ostracoderm(n = noun.animal) - extinct fish-like jawless vertebrate having a heavily armored body; of the Paleozoic;
is a kind of agnathan, jawless fish, jawless vertebrate
is a member of order ostracodermi, ostracodermi

   Any of several extinct fishlike jawless vertebrates having a heavily armored body; of the Paleozoic. [WordNet 1.5]