Dunkleosteus hunting Cladoselache sharks, illustration. Dunkleosteus measured up to 10 metres long and was the largest animal in the Devonian seas. Massive bony plates protected its head, and a hinge at the back of its skull allowed the head to rise and the jaws to lower, giving it an enormously powerful bite. What appear to be teeth are actually sharp, bony plates at the front of its jaw. It existed from 382 to 358 million years ago. Cladoselache sharks existed during the Late Devonian period (roughly 370 million years ago), and measured up to 2 metres in length.