Meet Dunkleosteus. A late Devonian placoderm that roamed the open oceans and ruled its domain 400 million years ago. 

Imagine pulling this up on your fishing line. Its teeth were actually part of its skull and it did not have jaws.  Sharks, our cartilaginous friends, showing up some twenty million years later were among the first to introduce that rather important feature.  

Here are the jaws of Carcharadon Megalodon. It lived ten million years ago, was the largest shark that we know of and is thought to have reached lengths of sixty feet or more.