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Armor (or armour) in animals is external or superficial protection against attack by predators, formed as part of the body (rather than the behavioural use of protective external objects), usually through the hardening of body tissues, outgrowths or secretions. It has therefore mostly developed in 'prey' species. Armored structures are usually composed of hardened mineral deposits, chitin, bone or keratin.
Armour is evident on numerous animal species from both current and prehistoric times. Dinosaurs such as Ankylosaurus, as well as other Thyreophora (armored dinosaurs such as Ankylosauria and Stegosauria), grew thick plate-like armor on their bodies as well as offensive armor appendages such as the thagomizer or a club. The armor took many forms, including osteoderms, spikes, horns and plates. Other dinosaurs such as ceratopsian dinosaurs as well as some sauropods such as Saltasaurus and Agustinia, grew armor to defend themselves, although armor in sauropods overall is uncommon.
In modern times, some molluscs employ the use of shells as armor and armor is evident in the chitinous exoskeleton of arthropods. Fish use armor in the form of scales, whether 'cosmoid', 'ganoid' or 'placoid' and in some cases spines, such as on fish such as the stickleback. The chalky plate, or cuttlebone, of cuttlefish also acts as armor. Most reptiles have scaly skin which protects them from predators in addition to water retention; the crocodile's exoskeleton and the shells of the Chelonia - tortoises, turtles and terrapins.
Numerous mammals, although not as sturdy as reptilian armor, employ the use of spines and body armor, like the spines of the Echidna and spiny anteaters and of porcupines and hedgehogs. The bony shell of the armadillos and the extinct Glyptodon were very much like Ankylosaurus' armor and modern armadillos curl up into a ball when threatened, making them unexposed due to their armor. Similarly, the hairy plate-like scales of the pangolin are employed in the same way and are constructed of the same material used in the offensive armor, the horn, of the rhinoceros.
Armour, although all used for the sole intent to ward off attackers, can be split into defensive and offensive armor. Examples of offensive armor are horns, hooves, antlers, claws and beaks, clubs and pincers, as developed in some mammals, birds, reptiles (including dinosaurs, such as the Dromaeosaurids claw and the Ceratopsian horn) and arthropods. Offensive armor is often used in conjunction with defensive armor and in some cases makes an animal almost unassailable.
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