The placoderms appear in the fossil record during the Silurian, but the apex predator of the ocean that we call Dunkleosteus began to appear in the Late Devonian, near the decline and extinction of all of the placoderms. Lasting about 50 million years, the placoderms were fairly successful while they were around (not as successful as the 400 million year old lineage of sharks of course). The height of the evolution of jaw mechanics of placoderms is very evident in the articulated specimens of Dunkleosteus that have been discovered. The strength needed to open and close the jaw of the heavily armored skull alone was probably quite great. Add into the equation the amount of space available for additional muscle attachment, or simply strengthening the fewest required muscles, and the strength available for closing the massive jaws in a quick decisive snap is very impressive. Capable of up to 7400N of force along the "toothed" edge, the snap of that fish jaw could destroy most other fish in its sea and most likely many small man-made boats currently in the water these days. Part of the anatomy that helps this mechanism achieve the wide range of motion prior to the strong snapping of the jaws is the nuchal gap between the proper skull and thoracic shields.