Placoderms are characterized by their armour which is made of overlapping bony plates forming a head shield and a trunk shield. These mostly articulate by a knob and groove or may be fused to one composite shield. Each of the seven groups of placoderms has a unique pattern of bony plates and each plate has a specialized shape, overlap regions, surface texture (ornament) and possibly sensory-line grooves. Primitive placoderms have wart-like ornament (tubercles), advanced forms may have linear ridges and network patters.

Bothriolepis canadensis (ventral surface), from Escuminac  Bay, Quebec; courtesy of the Canadian Museum of Nature.  Photo by Annetta Markussen-Brown