It was from the lobe-finned fish that the tetrapods evolved, the four-limbed vertebrates, represented today by amphibians, reptiles, mammals, and birds.

Vertebrates, in other words the first fish, originated about 530 million years ago during the Cambrian explosion, which saw the rise in animal diversity.

Unlike the other fauna that dominated the Cambrian, these groups had the basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well-defined head and tail.

[8] These were followed by indisputable fossil vertebrates in the form of heavily armoured fish discovered in rocks from the Ordovician period (500–430 Mya).

The later radiations, such as those of fish in the Silurian and Devonian periods, involved fewer taxa, mainly with very similar body plans.

Many Ordovician, Silurian and Devonian agnathians were armoured with heavy, bony, and often elaborately sculpted, plates derived from mineralized scales.

The first armoured agnathans—the ostracoderms, precursors to the bony fish and hence to the tetrapods (including humans)—are known from the Middle Ordovician, and by the Late Silurian the agnathans had reached the high point of their evolution.

[14][15] Molecular data, both from rRNA[16] and from mtDNA[17] strongly supports the theory that living agnathans, known as cyclostomes, are monophyletic.

[18] In phylogenetic taxonomy, the relationships between animals are not typically divided into ranks, but illustrated as a nested "family tree" known as a cladogram.

The cladogram for jawless fish is based on studies by Philippe Janvier and others for the Tree of Life Web Project.

[23] Some researchers see them as vertebrates similar in appearance to modern hagfish and lampreys,[24] though phylogenetic analysis suggests that they are more derived than either of these groups.

The ostracoderm armour consisted of 3–5 mm polygonal plates that shielded the head and gills, and then overlapped further down the body like scales.

Unlike invertebrates that use ciliated motion to move food, ostracoderms used their muscular pharynx to create a suction that pulled small and slow-moving prey into their mouths.

He compared them at first with extant armored fish such as catfish and sturgeons but later, realizing that they had no movable jaws, classified them in 1844 into a new group "ostracoderms".

[28] The vertebrate jaw probably originally evolved in the Silurian period and appeared in the Placoderm fish, which further diversified in the Devonian.

Already long assumed to be a paraphyletic assemblage leading to more derived gnathostomes, the discovery of Entelognathus suggests that placoderms are directly ancestral to modern bony fish.

Placoderms, class Placodermi ('plate-skinned'), are extinct armoured prehistoric fish, which appeared about 430 Ma in the Early to Middle Silurian.

The chart on the right shows the rise and demise of the separate placoderm lineages: Acanthothoraci, Rhenanida, Antiarchi, Petalichthyidae, Ptyctodontida and Arthrodira.

Eventually competition from bony fish proved too much, and the spiny sharks died out in Permian times about 250 Ma.

In form they resembled sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteans (gars, bowfins).

Cartilaginous fish, class Chondrichthyes, consisting of sharks, rays and chimaeras, appeared by about 395 million years ago, in the Middle Devonian, evolving from acanthodians.

The radiation of elasmobranches in the chart on the right is divided into the following taxa: Cladoselache, Eugeneodontiformes, Symmoriida, Xenacanthiformes, Ctenacanthiformes, Hybodontiformes, Galeomorphii, Squaliformes and Batoidea.

The bony (and cartilaginous) fish groups that emerged after the Devonian were characterised by steady improvements in foraging and locomotion.

[38] The fin-limbs of lobe-finned fish such as the coelacanths show a strong similarity to the expected ancestral form of tetrapod limbs.

The first lobe-finned fish, found in the uppermost Silurian (c. 418 Mya), closely resembled spiny sharks, which became extinct at the end of the Paleozoic.

In the Early to Middle Devonian (416–385 Mya), while the predatory placoderms dominated the seas, some lobe-finned fish came into freshwater habitats.

[41] The lungfish evolved the first proto-lungs[citation needed] and proto-limbs, developing the ability to live outside a water environment in the Middle Devonian (397–385 Mya).

Lobe-finned fish continued until towards the end of Paleozoic era, suffering heavy losses during the Permian-Triassic extinction event (251 Mya).

They inhabit abyssal depths in the sea, coastal inlets and freshwater rivers and lakes, and are a major source of food for humans.