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Osteoderms are bony deposits forming scales, plates or other structures in the dermal layers of the
skinThe skin is the outer covering of the body. In humans, it is the largest organ of the integumentary system made up of multiple layers of mesodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs. Skin of a different nature exists in amphibians, reptiles, birds...
. Osteoderms are found in many groups of extant and extinct
reptileReptiles, or members of the class Reptilia, are air-breathing, generally "cold-blooded" amniotes that generally have skin covered in scales or scutes. They are tetrapods and lay amniote eggs, whose embryos are surrounded by the amnion membrane...
s, including
lizardLizards are a very large and widespread group of squamate reptiles, with nearly 5,000 species, ranging across all continents except Antarctica as well as most oceanic island chains...
s, various groups of
dinosaur{{Otheruses}}{{pp-semi-protected|small=yes}}{{Otheruses}}{{pp-semi-protected|small=yes}}{{Taxobox|name = Dinosaurs|fossil_range = {{Fossil range|230|65|earliest=230|latest=0|PS=
Descendant taxon Aves survives to present.}}|image = field_dinos_2.jpg...
s (most notably ankylosaurs and
stegosauriaKnown colloquially as stegosaurs, the Stegosauria are a group of herbivorous dinosaurs of the Jurassic and Early Cretaceous Periods, being found mostly in the Northern Hemisphere, predominantly in what is now North America and China....
ns),
crocodiliaCrocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...
ns,
phytosaurPhytosaurs - family Phytosauridae or Parasuchidae - were a group of large semi-aquatic predatory archosaurs that flourished during the Late Triassic period...
s,
aetosaurThe aetosaurs are an extinct clade of heavily armoured, medium- to large-sized Late Triassic herbivorous archosaurs. Two distinct subdivisions of aeotosaurs are currently recognized, the Desmatosuchinae and the Aetosaurinae, based primarily on differences in the morphology of the bony scutes of...
s,
placodontPlacodonts were a group of marine reptiles that lived during the Triassic period, becoming extinct at the end of the period. It is believed that they were related to the Sauropterygia, the group that includes Plesiosaurs...
s, and
NanchangosaurusNanchangosaurus was a large aquatic reptile native to the middle Triassic of China. It was named after the area in China, Nanchang, where it was found. It was about three feet in length, and probably fed on fish or used its long jaws to probe for aquatic invertebrates...
(a marine reptile with possible
ichthyosaurIchthyosaurs were giant marine reptiles that resembled fish and dolphins...
affinities).
Osteoderms are uncommon in
mammalMammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...
s, but do occur in
pangolinA pangolin , also scaly anteater or trenggiling, is a mammal of the order Pholidota. There is only one extant family and one genus of pangolins, comprising eight species. There are also a number of extinct taxa. Pangolins have large keratin scales covering their skin and are the only mammals with...
s and many
xenarthraThe superorder Xenarthra is a group of placental mammals , extant today only in the Americas. The origins of the order can be traced back as far as the early Tertiary...
ns (glyptodonts,
armadilloArmadillos are small placental mammals, known for having a leathery armor shell. The Dasypodidae are the only surviving family in the order Cingulata, part of the superorder Xenarthra along with the anteaters and sloths...
s,
ground slothGround sloths are a diverse group of extinct sloths, in the mammalian superorder Xenarthra. Their most recent survivors lived in the Antilles, where it has been proposed they may survived until 1550 AD; however, the youngest AMS radiocarbon date reported is 4190 BP, calibrated to c. 4700 BP for...
s). Osteoderms have clearly evolved independently in many different lineages, and these varied structures should be thought of anatomical analogues, not
homologuesIn evolutionary biology, homology refers to any similarity between characteristics of organisms that is due to their shared ancestry. The word homologous derives from the ancient Greek ομολογειν, 'to agree'. There are examples in different branches of biology...
, and do not necessarily indicate
monophylyIn common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it consists of an ancestor and all its descendants. The term is synonymous with the uncommon term holophyly...
. In many cases osteoderms may function as defensive
armorArmour in animals is external or superficial protection against attack by predators, formed as part of the body , usually through the hardening of body tissues, outgrowths or secretions. It has therefore mostly developed in 'prey' species...
.
The osteoderms of modern crocodylians are heavily vascularized, and can function as both armor and as heat-exchangers, allowing these large reptiles to rapidly raise or lower their temperature.