Dunkleosteus prehistoric fish hunting a Cladoselache shark, artwork. These two species lived in the Devonian period (408-360 million years ago). Dunkleosteus was a large predatory fish that had plates of sharp bone within its jaws instead of teeth. It was a placoderm fish, and had a heavily-armoured head and a scaleless body. Some species of Dunkleosteus are thought to have reached lengths approaching ten metres. Cladoselache was a genus of early shark that was very common during the Devonian period. It had a long streamlined body and deeply forked tail that made it a fast swimmer.