Class of archaic, jawed (Gnathostomata), and heavily armoured fish, which appeared in the Devonian and which were virtually extinct by the end of that period. They were rather diverse in body form. They all possessed a head shield formed by bony plates. Pectoral and pelvic fins appear to have been present. Most of them (e.g. Coccosteus) were bottom-dwelling fish, with a depressed body terminating in a heterocercal tail.