- any of several extinct jawless fishes of the Ordovician, Silurian, and Devonian periods, having the body enclosed in an armor of bony plates.
- any extinct Palaeozoic fishlike jawless vertebrate of the group Ostracodermi, characterized by a heavily armoured body
- Any of various 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).