PLACODERM

any member of an extinct class (Placodermi) of primitive fishes known only from fossil remains. Placoderms existed throughout the Devonian period (408,000,000360,000,000 years ago), but only two species persisted into the succeeding Carboniferous period. During the Devonian they were a dominant group, occurring in all continents except South America in a variety of marine and freshwater sediments. Most placoderms were small or moderate in size, but a few may have reached a length of 13 feet (4 metres). The name is derived from their characteristic armour of dermal, or skin, bones. This armour formed a head shield and a trunk shield, the two commonly connected by a paired joint in the neck region. The arrangement of bones is so different from that of modern fishes with bony skeletons that it is unlikely that the bones of the two groups are homologous (similar in origin). The earliest placoderms were heavily armoured, bottom-dwelling (benthic) fishes; they were poor swimmers. Many later forms became highly specialized for this way of life. Others became adapted for fast swimming between the surface and the bottom (i.e., were nektonic). The sluggish benthic placoderms had small, ventrally placed mouths and presumably fed on bottom detritus and small invertebrates. Fossil remains indicate that some species had heavy, blunt jaw plates adapted for crushing hard-shelled invertebrates, while others were able to open their jaws wide enough to swallow smaller fish. The origin of the placoderms is unknown, although it is possible that they may have shared a common ancestor with the sharks, skates, and rays. Some authorities speculate that one order (Ptyctodontida) may have given rise to the modern class Holocephali, of which the chimaeras are the only living representatives.