Ocean Life Diversifies
An increasing variety and abundance of marine life populated the warm seas. While sponges, corals, and brachiopods occupied the seafloor, trilobites, cephalopods (related to squid and octopi), and jawed fishes swam in the waters above. Despite this diversity only 20% of all known species on Earth today live in water.
The first fish were jawless and belonged to the “Agnatha” superclass of fish, with characteristically defined skulls, made of either bone or cartilage. An example of this type of fish is the Haikouichthys which is now extinct but lived during the Cambrian Explosion.
In Devonian time, from about 416 to 355 million years ago, fishes of many different types swam and hunted in the seas. Lobe-finned fishes — ancestors to the amphibians — and the early sharks made their appearance by this time.