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Any of three orders of small, extinct primitive jawless fishes of the Ordovician through the Devonian Period. Most ostracoderms had their heads or entire bodies encased in fused bony plates. Some or all ostracoderms are sometimes classified together with the modern cyclostomes (lampreys and hagfish).
an archaic and informal term for a member of the group of armoured, jawless, fishlike vertebrates that emerged during the early part of the Paleozoic Era (542-251 million years ago). Ostracoderms include both extinct groups, such as the heterostracans and osteostracans, and living forms, such as hagfishes and lampreys.