Timeline of fish evolution

The evolution of fishes took place over a timeline which spans the Cambrian to the Cenozoic, including during that time in particular the Devonian, which has been dubbed the "age of fishes" for the many changes during that period.

The Late Devonian extinctions played a crucial role in shaping the evolution of fish, or vertebrates in general. Fishes evolved during the Early Paleozoic; by the Devonian, all modern groups (Agnatha, Chondrichthyes, and Osteichthyes) were already present. Devonian aquatic environments were also marked by placoderms and acanthodians, which are only known from fossils, however. After suffering large losses during the Late Devonian extinctions, cartilaginous fishes (Chondrichthyes) and the Actinopterygii among the bony fishes (Osteichthyes) diversified.

The sections below describe the pre-Devonian origin of fish, their Devonian radiation, including the conquest of land by early tetrapods, and the post-Devonian evolution of fishes.

The Devonian period is broken into the Early, Middle and Late stages. By the start of the Early Devonian 419 mya, jawed fishes had divided into four distinct clades: the placoderms and spiny sharks, both of which are now extinct, and the cartilaginous and bony fishes, both of which are still extant. The modern bony fish, class Osteichthyes, appeared in the late Silurian or early Devonian, about 416 million years ago. Both the cartilaginous and bony fish may have arisen from either the placoderms or the spiny sharks. A subclass of bony fish, the ray-finned fishes (Actinopterygii), have become the dominant group in the post-Paleozoic and modern world, with some 30,000 living species. Meanwhile, another subclass of bony fish, the lobe-finned fishes, became the dominant group on land.

Sea levels in the Devonian were generally high. Marine faunas were dominated by bryozoa, diverse and abundant brachiopods, the enigmatic hederelloids, microconchids and corals. Lily-like crinoids were abundant, and trilobites were still fairly common. Among vertebrates, jawless armoured fish (ostracoderms) declined in diversity, while the jawed fish (gnathostomes) simultaneously increased in both the sea and fresh water. Armoured placoderms were numerous during the lower stages of the Devonian Period but became extinct in the Late Devonian, perhaps because of competition for food against the other fish species. Early cartilaginous (Chondrichthyes) and bony fish (Osteichthyes) also become diverse and played a large role within the Devonian seas. The first abundant genus of shark, Cladoselache, appeared in the oceans during the Devonian Period. The great diversity of fish around at the time have led to the Devonian being given the name "The Age of Fish" in popular culture.

The first ray-finned and lobe-finned bony fish appeared in the Devonian, while the placoderms began dominating almost every known aquatic environment. However, another subclass of Osteichthyes, the Sarcopterygii( which includes lobe-finned fish like coelacanths and lungfish, as well as and tetrapods), was the most diverse group of bony fish in the Devonian. Sarcopterygians are basally characterized by internal nostrils, lobe fins containing a robust internal skeleton, and cosmoid scales.

During the Middle Devonian 393–383 Ma, the armoured jawless ostracoderm fish were declining in diversity; the jawed fish were thriving and increasing in diversity in both the oceans and freshwater. The shallow, warm, oxygen-depleted waters of Devonian inland lakes, surrounded by primitive plants, provided the environment necessary for certain early fish to develop essential characteristics such as well developed lungs and the ability to crawl out of the water and onto the land for short periods of time. Cartilaginous fish, class Chondrichthyes, consisting of sharks, rays and chimaeras, appeared by about 395 million years ago, in the middle Devonian

During the Late Devonian the first forests were taking shape on land. The first tetrapods appeared in the fossil record over a period, the beginning and end of which are marked with extinction events. This lasted until the end of the Devonian 359 mya. The ancestors of all tetrapods began adapting to walking on land, their strong pectoral and pelvic fins gradually evolved into legs (see Tiktaalik). In the oceans, primitive sharks became more numerous than in the Silurian and the late Ordovician. The first ammonite mollusks appeared. Trilobites, the mollusk-like brachiopods and the great coral reefs, were still common.

The Late Devonian extinction occurred at the beginning of the last phase of the Devonian period, the Famennian faunal stage, (the Frasnian-Famennian boundary), about 372.15 Ma. Many fossil agnathan fish, save for the psammosteid heterostracans, make their last appearance shortly before this event. The Late Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms. The most important group affected by this extinction event were the reef-builders of the great Devonian reef-systems.

A second extinction pulse, the Hangenberg event closed the Devonian period and had a dramatic impact on vertebrate faunas. Placoderms mostly became extinct during this event, as did most members of other groups including lobe-finned fish, acanthodians and early tetrapods in both marine and terrestrial habitats, leaving only a handful of survivors. This event has been related to glaciation in the temperate and polar zones as well as euxinia and anoxia in the seas.

The first tetrapods are four-legged, air-breathing, terrestrial animals from which the land vertebrates descended, including humans. They evolved from lobe-finned fish of the clade Sarcopterygii, appearing in coastal water in the middle Devonian, and giving rise to the first amphibians.

The group of lobe-finned fishes that were the ancestors of the tetrapod are grouped together as the Rhipidistia, and the first tetrapods evolved from these fish over the relatively short timespan 385–360 Ma. The early tetrapod groups themselves are grouped as Labyrinthodontia. They retained aquatic, fry-like tadpoles, a system still seen in modern amphibians. From the 1950s to the early 1980s it was thought that tetrapods evolved from fish that had already acquired the ability to crawl on land, possibly so they could go from a pool that was drying out to one that was deeper. However, in 1987, nearly complete fossils of Acanthostega from about 363 Ma showed that this Late Devonian transitional animal had legs and both lungs and gills, but could never have survived on land: its limbs and its wrist and ankle joints were too weak to bear its weight; its ribs were too short to prevent its lungs from being squeezed flat by its weight; its fish-like tail fin would have been damaged by dragging on the ground. The current hypothesis is that Acanthostega, which was about 1 metre (3.3 ft) long, was a wholly aquatic predator that hunted in shallow water. Its skeleton differed from that of most fish, in ways that enabled it to raise its head to breathe air while its body remained submerged, including: its jaws show modifications that would have enabled it to gulp air; the bones at the back of its skull are locked together, providing strong attachment points for muscles that raised its head; the head is not joined to the shoulder girdle and it has a distinct neck.

The Devonian proliferation of land plants may help to explain why air-breathing would have been an advantage: leaves falling into streams and rivers would have encouraged the growth of aquatic vegetation; this would have attracted grazing invertebrates and small fish that preyed on them; they would have been attractive prey but the environment was unsuitable for the big marine predatory fish; air-breathing would have been necessary because these waters would have been short of oxygen, since warm water holds less dissolved oxygen than cooler marine water and since the decomposition of vegetation would have used some of the oxygen.

There are three major hypotheses as to how tetrapods evolved their stubby fins (proto-limbs). The traditional explanation is the "shrinking waterhole hypothesis" or "desert hypothesis" posited by the American paleontologist Alfred Romer. He believed limbs and lungs may have evolved from the necessity of having to find new bodies of water as old waterholes dried up.

The second hypothesis is the "inter-tidal hypothesis" put forward in 2010 by a team of Polish paleontologists led by Grzegorz Niedźwiedzki. They argued that sarcopterygians may have first emerged unto land from intertidal zones rather than inland bodies of water. Their hypothesis is based on the discovery of the 395 million-year-old Zachełmie tracks in Zachełmie, Poland, the oldest ever discovered fossil evidence of tetrapods.

The third hypothesis, the "woodland hypothesis", was proposed by the American paleontologist Gregory J. Retallack in 2011. He argues that limbs may have developed in shallow bodies of water in woodlands as a means of navigating in environments filled with roots and vegetation. He based his conclusions on the evidence that transitional tetrapod fossils are consistently found in habitats that were formerly humid and wooded floodplains.

Research by Jennifer A. Clack and her colleagues showed that the very earliest tetrapods, animals similar to Acanthostega, were wholly aquatic and quite unsuited to life on land. This is in contrast to the earlier view that fish had first invaded the land — either in search of prey (like modern mudskippers) or to find water when the pond they lived in dried out — and later evolved legs, lungs, etc.

Two ideas about the homology of arms, hands and digits have existed in the past 130 years. First that digits are unique to tetrapods and second that antecedents were present in the fins of early sarcopterygian fish. Until recently it was believed that "genetic and fossil data support the hypothesis that digits are evolutionary novelties".^p. 640. However new research that created a three-dimensional reconstruction of Panderichthys, a coastal fish from the Devonian period 385 million years ago, shows that these animals already had many of the homologous bones present in the forelimbs of limbed vertebrates. For example, they had radial bones similar to rudimentary fingers but positioned in the arm-like base of their fins. Thus there was in the evolution of tetrapods a shift such that the outermost part of the fins were lost and eventually replaced by early digits. This change is consistent with additional evidence from the study of actinopterygians, sharks and lungfish that the digits of tetrapods arose from pre-existing distal radials present in more primitive fish. Controversy still exists since Tiktaalik, a vertebrate often considered the missing link between fishes and land-living animals, had stubby leg-like limbs that lacked the finger-like radial bones found in the Panderichthys. The researchers of the paper commented that it "is difficult to say whether this character distribution implies that Tiktaalik is autapomorphic, that Panderichthys and tetrapods are convergent, or that Panderichthys is closer to tetrapods than Tiktaalik. At any rate, it demonstrates that the fish–tetrapod transition was accompanied by significant character incongruence in functionally important structures.".^p. 638.

From the end of the Devonian to the Mid Carboniferous a 30 million year gap occurs in the fossil record. This gap, called Romer's gap, is marked by the absence of ancestral tetrapod fossils and fossils of other vertebrates that look well-adapted for life on land.

By the late Devonian, land plants had stabilized freshwater habitats, allowing the first wetland ecosystems to develop, with increasingly complex food webs that afforded new opportunities. Freshwater habitats were not the only places to find water filled with organic matter and choked with plants with dense vegetation near the water's edge. Swampy habitats like shallow wetlands, coastal lagoons and large brackish river deltas also existed at this time, and there is much to suggest that this is the kind of environment in which the tetrapods evolved. Early fossil tetrapods have been found in marine sediments, and because fossils of primitive tetrapods in general are found scattered all around the world, they must have spread by following the coastal lines — they could not have lived in freshwater only.

• Fossil Illuminates Evolution of Limbs from Fins Scientific American, 2 2 April 2004.

• During the Carboniferous period, fish diversity seemingly declined and reached low levels during the Permian period.
• The Mesozoic Era began about 252 million years ago in the wake of the Permian-Triassic event, the largest mass extinction in Earth's history, and ended about 66 million years ago with the Cretaceous–Paleogene extinction event, another mass extinction that killed off non-avian dinosaurs, as well as other plant and animal species. It is often referred to as the Age of Reptiles because reptiles were the dominant vertebrates of the time. The Mesozoic witnessed the gradual rifting of the supercontinent Pangaea into separate landmasses. The climate alternated between warming and cooling periods; overall the Earth was hotter than it is today. The impact of the end-Permian event on Bony fishes diversity remains elusive. It appears that there is no major diversity loss, but that can be the result of poor fossil record.

The Mesozoic saw the diversification of neopterygian fishes, the clade that consists of holostean and teleost fishes. Most of them were small in size. The diversity of body shape variety in Triassic, Jurassic, and Early Cretaceous neopterygian fishes has been documented, revealing that the accumulation of novel body shapes in teleost fishes was predominantly gradual throughout this 150 million year period (250Mya - 100Mya). Holostean fishes appear to accumulate body shape variety (so called disparity) between the early Triassic and Toarcian, after which the amount of variety seen among their body shapes remained stable until the end of the Early Cretaceous.

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