Funny Fish Face

Evolution
Fish do not represent a monophyletic group, and therefore the "evolution of fish" is not studied as a single event.
Proliferation of fish was apparently due to the hinged jaw, because jawless fish left very few descendants. Lampreys may approximate 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, improved respiration, or a combination of factors.