Origin and Evolution
The early fossil record on fish is not very clear. It appears it was not a successful enough animal early in its evolution to leave many
fossils. However, this would eventually change over time as it became a dominant form of sea life and eventually branched
to create land vertebrates such as amphibians, reptiles, and mammals.
The proliferation was apparently due to the formation of the hinged jaw because jawless fish left very few descendants. Lampreys may be a rough representative of pre-jawed fish. The first jaws are found in Placodermi fossils. It is unclear if the advantage of a hinged jaw is greater biting force, respiratory-related, or a combination.
Some speculate that fish may have evolved from a creature similar to a coral-like Sea squirt, whose larvae resemble primitive fish in some key ways. The first ancestors of fish may have kept the larval form into adulthood (as some sea squirts do today), although maybe the reverse of this is case. Candidates for early fish include Agnatha such as Haikouichthys, Myllokunmingia, Pikaia, and Conodonts.