Picture yourself submerged in an ancient sea, 400 million years ago. Above, a large shape appears, gliding powerfully through the water. Its head and trunk are covered with bony plates of armor, and the great, jagged jaws are like a primitive slicing machine, roughly made but clearly effective.

This is Dunkleosteus, one of the first jawed vertebrates, and one of the largest of the armored fishes called placoderms. The fossil record indicates that this fish was an aggressive predator. Some placoderm fossils contain evidence of gouges and scrapes and cuts in the surface of the large bones, matching fairly well to the serrated edges of Dunkleosteus jaw bones; others show that puncture wounds go right through the bones.

The serrated, razor-sharp edges of bones in Dunkleosteus' jaws served as cutting edges. As they rubbed against each other, the opposing jaw blades were like self-sharpening shears. These bones grew continually, regenerating as they were worn down by usage.

Dunkleosteus was undoubtedly a powerful swimmer, much like today's large sharks. Its twenty-foot-long, muscular body ended in a shark-like tail. Its features are clearly those of a predator, and there were plenty of fish to feed on in the Devonian seas. Its prey may well have included primitive sharks, which did not achieve great size and diversity until Dunkleosteus and the other placoderms had disappeared from the oceans.