Evolution of fish
（重定向自Paleoichthyologist）
![Vertebrate classesSpindle diagram for the evolution of fish and other vertebrate classes. The diagram is based on Michael Benton, 2005.[2]
Conventional classification has living vertebrates as a subphylum grouped into eight classes based on traditional interpretations of gross anatomical and physiological traits. In turn, these classes are grouped into the vertebrates that have four limbs (the tetrapods) and those that do not: fishes. The extant vertebrate classes are:[3]
Fish:
jawless fishes (Agnatha)
cartilaginous fishes (Chondrichthyes)
ray-finned fishes (Actinopterygii)
lobe-finned fishes (Sarcopterygii)
Tetrapods:
amphibians (Amphibia)
reptiles (Reptilia)
birds (Aves)
mammals (Mammalia)
In addition to these are two classes of extinct jawed fishes, the armoured placoderms and the spiny sharks.](/uploads/202502/01/Fish_evolution2927.png)
![The lancelet, a small, translucent, fish-like animal, is the closest living invertebrate relative of the olfactoreans (vertebrates and tunicates).[5][6]The early vertebrate Haikouichthys, from about 518 million years ago in China, may be the](/uploads/202502/01/Amphioxus2927.png)
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).