Placoderms had heavy armour that covered their head and thorax. The rest of the body was covered with small bony scales or was without dental armour. Primitive bodies were covered with thick bony ornamented platelets. On the more advanced bodies, the body scales were reduced or absent, corresponding with the overall trend seen throughout the evolution of these fish to reduce body weight. The head and trunk shields of most placoderms were articulated by bony joints. This joint allowed the forward part of the skull to tilt up, increasing the gap and allowing for more mobility. Placoderms lacked teeth but biting or grinding structures are often found in the dermal bones lining their mouths. They had one dorsal fin, a pair of pectoral fins and a pair of pelvic fins with shark-like shape and internal structure, many species also had a single anal fin. These strange looking fish evolved into a variety of body forms in a relatively short time. Many were torpedo-shaped except for the flattened Phyllolepidia and the bottom-feeding Antiarch.

John A. Long, 1995