Evolution of fish
Evolution of fish
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The Devonian period (419–359 Mya), also known as the Age of Fishes, saw the development of early sharks, armoured placoderms and various lobe-finned fish, including the tetrapod transitional species

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 (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 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

[edit]
Further information: Vertebrate paleontology
Vertebrate classes
Spindle diagram for the evolution of fish and other vertebrate classes. The diagram is based on Michael Benton, 2005.[1] 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: fish. The 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.

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 Haikouichthys and Myllokunmingia.[6][3] 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.[7] All of these early vertebrates lacked jaws, relying instead 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 Ordovician period (500–430 Mya).

The first jawed vertebrates appeared in the late Ordovician and became common in the Devonian, often known as the "Age of Fishes".[9] The two groups of bony fish, the Actinopterygii and Sarcopterygii, evolved and became common.[10] 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.[10]

Idealised vertebrate body plan, showing key characteristics

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 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

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

Jawless fish belong to the superclass Agnatha in the phylum Chordata, subphylum Vertebrata. Agnatha means 'un-jawed, without jaws' (from Ancient Greek).[11] 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 (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 paraphyletic,[13] because most extinct agnathans belong to the stem group of the gnathostomes, the jawed fish that evolved from them.[14][15] Molecular data, both from rRNA[16] and from mtDNA[17] strongly supports the theory that living agnathans, known as cyclostomes, 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

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†Conodonts (extinct) resembled primitive jawless eels

Conodonts resembled primitive jawless eels. They appeared 520 Ma ago and were wiped out 200 Ma ago.[21] 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".[22] 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 (Promissum at least) were efficient cruisers but incapable of bursts of speed.[22] In 2012 researchers classified the conodonts in the phylum Chordata on the basis of their fins with fin rays, chevron-shaped muscles and notochord.[23] Some researchers see them as vertebrates similar in appearance to modern hagfish and lampreys,[24] though phylogenetic analysis suggests that they are more derived than either of these groups.[25]

†Ostracoderms

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†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 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".[27]

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.[28]

Jawed fish

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Dunkleosteus, a Placoderm
Further information: Jawed fish and Evolution of jaws

The vertebrate jaw probably originally evolved in the Silurian period and appeared 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 vertebrates, fish jaws are bony or cartilaginous and oppose vertically, comprising an upper jaw and a lower jaw. The jaw is derived from the most anterior two pharyngeal arches 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.[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 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 teleost 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)

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)

†Tristichopteridae (tetrapodomorphs)

Tetrapods (four-legged animals)


†Placoderms

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

Placoderms, class Placodermi ('plate-skinned'), are extinct armoured prehistoric fish, which appeared about 430 Ma in the Early to Middle Silurian. They were mostly wiped out during the Late Devonian extinction event, 378 Ma, though some survived and made a slight recovery in diversity during the Famennian epoch before dying out entirely at the close of the Devonian, 360 mya; they are ultimately ancestral to modern gnathostome vertebrates.[30][31] Their head and thorax were covered with massive and often ornamented armoured plates. The rest of the body was scaled or naked, depending on the species. The armour shield was articulated, with the head armour hinged to the thoracic armour. This allowed placoderms to lift their heads, unlike ostracoderms. Placoderms were the first jawed fish; their jaws likely evolved from the first of their gill arches. The chart on the right shows the rise and demise of the separate placoderm lineages: Acanthothoraci, Rhenanida, Antiarchi, Petalichthyidae, Ptyctodontida and Arthrodira.

†Spiny sharks

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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

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

Cartilaginous fish, class Chondrichthyes, consisting of sharks, rays and chimaeras, appeared by about 395 million years ago, in the Middle Devonian, evolving from acanthodians. The class contains the subclasses Holocephali (chimaeras) and Elasmobranchii (sharks and rays). The radiation of elasmobranches in the chart on the right is divided into the following taxa: Cladoselache, Eugeneodontiformes, Symmoriida, Xenacanthiformes, Ctenacanthiformes, Hybodontiformes, Galeomorphi, Squaliformes and Batoidea.

Bony fish

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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

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Further information: Sarcopterygii
The 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 Sarcopterygii, are mostly extinct bony fish, basally characterised by robust and stubby lobe fins containing a robust internal skeleton, cosmoid scales and internal nostrils. Their fins are fleshy, lobed, and paired, joined to the body by a single bone.[37] 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 and pelvic fins 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, amphibians. 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 enamel.

Lobe-finned fish, such as coelacanths 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.[38] 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 spiny sharks, which became extinct at the end of the Paleozoic. In the Early to Middle Devonian (416–385 Mya), while the predatory placoderms dominated the seas, some lobe-finned fish came into 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 coelacanths and the rhipidistians. 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 oceans (genus Latimeria). The Rhipidistians, whose ancestors probably lived in estuaries, migrated into freshwater habitats. They in turn split into two major groups: the lungfish and the tetrapodomorphs. The lungfish's greatest diversity was in the Triassic period; today there are three genera left.[41] The lungfish evolved the first proto-lungs[citation needed] 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 tetrapods (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

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Ray-finned fish, class Actinopterygii, differ from lobe-finned fish in that their fins consist of webs of skin supported by spines ("rays") made of bone or horn. There are other differences in respiratory and circulatory structures. Ray-finned fish normally have skeletons made from true bone, though this is not true of sturgeons and paddlefish.[42]

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.[42]

Timeline

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Main article: Timeline of fish evolution

See also

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References

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Sources

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Evolution of fish

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The evolution of fish encompasses the origins and diversification of the earliest vertebrates, beginning with primitive, jawless forms approximately 530 million years ago during the late Cambrian period and progressing through key innovations such as the development of jaws, paired fins, and bony skeletons that enabled predation, locomotion, and adaptation to diverse aquatic environments.[1] Over hundreds of millions of years, fish lineages radiated into two major groups—cartilaginous fish (chondrichthyans) and bony fish (osteichthyans)—with the latter further dividing into ray-finned (actinopterygians) and lobe-finned (sarcopterygians) forms, the latter of which gave rise to tetrapods including amphibians and eventually land vertebrates.[2] Today, fish represent the most species-rich class of vertebrates, with over 37,000 described species as of 2025 inhabiting nearly every aquatic habitat from freshwater rivers to the deep ocean, though the term "fish" is not a strict monophyletic group but a grade of aquatic vertebrates excluding tetrapods.[3] The earliest fish-like vertebrates, often called agnathans or jawless fish, emerged in the late Cambrian around 530 million years ago, many resembling eel-like forms with cartilaginous or armored bodies, no true jaws, and a notochord for support rather than a vertebral column.[1] These forms, such as the conodonts and ostracoderms, filtered food through gill-like structures and lacked paired fins, relying on undulating body movements for propulsion; their fossils indicate an initial colonization of shallow marine waters before spreading to freshwater systems.[4] A pivotal evolutionary milestone occurred in the Silurian period (about 440–419 million years ago), when jaws evolved from the gill arches of ancestral jawless fish, allowing early gnathostomes (jawed vertebrates) like placoderms and acanthodians to become active predators and diversify rapidly during the Devonian "Age of Fishes."[1] Bony fish (osteichthyans) arose in the late Silurian, featuring ossified skeletons, swim bladders derived from lung-like structures for buoyancy control, and cycloid scales, which provided advantages in respiration and mobility compared to the cartilaginous skeletons of sharks and rays that evolved concurrently.[1] Lobe-finned fish, including ancient coelacanths and lungfish, developed fleshy, muscular fins supported by bones that prefigured the limbs of tetrapods, with transitional forms like Tiktaalik appearing around 375 million years ago in the late Devonian as evidence of the water-to-land transition.[2] Ray-finned fish, by contrast, underwent explosive diversification following the Cretaceous–Paleogene extinction around 66 million years ago, driven by innovations in fin rays for precise maneuvering and the radiation of teleosts, which now comprise over 96% of all fish species and dominate modern ecosystems through adaptations to varied diets, depths, and salinities.[5]

Introduction

Definition and Scope

In evolutionary biology, "fish" refers to a paraphyletic grade of aquatic vertebrates that excludes tetrapods and their descendants, encompassing jawless fish (agnathans, such as lampreys and hagfish), cartilaginous fish (chondrichthyans, including sharks and rays), and bony fish (osteichthyans, comprising ray-finned and lobe-finned fishes).[2][6] This grouping highlights a basal stage in vertebrate evolution where these animals share common ancestry but do not form a monophyletic clade, as the lobe-finned osteichthyans gave rise to land vertebrates.[2] Historically, fish were classified as a single taxonomic class under Linnaeus's system in Systema Naturae (1758), denoted as Pisces, based on superficial morphological similarities like body shape and habitat. In contrast, modern cladistic approaches, rooted in phylogenetic systematics, reject this monophyletic treatment, recognizing fish as a non-natural assemblage because it omits derived groups like tetrapods that evolved from within the osteichthyans.[7] Cladistics emphasizes shared derived characters (synapomorphies) and common ancestry, rendering "fish" a grade rather than a clade for descriptive convenience in evolutionary studies.[2] The scope of this article focuses on the phylogenetic history of these vertebrates, from their chordate origins through diversification into major lineages, emphasizing aquatic forms with defining traits such as gill-based respiration, finned appendages for locomotion, and often scaley integument.[2][8] This excludes non-vertebrate aquatic animals resembling fish, such as certain echinoderms or arthropods, which lack vertebral columns and true craniates.[9] The term "fish" derives from Old English fisc, tracing to Proto-Indo-European dhgwh- meaning "fish," while Linnaeus's Pisces stems from Latin piscis, reflecting an ancient recognition of these swimmers as a distinct biological category.[10][11]

Significance in Vertebrate Evolution

Fish represent the earliest vertebrates, emerging approximately 530 million years ago during the Early Cambrian period. Fossils such as Haikouichthys from the Chengjiang biota illustrate this origin, showcasing primitive features like a notochord, dorsal nerve cord, and segmented musculature that formed the basic vertebrate architecture.[12] These early fish-like forms, including jawless agnathans, marked the transition from invertebrate chordates to true vertebrates, setting the stage for the evolutionary radiation of craniates. Central innovations in early fish, such as the notochord for axial support, neural crest cells for craniofacial development, and pharyngeal slits for respiration and feeding, originated in these basal vertebrates and became hallmarks of the entire phylum Chordata.[13] The neural crest, in particular, enabled the formation of diverse structures like the skull and peripheral nervous system, providing evolutionary flexibility that persists in all vertebrates today.[14] Pharyngeal slits, evident in Cambrian agnathans, facilitated gill-based oxygen extraction in aquatic environments and later adapted into diverse roles, underscoring fish as the cradle for these conserved traits. During the Paleozoic era, fish achieved ecological dominance in marine ecosystems, particularly from the Ordovician onward, where their proliferation as predators reshaped food webs and intensified predator-prey dynamics.[15] This radiation coincided with rising oceanic oxygenation levels, which supported larger body sizes and active predation strategies among early fish, contributing to biodiversity surges like the Great Ordovician Biodiversification Event by enabling more complex trophic interactions.[16] Jawless fish, as the earliest forms, initiated this shift, with their filter-feeding and scavenging roles evolving into more aggressive pursuits in later groups. The evolutionary legacy of fish profoundly influenced tetrapod emergence, providing ancestral traits such as robust pectoral fins that prefigured limbs and primitive lungs or swim bladders for air breathing in sarcopterygian lineages.[17] Genomic analyses of lungfish, the closest living relatives to tetrapods, reveal that genetic underpinnings for limb development and pulmonary respiration were present in Devonian fish, facilitating the transition to terrestrial habitats around 400 million years ago.[18] These adaptations in lobe-finned fish underscore how aquatic vertebrate innovations laid the groundwork for conquering land.

Origins of Vertebrates

Early Chordates

The phylum Chordata is defined by the presence of four key synapomorphies that appear at some stage during an organism's development: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.[19] These features provide structural support, nervous coordination, respiratory or feeding functions, and propulsion, respectively, and distinguish chordates from other deuterostomes like echinoderms.[19] While modern chordates include tunicates, lancelets, and vertebrates, the fossil record reveals that these traits originated in soft-bodied, worm-like ancestors during the Cambrian explosion, a period of rapid metazoan diversification.[19] The earliest potential proto-chordates appear in the Early Cambrian Chengjiang biota of Yunnan Province, China, dated to approximately 518 million years ago (Mya). Yunnanozoon lividum, a slender, worm-shaped fossil about 3 cm long, exhibits branchial arches suggestive of pharyngeal slits and a possible notochord-like structure, positioning it as a stem-group chordate close to the vertebrate lineage.[20] Similarly, fossils from the same deposit, such as Haikouella lanceolata, display a proto-cranium with sensory structures and segmental myomeres, indicating an active swimmer but lacking true vertebrae. These Chengjiang specimens, preserved in anoxic mudslides that captured soft tissues, highlight the transition from simple bilaterian worms to more organized body plans with bilateral symmetry and serial segmentation. Further evidence comes from the Middle Cambrian Burgess Shale lagerstätte in British Columbia, Canada, around 505 Mya, which famously preserves soft-bodied marine life from the Miaolingian stage. Pikaia gracilens, an eel-like fossil reaching up to 6 cm, features a prominent notochord, dorsal nerve cord impressions, and chevron-shaped myomeres for undulatory swimming, marking it as a basal chordate and potential ancestor to all vertebrates.[21] The Burgess Shale's exceptional preservation, due to rapid burial in underwater landslides, reveals a diverse ecosystem where proto-chordates coexisted with arthropods and other early bilaterians. Among the Chengjiang finds, Myllokunmingia fengjiaoa stands out as one of the earliest known craniates, with proto-vertebral elements along the notochord, branchial baskets for gill support, and a distinct head region, suggesting enhanced sensory capabilities around 520 Mya.[22] These basal forms, emerging during the Ediacaran-Cambrian transition (541–520 Mya), reflect increasing oxygen levels and ecological pressures that favored filter-feeding and predatory lifestyles in shallow marine environments. The lagerstätten of Chengjiang and Burgess Shale provide critical windows into this pivotal interval, bridging non-vertebrate chordates to the subsequent Ordovician radiation of jawless fish.

Emergence of Jawless Fish

The earliest jawless vertebrates (agnathans), including soft-bodied forms like Myllokunmingia and Metaspriggina, appeared during the Cambrian period around 520–500 million years ago (Mya).[23] More derived, armored jawless fish known as ostracoderms—such as Arandaspis prionotolepis from the Stairway Sandstone of Australia—emerged during the early Ordovician period approximately 480 Mya, marking a key phase in the diversification of true vertebrates.[24] These primitive ostracoderms were small, typically 10-15 cm long, and characterized by heavily armored heads composed of bony plates that provided protection in predator-scarce but abrasive shallow marine environments. Their bodies lacked paired fins, relying instead on undulating movements for locomotion, and featured heterocercal tails that enhanced propulsion in low-oxygen bottom waters.[15] Anatomically, these early ostracoderms resembled modern cyclostomes (hagfishes and lampreys) in possessing a circular, jawless mouth equipped with rasping or scraping structures for feeding, though adapted primarily for bottom-dwelling habits rather than active predation.[25] Examples include the arandaspids, with ventral mouths suited for ingesting detritus and microorganisms from the seafloor, and thelodonts, whose shark-like scales covered elongated bodies for streamlined movement over sediments.[15] This ectoparasitic or scavenging lifestyle, inferred from mouth morphology and associated microfossils, allowed them to exploit organic-rich substrates in nearshore habitats, avoiding competition with more mobile invertebrates.[26] Over the Ordovician and into the Silurian, jawless fish diversified in shallow marine settings, evolving adaptations for filter-feeding on planktonic particles and scavenging organic matter, which supported their radiation into various ecological niches.[27] Groups like the pteraspidomorphs exhibited regional variations in head shield shapes—ranging from crescentic to box-like—for optimized sediment sifting, contributing to their abundance in epicontinental seas.[28] Phylogenetically, these ostracoderms represent a paraphyletic assemblage of stem-gnathostomes, serving as crucial intermediates between soft-bodied chordates and the later jawed vertebrates by developing vertebral columns, crania, and sensory systems that foreshadowed gnathostome innovations.[15] Brief references to conodont elements in some deposits suggest complementary rasping tools, though full details pertain to specialized forms.

Jawless Fish

Conodonts

Conodonts represent an extinct group of early jawless vertebrates known primarily from their phosphatic, tooth-like microfossils called elements, which constitute the earliest evidence of a mineralized skeleton in the vertebrate lineage.[29] These elements, composed mainly of apatite, first appeared around 500 million years ago during the Late Cambrian and persisted until their extinction approximately 200 million years ago in the Late Triassic, spanning over 300 million years of marine environments.[30] The fossil record consists almost exclusively of these isolated elements, which are typically 0.5–1.0 mm in size and recovered from sedimentary rocks through acid dissolution techniques, providing a rich archive for studying early vertebrate evolution. Rare soft-tissue fossils, preserved in exceptional lagerstätten, have confirmed conodonts as vertebrates with elongate, eel-like bodies adapted for a nektonic lifestyle.[31] These specimens reveal segmented myomeres indicative of muscular propulsion, large eyes supported by sclerotic cartilages for enhanced vision in dim waters, and a notochord extending into a finned tail, features aligning them closely with other early chordates.[32] Recent analyses in the 2020s, including redescription of Silurian material from the Waukesha Lagerstätte, have further detailed these soft parts, showing transverse myomeres and cartilaginous structures that underscore their predatory adaptations, with only about a dozen such preservations documented to date.[32] Unlike later armored jawless fish such as ostracoderms, conodonts lacked extensive dermal armor, suggesting a more agile, soft-bodied form.[31] The feeding apparatus of conodonts, assembled from multiple elements arranged in bilateral symmetry within the mouth region, functioned to grasp and process prey, positioning them as active predators or scavengers in Paleozoic marine ecosystems.[33] Isotopic studies of elements from Devonian species indicate a top-trophic-level position, with wear patterns on the elements suggesting they endured mechanical stress from tearing soft-bodied prey or scavenging carrion.[34] Early coniform elements likely served in a macrophagous role from ontogeny, evolving into more complex arrays in later forms for efficient prey capture.[35] Conodont elements exhibit a cosmopolitan distribution across ancient oceans, from low-latitude shallow seas to high-latitude deep waters, reflecting their broad ecological tolerance and utility in global correlation.[36] Their rapid evolutionary turnover and precise stratigraphic occurrence make them indispensable for biostratigraphy, particularly in Paleozoic carbonate and clastic rocks, enabling fine-scale dating and paleoenvironmental reconstructions worldwide.[37]

Ostracoderms

Ostracoderms represent a diverse paraphyletic assemblage of stem-gnathostome vertebrates characterized by extensive dermal armor, which dominated aquatic ecosystems during the Silurian and Devonian periods from approximately 443 to 359 million years ago.[27] This radiation marked the peak of jawless fish (agnathan) diversity, with major clades achieving high generic richness in shallow-water environments, particularly during the Early Devonian Lochkovian stage.[27] Their fossil record, preserved in marine and marginal sediments, reveals a succession of forms that filled ecological niches before the rise of jawed vertebrates.[27] Key groups within ostracoderms include the pteraspidomorphs, such as heterostracans, which featured large, fused cephalic shields composed of dorsal and ventral plates covering the head and anterior trunk, and the anaspids, eel-like forms with elongated bodies and lighter scalation.[38] Pteraspidomorphs, including taxa like pteraspids, originated in the Silurian and persisted into the Devonian, often lacking paired fins and exhibiting a heterocercal tail for propulsion.[38] Anaspids, in contrast, ranged from the Early Silurian (Wenlock) to Late Silurian (Přídolí), with genera such as Birkenia and Jamoytius displaying thin, odontode-covered scales and no heavy plating, suggesting a more agile body plan.[39] Osteostracans, another prominent pteraspidomorph subgroup, had robust head shields and are known from mid-Silurian to Late Devonian deposits, primarily in the Northern Hemisphere.[40] The armor of ostracoderms consisted of dermal bone plates and scales formed from acellular bone overlaid with odontodes—superficial dentine-enameloid structures—that provided robust protection against invertebrate and early vertebrate predators in Paleozoic seas.[39] These plates often incorporated sensory line systems, such as the lateral line canals in osteostracans, which were integrated into the dermoskeleton via pores and grooves to detect water movements and prey, enhancing navigation in low-visibility habitats.[41] In anaspids, the armor was less extensive, featuring superficial tubercles on parallel-fibered bone, while pteraspidomorphs displayed polyodontode scales with vascularized layers for added durability.[39] Most ostracoderms led benthic lifestyles as bottom-dwellers in reefs, lagoons, and nearshore environments, functioning as filter-feeders that sifted organic particles from sediments, as inferred from their ventral mouth positions and low-energy swimming adaptations.[40] Some anaspids may have exhibited more nektonic behaviors, migrating through open waters based on their streamlined forms and reduced armor.[38] Their decline began in the Middle Devonian, culminating in extinction by the Late Devonian, driven primarily by eustatic sea-level rises that reduced shallow-water habitats rather than direct competition with jawed fishes, though the latter contributed to long-term replacement.[27] This event underscores the vulnerability of specialized agnathan ecologies to environmental shifts during the Devonian.[27]

Jawed Fish

Placoderms

Recent discoveries in 2022 have identified the earliest jawed vertebrates (gnathostomes) from approximately 436 million years ago in the Early Silurian of China, including stem representatives of placoderms, acanthodians, and chondrichthyans.[42] Placoderms represent the earliest known jawed vertebrates (gnathostomes), with the oldest fossils dating to the Early Silurian period approximately 436 million years ago, discovered in well-preserved deposits in China.[42] These primitive forms quickly diversified, achieving peak abundance and morphological variety during the Devonian period (419–359 million years ago), when they dominated both marine and freshwater ecosystems worldwide. By the Late Devonian, around 359 million years ago, placoderms underwent a sharp decline, ultimately becoming extinct during the Hangenberg event, one of the major mass extinction pulses that reshaped vertebrate faunas. Their defining feature was a robust dermal armor of large, interlocking bony plates covering the head and anterior trunk, providing substantial protection while allowing flexibility in movement; this armor typically extended to the midpoint of the body, with the posterior region covered in smaller scales. A pivotal evolutionary innovation in placoderms was the development of true jaws, which originated from the modification of the anterior-most gill arches into hinged structures capable of biting and shearing. This adaptation transformed feeding strategies from the suction-based mechanisms of jawless ancestors to more versatile predation, enabling placoderms to capture and process tougher prey. For instance, the arthrodire Dunkleosteus terrelli, a formidable Late Devonian predator, exemplifies this capability; reaching lengths of about 3.4–4.1 meters, it wielded massive, self-sharpening jaw plates akin to bony scissors, exerting bite forces estimated at up to 8,000 Newtons to crush armored contemporaries.[43] Such jaw morphology not only facilitated ecological dominance but also laid foundational anatomical patterns for subsequent gnathostome radiation. Placoderm diversity was extraordinary, encompassing over 300 genera across several orders adapted to varied niches. Antiarchs, for example, featured unique pectoral fins reinforced with thorny spines and jointed like arthropod limbs, aiding in benthic locomotion and sediment sifting in shallow waters. In contrast, arthrodires possessed a distinctive double articulation between the head and thoracic armor, permitting enhanced mobility and predatory strikes, with some forms evolving streamlined bodies for open-water hunting. This morphological breadth underscores their adaptability to both coastal marine and inland freshwater habitats, from rivers to reefs. Phylogenetically, placoderms are regarded as a paraphyletic assemblage of basal gnathostomes, with subgroups like antiarchs and arthrodires branching as successive outgroups to the crown gnathostomes (comprising chondrichthyans and osteichthyans). Fossil evidence, such as osteichthyan-like marginal jaw bones in Silurian forms like Entelognathus, reveals shared dermal and endoskeletal traits that influenced the divergence of cartilaginous and bony fish lineages, highlighting placoderms' role in early vertebrate body plan experimentation.

Acanthodians

Acanthodians, often referred to as "spiny sharks," represent an early group of jawed fishes that emerged during the Silurian period and persisted until the late Permian, spanning approximately 443 to 252 million years ago. These small-bodied vertebrates, typically measuring 5 to 30 centimeters in length, exhibited a streamlined, shark-like form covered in ganoid scales composed of cosmine and dentine layers, providing lightweight protection compared to the heavier armor of contemporaneous groups. Their most distinctive feature was the presence of robust, dentine-reinforced spines preceding each fin, which likely enhanced maneuverability and defense in aquatic environments. Fossils indicate they were among the first gnathostomes to diversify widely, with over 60 genera documented from marine and freshwater deposits.[44][45][46] Anatomically, acanthodians possessed a cartilaginous endoskeleton with calcified elements, paired fins supported by broad bony bases, and a single dorsal fin preceded by a prominent spine, distinguishing them from later actinopterygians that often had multiple dorsal spines. They featured multiple external gill slits—typically five—opening independently without a full operculum in primitive forms, allowing efficient respiration in oxygen-variable waters. Jaws were equipped with sharp, conical teeth or tooth whorls in predatory species, while filter-feeding forms had elongated gill rakers for capturing small particles. Diets varied across taxa: some, like acanthodiforms, were planktivores relying on suspension feeding, whereas ischnacanthiforms consumed larger prey such as small invertebrates and fish, as evidenced by stomach contents in well-preserved specimens.[47][48][49] Ecologically, acanthodians achieved peak diversity during the Devonian period (419–359 million years ago), inhabiting shallow marine seas, reefs, and freshwater rivers across Gondwana and Laurussia, where they occupied mid-trophic levels as both predators and prey. Their adaptability to euryhaline conditions facilitated colonization of estuarine and lacustrine habitats, contributing to their global distribution in deposits from Europe, North America, and Asia. Phylogenetic analyses suggest acanthodians form a paraphyletic grade rather than a monophyletic clade, with some lineages representing stem-group chondrichthyans (cartilaginous fishes) due to shared features like dentine spines and multiple gill slits, while others align as stem osteichthyans (bony fishes) based on endoskeletal and scale histology. This mosaic evolutionary position underscores their role as transitional forms in gnathostome radiation, potentially ancestral to modern shark and ray lineages.[46][50][44][51]

Chondrichthyes

Chondrichthyes, the class of cartilaginous fishes including modern sharks, rays, and chimaeras, originated in the Early Devonian Period around 419 million years ago, evolving from acanthodian-like stem-group ancestors that lacked extensive bony ossification.[52] These early forms retained a predominantly cartilaginous endoskeleton throughout their evolutionary history, a trait that distinguishes them from bony fishes and likely contributed to their lightweight, flexible body plans suited for agile predation.[53] The fossil record indicates that the earliest unequivocal chondrichthyan remains, such as isolated teeth and scales, appear in Early Devonian deposits, marking the divergence of this lineage from other jawed vertebrates.[54] Key anatomical features of Chondrichthyes emerged early in their history, including placoid scales composed of dentin and enameloid for hydrodynamic efficiency and protection, a cloaca for waste and reproductive functions, and internal fertilization via claspers in males, which enhanced reproductive success in marine environments.[55] Among primitive groups, symmoriiforms represent early shark-like chondrichthyans, known from Late Devonian fossils exhibiting dorsal fin spines and streamlined bodies adapted for fast swimming; these forms, such as those from Moroccan Famennian deposits, illustrate the initial diversification of predatory morphologies around 372 million years ago.[56] The absence of bony elements in their skeletons, confirmed by exceptional preservations, underscores the class's reliance on cartilage reinforced by calcified prisms rather than true bone.[57] The fossil record provides iconic examples of early Chondrichthyes, such as Cladoselache from Late Devonian strata in North America, dating to approximately 380 million years ago, which preserved nearly complete skeletons showcasing a torpedo-shaped body, multiple gill slits, and heterocercal tails without ossified structures.[58] This genus exemplifies the predatory adaptations that defined the group, with no evidence of swim bladders or heavy skeletal mineralization, allowing for buoyant, energy-efficient cruising.[59] Diversification accelerated during the Carboniferous and Permian Periods, with genus richness peaking in the mid-Carboniferous (Serpukhovian stage, ~330 million years ago) as chondrichthyans radiated into diverse niches, including deeper waters post-Devonian extinctions; this radiation included symmoriiforms and early holocephalans before a decline at the Carboniferous-Permian boundary.[54] By the Mesozoic Era, particularly the Jurassic Period (~200 million years ago), modern neoselachian forms—encompassing galeomorph sharks, squalomorph sharks, and batoid rays—emerged and proliferated, building on ancestral traits like the ampullae of Lorenzini, jelly-filled electroreceptors that detect prey bioelectric fields for precise hunting.[60] These adaptations, present in fossils from the Devonian onward, facilitated the group's persistence as apex marine predators alongside the parallel evolution of osteichthyan fishes.[61]

Osteichthyes

Osteichthyes, or bony fishes, represent the most diverse group of jawed vertebrates, comprising over 30,000 extant species and originating in the late Silurian to early Devonian period around 425 million years ago.[62] This clade is characterized by the development of endochondral bone, a skeletal tissue that forms through the ossification of cartilage models, providing structural support and rigidity distinct from the cartilaginous skeletons of earlier gnathostomes.[63] A key innovation was the evolution of a lung-like structure in their common ancestor, which later modified into the swim bladder in many lineages, aiding buoyancy control in aquatic environments; this organ is homologous to tetrapod lungs and likely facilitated early adaptations to varying oxygen levels in freshwater habitats.[64] Early representatives of Osteichthyes, such as Cheirolepis from the Middle Devonian approximately 390–400 million years ago, exemplify basal forms with a streamlined body covered in ganoid scales—rhomboid-shaped structures composed of layers of bone, dentine, and enamel-like ganoine for protection and flexibility.[65] These scales contrast with the smoother cycloid patterns seen in more derived bony fishes, reflecting an evolutionary progression toward lighter, more efficient integument. Cheirolepis inhabited freshwater and estuarine systems, highlighting the initial radiation of Osteichthyes into continental waters during the Devonian, where they diversified amid the "Age of Fishes" and outcompeted many jawless and armored predecessors.[66] The monophyly of Osteichthyes is supported by shared derived traits, including an operculum—a bony flap covering the gills that enhances respiratory efficiency by allowing water flow over gill arches without constant swimming—and teeth capped with true enamel, a hypermineralized tissue providing durability for feeding on diverse prey.[67] These features underpinned a major evolutionary radiation, with Osteichthyes invading freshwater ecosystems in the Devonian before achieving global dominance in marine and freshwater realms from the Permian through the Mesozoic eras, coinciding with the decline of chondrichthyans and the rise of modern subgroups. This diversification culminated in the split into two primary lineages: Sarcopterygii (lobe-finned fishes) and Actinopterygii (ray-finned fishes), each adapting to distinct ecological niches.[67]

Diversification of Osteichthyes

Sarcopterygii

Sarcopterygii, commonly known as lobe-finned fishes, represent a major clade of bony fishes (Osteichthyes) that originated during the late Silurian period, approximately 425 million years ago (Mya), with the earliest fossils appearing in late Silurian strata.[68] These fishes are characterized by their fleshy, lobed paired fins supported by robust endochondral bones, which include elements homologous to the limb bones of tetrapods, such as the humerus, radius, and ulna in the pectoral fins.[69] This skeletal structure provided greater structural support and mobility compared to the fin rays of other osteichthyans, enabling adaptations to diverse environments, including shallow, oxygen-poor waters.[70] The Sarcopterygii diversified into several key groups during the Devonian, including the coelacanths (Actinistia), lungfishes (Dipnoi), and tetrapodomorphs. Coelacanths, represented today by the "living fossil" genus Latimeria, feature rounded tails and robust fins, with fossils dating back to the Devonian and modern sightings confirming their persistence in deep marine habitats.[69] Lungfishes, or dipnoans, evolved air-breathing lungs derived from the swim bladder, allowing survival in marginal aquatic environments through estivation during dry periods; their fossil record shows peak diversity during the Devonian period, with subsequent decline through the Mesozoic.[71] Tetrapodomorphs, such as the Late Devonian Eusthenopteron, exhibited advanced fin structures and sensory adaptations that foreshadowed terrestrial locomotion.[69] Key adaptations in sarcopterygians included the development of lungs for aerial respiration in low-oxygen settings and cosmine-covered scales, a multilayered tissue combining dentine, bone, and pore canals that enhanced protection and possibly sensory function.[72] These traits peaked in diversity during the Devonian and Carboniferous periods, with sarcopterygians occupying freshwater and coastal niches before many lineages declined in the Mesozoic. In contrast to the lightweight, ray-supported fins of actinopterygians, sarcopterygian fins emphasized strength and flexibility.[68] Sarcopterygii played a pivotal role in the transition to tetrapods, exemplified by Tiktaalik roseae, a tetrapodomorph from approximately 375 Mya in the Late Devonian, whose pectoral fins featured a functional wrist-like joint and robust limb bones, bridging aquatic fins to terrestrial limbs.[73] This fin-to-limb evolution involved enhancements in girdle strength and muscle attachment, facilitating weight-bearing on substrates.[74] Recent observations, including the first in situ sightings of the Indonesian coelacanth (Latimeria menadoensis) in North Maluku waters in 2025, underscore the clade's enduring survival and provide new insights into their behavior at depths of 140 meters.[75]

Actinopterygii

Actinopterygii, commonly known as ray-finned fishes, represent the most diverse clade of vertebrates, originating in the late Silurian period around 425 million years ago with the divergence from sarcopterygian lineages.[76] The earliest confirmed actinopterygian fossils, such as Andreolepis hedei from deposits in Sweden and Russia dated to approximately 420 million years ago, exhibit primitive skeletal features including ganoid scales and a heterocercal tail.[76] A defining characteristic of this group is the presence of lepidotrichia—bony fin rays composed of segmented, jointed structures that support the fins and enable precise maneuverability and propulsion in water, distinguishing them from the fleshy, lobe-like fins of other osteichthyans.[76] Additionally, actinopterygians lack internal nostrils, or choanae, which are absent in their nasal passages, a trait that sets them apart from lobe-finned fishes and reflects their early evolutionary specialization for aquatic respiration.[76] The evolutionary history of Actinopterygii spans from Paleozoic stem-group forms to the dominance of modern subgroups, with increasing morphological and ecological diversity through the Mesozoic. Basal actinopterygians gave rise to chondrosteans, including extant sturgeons and paddlefishes, which retain primitive features like cartilaginous skeletons and spiracle remnants, appearing prominently in the Triassic.[76] Holosteans, such as gars and bowfins, emerged as a paraphyletic group in the Early Triassic, characterized by intermediate traits like ganoid scales and robust jaws adapted for ambush predation in shallow waters.[76] The teleosts, the most speciose subclass, encompass over 30,000 living species and account for approximately 96% of all extant fish diversity, undergoing an explosive radiation during the Late Cretaceous that coincided with the fragmentation of Gondwana and the expansion of shallow marine and freshwater habitats.[77][78] This radiation was facilitated by innovations in the neopterygian lineage, including the rostral position of the maxilla and enhanced caudal fin structures, allowing teleosts to exploit a wide array of niches from deep oceans to coral reefs.[76] Several key adaptations underscore the success of Actinopterygii, particularly in teleosts, where refinements to the swim bladder evolved into a hydrostatic organ with gas glands for precise buoyancy regulation, enabling energy-efficient swimming in varied depths.[76] Scale evolution progressed from heavy, enamel-covered ganoid types in basal forms to lighter cycloid and ctenoid scales in teleosts, reducing drag and allowing for sleeker body shapes suited to fast swimming and complex environments.[76] These changes supported multiple invasions of freshwater systems, where teleosts adapted to low-oxygen conditions through accessory air-breathing structures, and coral reef ecosystems, where percomorph teleosts developed vibrant colorations and specialized fin morphologies for symbiotic interactions and predation.[79] Such invasions, occurring repeatedly since the Jurassic, have led to teleosts comprising the majority of freshwater fish species today.[79] Recent paleontological insights have further illuminated the sensory evolution within Actinopterygii, particularly among otophysan teleosts. In 2025, the discovery of Acronichthys maccognoi, a Late Cretaceous otophysan fossil from freshwater deposits in Alberta, Canada (North America), revealed key evidence of the Weberian apparatus—a chain of ossicles connecting the swim bladder to the inner ear for enhanced sound detection.[79] This finding demonstrates that the sophisticated hearing mechanism, with modern otophysans capable of detecting frequencies up to approximately 15 kHz for navigation and communication, originated in marine ancestors around 154 million years ago, rather than evolving post-transition to freshwater as previously hypothesized.[79] The fossil indicates at least two independent radiations into freshwater habitats during the Cretaceous, challenging models of otophysan diversification and highlighting how marine innovations facilitated conquest of riverine and lacustrine niches, where otophysans now dominate with over 10,000 species including carps and catfishes.[79]

Evolutionary Timeline and Recent Insights

Key Events by Geological Period

The earliest evidence of chordate origins dates to the Cambrian period, with fossils such as Pikaia gracilens from the Burgess Shale formation representing primitive chordates possessing a notochord and segmented muscles around 505 million years ago (Mya). Conodonts, soft-bodied, eel-like chordates identified primarily through their microscopic phosphatic feeding apparatuses, emerged in the late Cambrian approximately 500 Mya, potentially representing the first vertebrates or craniates.[80] By the Ordovician period (485–443 Mya), jawless fish (agnathans) like ostracoderms began to appear in greater numbers, with armored forms such as Arandaspis indicating early diversification in shallow marine environments.[1] During the Silurian and Devonian periods (443–359 Mya), jawless fish reached their peak diversity, particularly with ostracoderms dominating as filter-feeders in reefs and nearshore habitats. The evolution of jaws in early gnathostomes triggered a rapid radiation of jawed fishes around 420 Mya, with placoderms emerging as dominant armored predators by the Early Devonian, particularly the order Arthrodira which comprises approximately 60% of known placoderm species.[81] This "Age of Fishes" saw the appearance of early chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes), alongside acanthodians, as jawless forms began to decline due to competitive pressures from more efficient feeders.[1] In the Carboniferous and Permian periods (359–252 Mya), acanthodians experienced a marked decline, likely due to intensified competition for resources and predation from emerging chondrichthyans and osteichthyans, leading to their extinction by the end of the Permian. Early chondrichthyans, such as sharks like Stethacanthus, proliferated in marine and freshwater systems with their cartilaginous skeletons and advanced sensory adaptations. Osteichthyans, including primitive ray-finned forms, began to spread widely, foreshadowing their future dominance in post-Paleozoic ecosystems. The Mesozoic and Cenozoic eras (252 Mya to present) witnessed the explosive diversification of teleost fishes, particularly following the Cretaceous-Paleogene (K-Pg) mass extinction event 66 Mya, which eliminated many non-teleost actinopterygians and created ecological vacancies filled by ray-finned species. This radiation led to teleosts achieving ecological dominance in marine and freshwater habitats, with innovations in fin structure and locomotion enabling occupation of diverse niches. Sarcopterygian fishes, once diverse, survived as relics, including coelacanths and lungfishes, representing less than 1% of modern fish diversity. As of 2025, over 35,000 fish species have been described, with approximately 96% belonging to actinopterygians, underscoring their evolutionary success.[3][82]

Modern Discoveries and Phylogenetic Updates

In recent years, advanced imaging techniques such as computed tomography (CT) scanning have revolutionized the study of fish fossils, enabling non-destructive analysis of internal structures and refining evolutionary interpretations. For instance, a 2025 study utilized CT scans to digitally reconstruct the anatomy of Norselaspis glacialis, a 400-million-year-old jawless fish from the Silurian period, revealing an enlarged heart and acute sensory systems adapted for evading predators in low-oxygen environments. These traits, previously thought to evolve later in jawed vertebrates, suggest that early agnathans possessed physiological advantages that facilitated the Devonian explosion of fish diversity.[83] Similarly, CT-based examinations in 2025 uncovered internal gill skeletons and pharyngeal tooth plates in a 310-million-year-old Carboniferous fish fossil, providing the earliest evidence of extraoral teeth for processing prey and challenging prior assumptions about the stepwise evolution of fish dentition. In the Triassic, amateur-discovered fossils from Anhui Province, China, dated to approximately 249 million years ago, were analyzed using high-resolution imaging to confirm a new coelacanth species, Whiteia anniae, highlighting post-extinction recovery patterns in sarcopterygians. These discoveries integrate with the broader evolutionary timeline by demonstrating how biomechanical adaptations persisted across mass extinction boundaries.[84][85] Phylogenetic revisions in the 2010s and beyond have clarified the position of enigmatic groups within vertebrate evolution. Conodonts, once debated as non-vertebrates, were confirmed as early chordates with vertebrate affinities through histological and synchrotron-aided analyses of soft tissues, including neural structures and mineralized elements resembling vertebrate dentine, placing them as stem-group cyclostomes around 500 million years ago. Acanthodians, long considered a polyphyletic assemblage, have been repositioned in recent phylogenies as a grade including stem-osteichthyans; for example, detailed tomographic studies of Acanthodes species reveal osteichthyan-like hyoid arches and braincase features, supporting their role as transitional forms between chondrichthyans and bony fishes.[86][51] Genomic comparisons have further updated understandings of trait evolution in extant fish lineages. The 2013 sequencing of the African coelacanth (Latimeria chalumnae) genome revealed conserved genes linked to limb development, affirming its position as the closest living sarcopterygian relative to tetrapods and providing a baseline for tracing sarcopterygian diversification. Updates through 2024, incorporating refined assemblies, have highlighted gene duplications in coelacanths that parallel those in lungfishes, refining models of air-breathing adaptations. In astyanax cavefish, genomic analyses from 2023 to 2025 dated the loss-of-function mutations in eye-development genes to approximately 11 million years ago in lineages like the Ozark cavefish (Troglichthys rosae), illustrating regressive evolution in isolated karst systems and enabling age estimates for subterranean ecosystems.[87][88][89] Studies on locomotion have also advanced, with skeletal analyses identifying at least 11 extant fish species capable of terrestrial walking, including mudskippers and climbing perch, based on robust pectoral girdles and fin morphologies that echo transitional forms in the sarcopterygian lineage. These findings underscore convergent evolution of ambulatory traits, informed by CT scans of fin-ray supports, and provide modern analogs for interpreting fossil evidence of fish-tetrapod transitions.[90]

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