The Devonian period, from 437 to 408 million years ago, was named for the English county where it was first identified. It has sometimes been called the Age of Fishes. Spectacular fish fossils abound in the massive Old Red Sandstone sediments that covered a large portion of Laurasia, the super-continent that would later split apart to form Europe, Greenland, and North America. These fossils indicate that a vast radiation (or divergence) in size and function was taking place among Devonian vertebrates. The jawless Agnathans had multiplied into many groups distributed around the world by the late Silurian (438 mya). Then, in the Devonian, came the fish, which developed jaws and were such successful competitors that the Agnathans were reduced almost to extinction, with only the lampreys and hagfish as their descendants.

Vast schools of eight-to-ten-inch spined fishes, the Acanthodians, swam in the mid-deep waters (beyond the continental shelf). Some were toothless...