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Anaspidiformes (subphylum Craniata, class Cephalaspidomorphi) An extinct order of fossil, fish-like vertebrates that are considered to be related to the lampreys (Petromyzoniformes). They had a fusiform body with a hypocercal tail, and up to 15 pairs of external gill openings. Extant during the Silurian and Devonian Periods, these small (up to 15 cm in length) animals lived predominantly in fresh water.