard tortoise

Scutes are similar to scales and serve the same function. Unlike the scales of lizards and snakes, which are formed from the epidermis, scutes are formed in the lower vascular layer of the skin and the epidermal element is only the top surface . Forming in the living dermis, the scutes produce a horny outer layer that is superficially similar to that of scales. Scutes will usually not overlap as snake scales (but see the pangolin). The outer keratin layer is shed piecemeal, and not in one continuous layer of skin as seen in snakes or lizards. The dermal base may contain bone and produce dermal armour. Scutes with a bony base are properly called osteoderms. Dermal scutes are also found in the feet of birds and tails of some mammals, and are believed to be the primitive form of dermal armour in reptiles.

The term is also used to describe the heavy armour of the armadillo and the extinct Glyptodon, and is occasionally used as an alternative to scales in describing snakes or certain fishes, such as sturgeons, shad, herring, and menhaden.

Mammals

Prehistoric ancestors of mammals, the synapsids, are thought to have scutes which were later reduced and replaced by hair. Excluding the attachment surface of fingernails, armoured scutes or scales are almost never seen in modern mammals. The horny scales of pangolins are only rarely called scutes, but "scute" is used to describe the heavy armour of the armadillo.

Turtles

The turtle's shell is covered by scutes formed mostly of keratin. They are built similarly to horn, beak, or nail in other species.

Fish

The pineconefish

Some fish, such as pineconefish, are completely or partially covered in scutes. River herrings and threadfins have an abdominal row of scutes, which are scales with raised, sharp points that are used for protection. Some jacks have a row of scutes following the lateral line on either side. Sturgeon have five rows of scutes instead of scales.

Birds

The tarsometatarsus 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.[1][2][3] Unblocking the feather suppression program results in feathers growing in place of scales along the tarsometatarsus and toes.[1][2][3] 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.[3] 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).[3] These data have led some researchers to suggest that reticulae are in fact highly truncated feathers.[3][4]

Insects and other arthropods

The term "scutum" is also used in insect anatomy, as an alternative name for the anterior portion of the mesonotum (and, technically, the metanotum, though rarely applied in that context).

Conspicuous scutum on a typical female hard tick before she has fed. Note the pale eye-spots near the edges of the scutum, roughly between the 2nd and 3rd legs
The same scutum is relatively less conspicuous after the tick has fed, because it has not changed in size, whereas the tick has swollen as it engorged
In the typical male hard tick, the conscutum covers practically the whole back

In the hard ticks, the Ixodidae, the scutum is a rigid, sclerotised plate on the anterior dorsal surface, just posterior to the head. In species with eyes, the eyes are on the surface of the scutum. The flexible exoskeleton posterior to the rigid scutum of the female tick, is called the alloscutum, the region that stretches to accommodate the blood with which the mature female tick becomes engorged. 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 the conscutum.[5]

In some species of Opiliones, fused abdominal segments are referred to as a scutum.[6]

See also

References

  1. 1 2 Sawyer, R.H., Knapp, L.W. 2003. Avian Skin Development and the Evolutionary Origin of Feathers. J. Exp. Zool (Mol Dev Evol) 298B:57–72.
  2. 1 2 Sawyer, R.H.; Rogers, L.; Washington, L.; Glenn, T.C.; Knapp, L.W. (2005). "Evolutionary Origin of the Feather Epidermis". Dev. Dyn. 232 (2): 256–267. doi:10.1002/dvdy.20291. PMID 15637693. S2CID 44477271.
  3. 1 2 3 4 5 Dhouailly, D (2009). "A new scenario for the evolutionary origin of hair, feather, and avian scales" (PDF). Journal of Anatomy. 214 (4): 587–606. doi:10.1111/j.1469-7580.2008.01041.x. PMC 2736124. PMID 19422430.
  4. ↑ Zheng, X.; Zhou, Z.; Wang, X.; Zhang, F.; Zhang, X.; Wang, Y.; Xu, X. (2013). "Hind wings in basal birds and the evolution of leg feathers". Science. 339 (6125): 1309–1312. Bibcode:2013Sci...339.1309Z. CiteSeerX 10.1.1.1031.5732. doi:10.1126/science.1228753. PMID 23493711. S2CID 206544531.
  5. ↑ Ivan G. Horak; Heloise Heyne; Roy Williams; G. James Gallivan; Arthur M. Spickett; J. Dürr Bezuidenhout; Agustín Estrada-Peña (14 February 2018). The Ixodid Ticks (Acari: Ixodidae) of Southern Africa. Springer. pp. 46–. ISBN 978-3-319-70642-9.
  6. ↑ Ricardo Pinto-da-Rocha; Rafael Fonseca-Ferreira; Maria Bichuette (18 November 2015). "A new highly specialized cave harvestman from Brazil and the first blind species of the genus: Iandumoema smeagol sp. n. (Arachnida, Opiliones, Gonyleptidae)". ZooKeys (537): 79–95. doi:10.3897/zookeys.537.6073. PMC 4714048. PMID 26798238.
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