structural:reformatting
Fossil history

About 530 million years ago during the Cambrian explosion, fishlike animals with a notochord and eyes at the front of the body, such as Haikouichthys, appear in the fossil record. During the late Cambrian, other jawless forms such as conodonts appear.

Jawed vertebrates appear in the Silurian, with giant armoured placoderms such as Dunkleosteus. Jawed fish, too, appeared during the Silurian: the cartilaginous Chondrichthyes and the bony Osteichthyes.

During the Devonian, fish diversity greatly increased, including among the placoderms, lobe-finned fishes, and early sharks, earning the Devonian the epithet "the age of fishes".