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Fish began evolving about 530 million years ago during the Cambrian explosion. It was during this time that the early developed the skull and the vertebral column, leading to the first and . The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the , and small mostly armoured fish known as , first appeared. Most jawless fish are now extinct; but the extant may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans.
The earliest Gnathostomata probably developed during the late Ordovician period. They are first represented in the fossil record from the Silurian by two groups of fish: the armoured fish known as Placodermi, which evolved from the ostracoderms; and the Acanthodii (or spiny sharks). The jawed fish that are still extant in modern days also appeared during the late Silurian: the Chondrichthyes (or cartilaginous fish) and the Osteichthyes (or bony fish). The bony fish evolved into two separate groups: the Actinopterygii (or ray-finned fish) and Sarcopterygii (which includes the lobe-finned fish).
During the Devonian period a great increase in fish variety occurred, especially among the ostracoderms and placoderms, and also among the lobe-finned fish and early sharks. This has led to the Devonian being known as the age of fishes. It was from the lobe-finned fish that the evolved, the four-limbed vertebrates, represented today by , , , and . Tetrapodomorpha first appeared during the early Devonian, and by the late Devonian the first tetrapods appeared. The diversity of jawed vertebrates may indicate the evolutionary advantage of a jawed mouth; but it is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of factors.
Fish, like many other organisms, have been greatly affected by throughout natural history. The earliest ones, the Ordovician–Silurian extinction events, led to the loss of many species. The Late Devonian extinction led to the extinction of the ostracoderms and placoderms by the end of the Devonian, as well as other fish. The spiny sharks became extinct at the Permian–Triassic extinction event; the conodonts became extinct at the Triassic–Jurassic extinction event. The Cretaceous–Paleogene extinction event, and the present day Holocene extinction, have also affected fish variety and Fish stocks.
, in other words the first fish, originated about 530 million years ago during the Cambrian explosion, which saw the rise in animal diversity.
The first ancestors of fish, or animals that were probably closely related to fish, were Haikouichthys and Myllokunmingia. These two genera all appeared around 530 Mya. 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. All of these early vertebrates lacked , relying instead on filter-feeding close to the seabed.
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 first Gnathostomata appeared in the late Ordovician and became common in the Devonian, often known as the "Age of Fishes". The two groups of Osteichthyes, the Actinopterygii and Sarcopterygii, evolved and became common. The Devonian saw the demise of virtually all jawless fish, save for lampreys and hagfish, as well as the Placodermi, a group of armoured fish that dominated much of the late Silurian, and the rise of the first Labyrinthodontia, transitional between fish and amphibians.
The colonisation of new Ecological niche resulted in diversification of body plans and sometimes an increase in size. The Devonian period (395 to 345 Mya) brought in such giants as the placoderm Dunkleosteus up to seven meters long, and early air-breathing fish that could remain on land for extended periods. Among this latter group were ancestral .
The reptiles appeared from labyrinthodonts in the subsequent Carboniferous period. The anapsid and synapsid amniotas were common during the late Paleozoic, while the became dominant during the Mesozoic. In the sea, the Osteichthyes became dominant.
The later radiations, such as those of fish in the Silurian and Devonian periods, involved fewer taxa, mainly with very similar body plans. The first animals to venture onto dry land were . Some fish had lungs and strong, bony fins and could crawl onto the land also.
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 Ostracodermi, precursors to the Osteichthyes and hence to the Tetrapoda (including humans)—are known from the Middle Ordovician, and by the Late Silurian the agnathans had reached the high point of their evolution. Most of the ostracoderms, such as Thelodonti, Osteostraci and Galeaspida, were more closely related to the gnathostomes than to the surviving agnathans, known as cyclostomes. Cyclostomes apparently split from other agnathans before the evolution of dentine and bone, which are present in many fossil agnathans, including . Agnathans declined in the Devonian and never recovered.
The agnathans as a whole are Paraphyly, because most extinct agnathans belong to the stem group of the gnathostomes, the jawed fish that evolved from them. Molecular data, both from rRNA and from mtDNA strongly supports the theory that living agnathans, known as cyclostomata, are monophyletic.Philippe Janvier 2010. "MicroRNAs revive old views about jawless vertebrate divergence and evolution." Proceedings of the National Academy of Sciences (USA) 107:19137-19138. [1] " Although I was among the early supporters of vertebrate paraphyly, I am impressed by the evidence provided by Heimberg et al. and prepared to admit that cyclostomes are, in fact, monophyletic. The consequence is that they may tell us little, if anything, about the dawn of vertebrate evolution, except that the intuitions of 19th century zoologists were correct in assuming that these odd vertebrates (notably, hagfishes) are strongly degenerate and have lost many characters over time." In Phylogenetics, the relationships between animals are not typically divided into ranks, but illustrated as a nested "family tree" known as a cladogram. Phylogenetic groups are given definitions based on their relationship to one another, rather than purely on physical traits such as the presence of a backbone. This nesting pattern is often combined with traditional taxonomy, in a practice known as evolutionary taxonomy.
The cladogram for jawless fish is based on studies by Philippe Janvier and others for the Tree of Life Web Project.Janvier, Philippe (1997) Vertebrata. Animals with backbones . Version 01 January 1997 in The Tree of Life Web Project (†=group is extinct)
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. The eyes were particularly shielded. Earlier used their for both respiration and feeding, whereas ostracoderms used their gills for respiration only. They had up to eight separate pharyngeal gill pouches along the side of the head, which were permanently open with no protective operculum. Unlike that use Cilium motion to move food, ostracoderms used their muscular pharynx to create a suction that pulled small and slow-moving prey into their mouths.
The first fossil fish that were discovered were ostracoderms. The Switzerland anatomist Louis Agassiz received some fossils of bony armored fish from Scotland in the 1830s. He had a hard time classifying them as they did not resemble any living creature. He compared them at first with extant armored fish such as catfish and but later, realizing that they had no movable jaws, classified them in 1844 into a new group "ostracoderms".
Ostracoderms existed in two major groups, the more primitive Heterostraci and the cephalaspids. Later, about 420 million years ago, the jawed fish evolved from one of the ostracoderms. After the appearance of jawed fish, most ostracoderm species underwent a decline, and the last ostracoderms became extinct at the end of the Devonian period. Vertebrate jaw design locked down early
As in most , fish jaws are bone or cartilage and oppose vertically, comprising an upper jaw and a lower jaw. The jaw is derived from the most anterior two supporting the gills, and usually bears numerous teeth. The skull of the last common ancestor of today's jawed vertebrates is assumed to have resembled sharks.
It is thought that the original selective advantages offered by the jaw were not related to feeding, but to increases in respiration efficiency. The jaws were used in the buccal pump (observable in modern fish and amphibians) that pumps water across the gills of fish or air into the lungs in the case of amphibians. Over evolutionary time the more familiar use of jaws (to humans) in feeding was selected for and became a very important function in vertebrates. Many Teleostei fish have substantially modified their jaws for suction feeding and jaw protrusion, resulting in highly complex jaws with dozens of bones involved.
Jawed vertebrates and jawed fish evolved from earlier jawless fish. The cladogram for jawed vertebrates is a continuation of the cladogram in the section above. (†=extinct)
Lobe-finned fish, fish belonging to the class Sarcopterygii, are mostly extinct bony fish, basally characterised by robust and stubby lobe fins containing a robust internal skeleton, and internal nostrils. Their fins are fleshy, lobed, and paired, joined to the body by a single bone. The fins of lobe-finned fish differ from those of all other fish in that each is borne on a fleshy, lobelike, scaly stalk extending from the body. The Pectoral fin and are articulated in ways resembling the tetrapod limbs they were the precursors to. The fins evolved into the legs of the first tetrapod land vertebrates, . They also possess two dorsal fins with separate bases, as opposed to the single dorsal fin of ray-finned fish. The braincase of lobe-finned fish primitively has a hinge line, but this is lost in tetrapods and lungfish. Many early lobe-finned fish have a symmetrical tail. All lobe-finned fish possess teeth covered with true tooth enamel.
Lobe-finned fish, such as and lungfish, were the most diverse group of bony fish in the Devonian. Taxonomists who subscribe to the cladistic approach include the grouping Tetrapoda within the Sarcopterygii, and the tetrapods in turn include all species of four-limbed vertebrates. The fin-limbs of lobe-finned fish such as the coelacanths show a strong similarity to the expected ancestral form of tetrapod limbs. The lobe-finned fish apparently followed two different lines of development and are accordingly separated into two subclasses, the Rhipidistia (including the lungfish, and the Tetrapodomorpha, which include the Tetrapoda) and the Actinistia (coelacanths). The first lobe-finned fish, found in the uppermost Silurian (c. 418 Mya), closely resembled , which became extinct at the end of the Paleozoic. In the Early to Middle Devonian (416–385 Mya), while the predatory Placodermi dominated the seas, some lobe-finned fish came into freshwater habitats.
In the Early Devonian (416-397 Mya), the lobe-finned fish split into two main lineages — the and the . The heyday of the former was the Late Devonian and Carboniferous, from 385 to 299 Mya, as they were more common during those periods than in any other period in the Phanerozoic; coelacanths still live today in the (genus Latimeria). The Rhipidistians, whose ancestors probably lived in Estuary, migrated into freshwater habitats. They in turn split into two major groups: the lungfish and the . The lungfish's greatest diversity was in the Triassic period; today there are three genera left. The lungfish evolved the first proto-lungs and proto-limbs, developing the ability to live outside a water environment in the Middle Devonian (397–385 Mya). The first tetrapodomorphs, which included the gigantic rhizodonts, had the same general anatomy as the lungfish, who were their closest kin, but they appear not to have left their water habitat until the Late Devonian epoch (385–359 Mya), with the appearance of Tetrapoda (four-legged vertebrates). Lobe-finned fish continued until towards the end of Paleozoic era, suffering heavy losses during the Permian-Triassic extinction event (251 Mya).
Ray-finned fish are a dominant vertebrate group, containing half of all known vertebrate species. They inhabit abyssal depths in the sea, coastal inlets and freshwater rivers and lakes, and are a major source of food for humans.
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