an archaic and informal term for a member of the group of armoured, jawless, fishlike (fish) vertebrates (vertebrate) that emerged during the early part of the Paleozoic Era (542–251 million years ago). Ostracoderms include both extinct (extinction) groups, such as the heterostracans and osteostracans, and living forms, such as hagfishes (hagfish) and lampreys (lamprey).