Placoderms (meaning "plated skin") were armored fish that evolved during the Silurian Period, about 420 million years ago. They diviersified and came to dominate the seas by the Devonian. They went extinct bout 355 million years ago. They had hinged bony armor on their head and thorax. They had no teeth, but did have bony reidges that acted like teeth. Some placoderms included Antiarchi, Bothriolepis, Dunkleosteus (the largest placoderm), Groenlandapsis, and Phyllolepis. Placoderms were early fish, but not sharks.