The
evolution of fish began about 530 million years ago during the
Cambrian explosion. Early
fish from the
fossil record are represented by a group of small, jawless, armoured fish known as
ostracoderms. Jawless fish lineages are mostly extinct. An extant
clade, the
lampreys may approximate ancient pre-jawed fish. The first jaws are found in
Placoderm fossils. The diversity of jawed vertebrates may indicate the evolutionary advantage of a jawed
mouth. It is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of factors. The evolution of fish is not studied as a single event since fish do not represent a
monophyletic group but a
paraphyletic one (by exclusion of the
tetrapods).