Evolution of fish
began evolving about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. 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 conodonts, and small mostly armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct; but the extant lampreys 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 jawed vertebrates 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 placoderms, which evolved from the ostracoderms; and the Acanthodii. The jawed fish that are still extant in modern days also appeared during the late Silurian: the Chondrichthyes and the Osteichthyes. The bony fish evolved into two separate groups: the Actinopterygii and Sarcopterygii.
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 tetrapods evolved, the four-limbed vertebrates, represented today by amphibians, reptiles, mammals, and birds. Transitional tetrapods 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 extinction events 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.
Overview
Fish may have evolved from an animal similar to a coral-like sea squirt, whose larvae resemble early fish in important ways. The first ancestors of fish may have kept the larval form into adulthood, as some sea squirts do today, although this path cannot be proven.Vertebrates, 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 jaws, 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.
The first jawed vertebrates appeared in the late Ordovician and became common in the Devonian, often known as the "Age of Fishes". The two groups of bony fish, 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 labyrinthodonts, transitional between fish and amphibians.
The colonisation of new niches resulted in diversification of body plans and sometimes an increase in size. The Devonian period 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 amphibians.
The reptiles appeared from labyrinthodonts in the subsequent Carboniferous period. The anapsid and synapsid amniotas were common during the late Paleozoic, while the diapsids became dominant during the Mesozoic. In the sea, the bony fish 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 arthropods. Some fish had lungs and strong, bony fins and could crawl onto the land also.
Jawless fish
Jawless fish belong to the superclass Agnatha in the phylum Chordata, subphylum Vertebrata. Agnatha means 'un-jawed, without jaws'. It excludes all vertebrates with jaws, known as gnathostomes. Although a minor element of modern marine fauna, jawless fish were prominent among the early fish in the early Paleozoic. Two types of Early Cambrian animal which apparently had fins, vertebrate musculature, and gills are known from the early Cambrian Maotianshan shales of China: Haikouichthys and Myllokunmingia. They have been tentatively assigned to Agnatha by Janvier. A third possible agnathan from the same region is Haikouella.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 —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 thelodonts, osteostracans and galeaspids, 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 conodonts. Agnathans declined in the Devonian and never recovered.
The agnathans as a whole are paraphyletic, 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 cyclostomes, are monophyletic. In phylogenetic taxonomy, 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.
File:Evolution of jawless fish.png|thumb|260px|right|Evolution of jawless fish. The diagram is based on Michael Benton, 2005.
The cladogram for jawless fish is based on studies by Philippe Janvier and others for the Tree of Life Web Project.
†Conodonts
s resembled primitive jawless eels. They appeared 520 Ma ago and were wiped out 200 Ma ago. Initially they were known only from tooth-like microfossils called conodont elements. These "teeth" have been variously interpreted as filter-feeding apparatuses or as a "grasping and crushing array". Conodonts ranged in length from a centimetre to the 40 cm Promissum. Their large eyes had a lateral position, which makes a predatory role unlikely. The preserved musculature hints that some conodonts were efficient cruisers but incapable of bursts of speed. In 2012 researchers classified the conodonts in the phylum Chordata on the basis of their fins with fin rays, chevron-shaped muscles and notochord. Some researchers see them as vertebrates similar in appearance to modern hagfish and lampreys, though phylogenetic analysis suggests that they are more derived than either of these groups.†Ostracoderms
s are armoured jawless fish of the Paleozoic. The term does not often appear in classifications today because the taxon is paraphyletic or polyphyletic, and has no phylogenetic meaning. However, the term is still used informally to group together the armoured jawless fish.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 chordates used their gills 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 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.
The first fossil fish that were discovered were ostracoderms. The Swiss 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 sturgeons 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 heterostracans 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.