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 branching to include land vertebrates such as amphibian, reptiles, and mammals.
The formation of the hinged jaw appears to be what resulted in the later proliferation of fish because un-jawed fish left very few ancestors. 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) known as neoteny, although
the reversal of this case is also possible. Candidates for early fish include Agnatha such as Haikouichthys, Myllokunmingia, Pikaia and Conodonts.
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