Evolution of fish

Source: Wikipedia, the free encyclopedia.

placoderms and various lobe-finned fish, including the tetrapod transitional species

The evolution of fish began about 530 million years ago during the

Cyclostomata, which includes the extant hagfish
, and this group may have split early on from other agnathans.

The earliest

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, like many other organisms, have been greatly affected by

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

Vertebrate classes
extant vertebrate classes are:[2]
Fish:
Tetrapods:
In addition to these are two classes of extinct jawed fish, the armoured
placoderms and the spiny sharks
.
Further information: Vertebrate paleontology

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, in other words the first fish, originated about 530 million years ago during the Cambrian explosion, which saw the rise in animal diversity.[3]

The lancelet, a small, translucent, fish-like animal, is the closest living invertebrate relative of the olfactoreans (vertebrates and tunicates).[4][5]

The first ancestors of fish, or animals that were probably closely related to fish, were

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.[7] All of these early vertebrates lacked jaws in the common sense and relied on filter feeding close to the seabed.[8]

These were followed by indisputable fossil vertebrates in the form of heavily armoured fish discovered in rocks from the

Mya
).

The first

amphibians.[10]

The colonisation of new niches 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, which could grow 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

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

A modern jawless fish, the lamprey, attached to a modern jawed fish
Lamprey mouth
Main article: Agnatha

Jawless fish belong to the

Haikouella
.

Many

tetrapods (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 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.[12] Agnathans declined in the Devonian
and never recovered.

The agnathans as a whole 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. 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
.

Evolution of jawless fish. The diagram is based on Michael Benton, 2005.[19]

The cladogram for jawless fish is based on studies by Philippe Janvier and others for the Tree of Life Web Project.[20] (†=group is extinct)


Jawless fish

Hyperoartia (lampreys)

?†

Euconodonta
(eel-like animals)

unnamed

†Pteraspidomorphi (jawless fish)

?†Thelodonti (jawless fish with scales)

unnamed

?†Anaspida

unnamed

†Galeaspida (jawless fish with bone head shields)

unnamed

?†Pituriaspida (armoured jawless fish with large rostrums)

†Osteostraci (bony armoured jawless fish with heads covered with a bony shield)

Jawed vertebrates → continued in section below

†Conodonts

Conodonts
(extinct) resembled primitive jawless eels

derived than either of these groups.[25]

†Ostracoderms

†Ostracoderms (extinct) were armoured jawless fish

Ostracoderms (lit. 'shell-skinned') 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.[26] 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

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".[27]

Ostracoderms existed in two major groups, the more primitive

Devonian period.[28]

Jawed fish

Placoderm
Further information:
Evolution of jaws

The vertebrate jaw probably originally evolved in the

Placoderm fish, which further diversified in the Devonian. The two most anterior pharyngeal arches are thought to have become the jaw itself and the hyoid arch, respectively. The hyoid system suspends the jaw from the braincase of the skull, permitting great mobility of the jaws. 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.

As in most

teeth. The skull of the last common ancestor of today's jawed vertebrates is assumed to have resembled sharks.[29]

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

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)

Jawed vertebrates

†

Placodermi (armoured fish)

unnamed

Acanthodians and Chondrichthyes (cartilaginous fish)

Bony fish

Actinopterygii (ray-finned fish), dominant class of fish today

Lobe-finned fish

?†Onychodontiformes (lobe-finned)

Actinistia (coelacanths)

unnamed

†Porolepiformes (lobe-finned)

Dipnoi (lungfish)

unnamed

†

Rhizodontimorpha (predatory lobe-finned)

†

tetrapodomorphs)

Tetrapods (four-legged animals)


†Placoderms

Further information:
List of placoderms
placoderms. The diagram is based on Michael Benton, 2005.[19]
Placoderms
(extinct) were armoured jawed fish (compare with the ostracoderms above)

Placoderms,

Petalichthyidae, Ptyctodontida and Arthrodira
.

†Spiny sharks

Main article: Acanthodii
†Spiny sharks (extinct) were the earliest known jawed fish. They resembled sharks and were ancestral to them.

Spiny sharks, class Acanthodii, are extinct fish that share features with both bony and cartilaginous fish, though ultimately more closely related to and ancestral to the latter. Despite being called "spiny sharks", acanthodians predate sharks, though they gave rise to them. They evolved in the sea at the beginning of the Silurian period, some 50 million years before the first sharks appeared. 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

Further information: List of prehistoric cartilaginous fish genera
Radiation of cartilaginous fish, derived from work by Michael Benton, 2005.[32]

Cartilaginous fish, class

Xenacanthiformes, Ctenacanthiformes, Hybodontiformes, Galeomorphii, Squaliformes and Batoidea
.

Bony fish

Further information: Osteichthyes and List of prehistoric bony fish genera

Bony fish, class Osteichthyes, are characterised by bony skeleton rather than cartilage. They appeared in the late Silurian, about 419 million years ago. The recent discovery of Entelognathus strongly suggests that bony fish (and possibly cartilaginous fish, via acanthodians) evolved from early placoderms.[33] A subclass of the Osteichthyes, the ray-finned fish (Actinopterygii), have become the dominant group of fish in the post-Paleozoic and modern world, with some 30,000 living species. The bony (and cartilaginous) fish groups that emerged after the Devonian were characterised by steady improvements in foraging and locomotion.[34]

Lobe-finned fish

Further information: Sarcopterygii
Queensland lungfish The lungfish is a lobe-finned fish loosely described as a living fossil. Lungfish evolved the first proto-lungs and proto-limbs. They developed the ability to live outside a water environment in the Middle Devonian (397–385 Mya), and have remained virtually the same for over 100 million years.[35]
Phylogenomic analysis has shown that "the closest living fish to the tetrapod ancestor is the lungfish, not the coelacanth".[36]

Lobe-finned fish, fish belonging to the class

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 enamel
.

Lobe-finned fish, such as

freshwater
habitats.

The coelacanth is another lobe-finned fish, loosely known as a "living fossil". The coelacanth body plan evolved roughly 408 million years ago, during the Early Devonian;[39] the two modern species have much the same shape.[40]

In the Early Devonian (416-397 Mya), the lobe-finned fish split into two main lineages — the

Permian-Triassic extinction event
(251 Mya).

Ray-finned fish

Ray-finned fish, class

sturgeons and paddlefish.[41]

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.[41]

Timeline

Main article: Timeline of fish evolution

See also

References

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Sources

Further reading

External videos
video icon Feeding Mechanism of Conodonts – YouTube
video icon Chordate evolution – YouTube

External links

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