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Any of various extinct fishes of the class Placodermi of the Silurian and Devonian Periods, characterized by bony plates of armor covering the head and flanks. The bodies of placoderms were spindle-shaped or flattened, and their skeletons were usually partially bony and included a cranium. Placoderms were the first group of fish to evolve jaws, but are not closely related to the jawed fish of today.
any member of an extinct group (Placodermi) of primitive jawed fishes known only from fossil remains. Placoderms existed throughout the Devonian period (416,000,000-359,000,000 years ago), but only two species persisted into the succeeding Carboniferous period. During the Devonian they were a dominant group, occurring in all continents except South America in a variety of marine and freshwater sediments.