Evolution of fish

The evolution of fish began 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.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:Hyperoartia (lampreys)?†Euconodonta (eel like animals)†Pteraspidomorphi (jawless fishes)?†Thelodonti (jawless fishes with scales)?†Anaspida (jawless ancestors of lampreys)†Galeaspida (jawless fishes with bone head shields)?†Pituriaspida (armoured jawless fishes with large rostrums)†Osteostraci (bony armoured jawless fish with bone head shields)Jawed vertebrates → continued in section below†Placodermi (armoured fishes)Acanthodians and Chondrichthyes (cartilaginous fishes)Actinopterygii (ray-finned fishes) <dominant class of fish today ?†Onychodontiformes (lobe-finned)Actinistia (coelacanths)†Porolepiformes (lobe-finned)Dipnoi (lungfishes)†Rhizodontimorpha (predatory lobe-finned)†Tristichopteridae (tetrapodomorphs)Tetrapods (four-legged animals)Pikaia, along with Myllokunmingia and Haikouichthys ercaicunensis immediately below, are all candidates in the fossil record for the titles of 'first vertebrate' and 'first fish'. Pikaia is a genus that appeared about 530 Ma during the Cambrian explosion of multicellular life. Pikaia gracilens (pictured) is a transitional fossil between invertebrates and vertebrates, and may be the earliest known chordate. In this sense it may have been the original ancestor of fishes. It was a primitive creature with no evidence of eyes, without a well defined head, and less than 2 inches (5 centimetres) long. Pikaia was a sideways-flattened, leaf-shaped animal that swam by throwing its body into a series of S-shaped, zig-zag curves, similar to movement of snakes. Fish inherited the same swimming movement, but they generally have stiffer backbones. It had a pair of large head tentacles and a series of short appendages, which may be linked to gill slits, on either side of its head. Pikaia shows the essential prerequisites for vertebrates. The flattened body is divided into pairs of segmented muscle blocks, seen as faint vertical lines. The muscles lie on either side of a flexible structure resembling a rod that runs from the tip of the head to the tip of the tail.Bothriolepis (pitted scale) was the most successful genus of antiarch placoderms, if not the most successful genus of any placoderm, with over 100 species spread across Middle to Late Devonian strata across every continent.Dunkleosteus is a genus of arthrodire placoderms that existed from 380 to 360 Ma. It grew up to 10 metres (33 ft) long and weighed up to 3.6 tonnes. It was a hypercarnivorous apex predator. Apart from its contemporary Titanichthys (below), no other placoderm rivalled it in size. Instead of teeth, Dunkleosteus had two pairs of sharp bony plates, which formed a beak-like structure. Apart from megalodon, it had the most powerful bite of any fish, generating bite forces in the same league as Tyrannosaurus rex and the modern crocodile.Materpiscis (mother fish) is a genus of ptyctodontid placoderm from about 380 Ma. Known from only one specimen, it is unique in having an unborn embryo present inside, and with remarkable preservation of a mineralised placental feeding structure (umbilical cord). This makes Materpiscis the first known vertebrate to show viviparity, or giving birth to live young. The specimen was named Materpiscis attenboroughi in honour of David Attenborough.Genus of extinct lobe-finned fishes that has attained an iconic status from its close relationships to tetrapods. Early depictions of this animal show it emerging onto land, however paleontologists now widely agree that it was a strictly aquatic animal. The genus Eusthenopteron is known from several species that lived during the Late Devonian period, about 385 Ma. It was the object of intense study from the 1940s to the 1990s by the paleoichthyologist Erik Jarvik.Until finds of other early tetrapods and closely related fishes in the late 20th century, Ichthyostega stood alone as the transitional fossil between fish and tetrapods, combining a fishlike tail and gills with an amphibian skull and limbs. It possessed lungs and limbs with seven digits that helped it navigate through shallow water in swamps.The Carboniferous seas were inhabited by many fish, mainly Elasmobranchs (sharks and their relatives). These included some, like Psammodus, with crushing pavement-like teeth adapted for grinding the shells of brachiopods, crustaceans, and other marine organisms. Other sharks had piercing teeth, such as the Symmoriida; some, the petalodonts, had peculiar cycloid cutting teeth. Most of the sharks were marine, but the Xenacanthida invaded fresh waters of the coal swamps. Among the bony fish, the Palaeonisciformes found in coastal waters also appear to have migrated to rivers. Sarcopterygian fish were also prominent, and one group, the Rhizodonts, reached very large size.As a result of the evolutionary radiation, carboniferous sharks assumed a wide variety of bizarre shapes—including sharks of the family Stethacanthidae, which possessed a flat brush-like dorsal fin with a patch of denticles on its top. Stethacanthus' unusual fin may have been used in mating rituals. Apart from the fins, Stethacanthidae resembled Falcatus (below).Pachycormiformes are an extinct order of ray-finned fish that existed from the Middle Triassic to the K-Pg extinction (below). They were characterized by serrated pectoral fins, reduced pelvic fins and a bony rostrum. Their relations with other fish are unclear.The family Ichthyodectidae (literally 'fish-biters') was a family of marine actinopterygian fish. They first appeared 156 Ma during the Late Jurassic and disappeared during the K-Pg extinction event 66 Ma. They were most diverse throughout the Cretaceous period. Sometimes classified in the primitive bony fish order Pachycormiformes, they are today generally regarded as members of the 'bulldog fish' order Ichthyodectiformes in the far more advanced Osteoglossomorpha. Most ichthyodectids ranged between 1 and 5 meters (3.5 and 16.5 ft) in length. All known taxa were predators, feeding on smaller fish; in several cases, larger Ichthyodectidae preyed on smaller members of the family. Some species had remarkably large teeth, though others, such as Gillicus arcuatus, had small ones and sucked in their prey. The largest Xiphactinus was 20 feet long, and appeared in the Late Cretaceous (below).Cretoxyrhina mantelli was a large shark that lived about 100 to 82 million years ago, during the mid Cretaceous period. It is commonly known as the Ginsu Shark.This shark was first identified by a famous Swiss Naturalist, Louis Agassiz in 1843, as Cretoxyhrina mantelli. However, the most complete specimen of this shark was discovered in 1890, by the fossil hunter Charles H. Sternberg, who published his findings in 1907. The specimen consisted of a nearly complete associated vertebral column and over 250 associated teeth. This kind of exceptional preservation of fossil sharks is rare because a shark's skeleton is made of cartilage, which is not prone to fossilization. Charles dubbed the specimen Oxyrhina mantelli. This specimen represented a 20-foot-long (6.1 m) shark.Enchodus is an extinct genus of bony fish. It flourished during the Upper Cretaceous and was small to medium in size. One of the genus' most notable attributes are the large 'fangs' at the front of the upper and lower jaws and on the palatine bones, leading to its misleading nickname among fossil hunters and paleoichthyologists, 'the saber-toothed herring'. These fangs, along with a long sleek body and large eyes, suggest Enchodus was a predatory species.Xiphactinus is an extinct genus of large predatory marine bony fish of the Late Cretaceous. They grew more than 4.5 metres (15 feet) long.Megalodon is an extinct species of shark that lived about 28 to 1.5 Ma. It looked much like a stocky version of the great white shark, but was much larger with fossil lengths reaching 20.3 metres (67 ft). Found in all oceans it was one of the largest and most powerful predators in vertebrate history, and probably had a profound impact on marine life.Jawless fishes The evolution of fish began 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 (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 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 do not represent a monophyletic group, but a paraphyletic one, as they exclude the tetrapods. Fish, like many other organisms, have been greatly affected by extinction events throughout natural history. 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. Fish may have evolved from an animal similar to a coral-like sea squirt (a tunicate), 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, among them the first fishes, originated about 530 million years ago during the Cambrian explosion, which saw the rise in organism diversity. The first ancestors of fish, or animals that were probably closely related to fish, were Pikaia, Haikouichthys and Myllokunmingia. These three genera all appeared around 530 Ma. Pikaia had a primitive notochord, a structure that could have developed into a vertebral column later. 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 in the common sense and relied on filter feeding close to the seabed. These were followed by indisputable fossil vertebrates in the form of heavily armoured fishes discovered in rocks from the Ordovician Period 500–430 Ma. 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 fishes, the actinopterygii and sarcopterygii, evolved and became common. The Devonian also saw the demise of virtually all jawless fishes, save for lampreys and hagfish, as well as the Placodermi, a group of armoured fish that dominated much of the late Silurian. The Devonian also saw the rise of the first labyrinthodonts, which was a transitional between fishes and amphibians.