Osteostraci

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Osteostraci An order of fossil, fish-like, jawless vertebrates that had a flattened head covered by a bony shield with a series of small gill openings on the side. Usually small (30 cm), they ranged from the Silurian to the Devonian Periods. The internal structures of the head resemble those of extant Petromyzonidae (lampreys).

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