Teleosts make up some 95% of all living fish species and around 45% of described vertebrates. The greatest mass of animals is found in the open ocean, and by this measure they come in second only to the crustaceans. But much more of their diversity is found in shallow seas, lakes, and streams all over the world.

The Palaeozoic era is famous for its variety of fossil fish, including the ancestors of land vertebrates, but this giant group is not found at all until the start of the Mesozoic. Here I have tried to give some examples to illustrate their natural history in this era, when they first diversified and began to take over the seas, shaping them at least as much as mammals and birds have the land and sky.

Drawings are based directly off the references given but are my own attempts to interpret them as living fish, for which I have no actual qualifications and may easily have made mistakes. All lengths are given as total lengths, i.e. include the tail fin or an estimate of it.

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Triassic		Jurassic		Cretaceous		Cenozoic
Teleosts		Heterocercal teleosts
		Homocercal teleosts		Ichthyodectiformes
				Crossognathiformes
				Osteoglossomorpha		→
				Elopomorpha		→
				Otomorpha		→
				Euteleostomorpha		→

The Triassic spans about 50 million years, broken down as follows:

Fossil teleosts make their first appearance in this period. Along with Halecomorphi like modern bowfins and Ginglymodi like gars, these are part of a group that share mobile upper jaws, where the maxilla on each side can be rotated forward to cover the sides of the mouth during feeding. Most also have a supporting supramaxilla except when the mouth is small.

Early teleosts are set apart by also having the premaxilla at the front of the upper jaw detached. Sometimes this can be further specialized like in early Marcopoloichthys, small fish with no teeth and only a few scales, which have a long curved premaxilla presumably involved in suction feeding.

Marcopoloichthys ani, to 4½ cm
Middle Anisian, Yunnan in SW China (Guanling Fm.)
Marine

In most others the premaxilla has a short ascending process and can be pushed forward. The main Triassic group are the Pholidophoridae, named for their covering of thick ganoid scales. These are small marine fish that probably fed on plankton except for the very largest, Pholidorhynchodon, which were predatory on other kinds.

Malingichthys wanfenglinensis, holotype 6 cm
Late Ladinian, Guizhou in SW China (Falang Fm.)
Marine

Pholidorhynchodon malzannii, to ~15 cm
Norian, Italy (Cene)
Marine

References for fish:

1. Tintori, Sun, Lombardo, et al. (2007). New specialized basal neopterygians (Actinopterygii) from Triassic of the Tethys realm.
2. Arratia (2013). Morphology, taxonomy, and phylogeny of Triassic pholidophorid fishes (Actinopterygii, Teleostei).
3. Tintori, Sun, Ni, et al. (2015). Oldest stem Teleostei from the Late Ladinian (Middle Triassic) of Southern China.
4. Arratia & Schultze (2024). The oldest teleosts (Teleostomorpha): their early taxonomic, phenotypic, and ecological diversification during the Triassic.

The Jurassic spans about 60 million years, broken down as follows:

During this period some teleosts start appearing in freshwater. The earliest known are the Archaeomaenidae, which have a free premaxilla but no ascending process. They first show up already on opposite ends of Pangaea, with Zaxilepis in Asia and Oreochima of similar size in Antarctica.

Zaxilepis qinglongensis, generally 4 to 7 cm
Early Jurassic, Yunnan in SW China (Xiangxi Fm.)
Freshwater

Archaeomaene tenuis, to ~22 cm
Aphnelepis australis, to ~16 cm
Probably Kimmeridgian, New South Wales (Talbragar)
Freshwater

Later Aphnelepis and Archaeomaene are found together in Australia. The first have thick scales in front but thinner ones toward the tail, while in the second the whole body has only leptoid scales. These take the form of thin overlapping plates without outer enamel or articulations as in ganoid scales, providing a more flexible and streamlined covering.

A few other families are known from the Late Jurassic of central Africa and possibly Brazil, including Catervariolidae, Ankylophoridae, and Pleuropholidae. I am hoping to find a copy of Saint-Seine (1955, 1962) before I try writing about them.

References for fish:

1. Su (1994). New Early Jurassic actinopterygians from Weixin, Yunnan. [N.B. the scale bars do not agree with the measurements.]
2. Bean (2021). Revision of the Mesozoic freshwater fish clade Archaeomaenidae. [N.B. misquoted size for Z.]
3. Bean (2024). A revision of the Late Jurassic fish Aphnelepis australis from the Talbragar Fossil Fish Bed of New South Wales, Australia.

The Early Jurassic marks a radiation of new teleosts with homocercal tails, where the vertebral column stops instead of extending along the rays into the fin. They also all share leptoid scales so are more flexible and built for propulsion than other relatives. The first known are Proleptolepis from England and some smaller kinds from Chile.

Proleptolepis elongata, holotype 12½ cm
Middle Sinemurian, England (Lyme Regis)
Marine

By the Late Jurassic these have split into several major groups, all but the first of which continue through at least the Cretaceous period:

1. Ascalaboidiformes
2. a. Ichthyodectiformes
   b. Crossognathiformes
3. a. Osteoglossomorpha (e.g. modern bonytongues)
   b. Elopomorpha (e.g. modern eels)
4. Otomorpha (e.g. modern herrings, carps, catfish)
5. Euteleostomorpha (e.g. modern trouts, dragonfish, cods, gobies, perches)

Ichthyodectiformes include early Occithrissops, which probably fed on small particles, and later types like Thrissops that were predators of smaller fish. The latter are also one of a few fish where fossils have preserved a colour pattern, with small dark marks on the scales.

Occithrissops willsoni, to ~24 cm
Late Bathonian, Wyoming (Lower Sundance Fm.)
Marine

Thrissops formosus, to 80 cm
Late Kimmeridgian to Early Tithonian, Germany and France (Cerin, Torleite to Mörnsheim Fms.)
Marine

Crossognathiformes are first represented by Varasichthys and relatives from Chile and Cuba. Then Luisiella and the orthogonikleithrids Cavenderichthys, both placed somewhere between surviving lineages (crown teleosts), are the oldest examples of homocercal teleosts found in freshwater habitats.

Varasichthys ariasi, to 25-30 cm
Oxfordian, Chile (Quebrada el Profeta)
Marine

Luisiella feruglioi, to 11-12 cm
Oxfordian, Argentina (Lower Cañadón Calcáreo Fm.)
Freshwater

Cavenderichthys talbragarensis, to 15 cm
Probably Kimmeridgian, New South Wales (Talbragar)
Freshwater

Otherwise these groups are mostly known from the shallow seas around Europe. Here both ascalaboids like Tharsis and smaller orthogonikleithrids are common. There are also several Elopomorpha like Anaethalion as well as some leptocephalus larvae characteristic of that group.

Tharsis dubius, to 25-30 cm
Late Kimmeridgian to Early Tithonian, Germany and France (Cerin, Nusplingen, Torleite to Mörnsheim Fms.)
Marine

Anaethalion knorri, neotype 18 cm
Late Kimmeridgian, France (Cerin) and Early Tithonian, Germany (Altmühltal Fm.)
Marine

Otomorpha had also started to diversify. In Europe they are only represented by relatively late Tischlingerichthys. After this though there are also southern Ancashichthys and Aijaichthys, kinds with dorsal and ventral scutes typical of the order Ellimmichthyiformes.

Tischlingerichthys viohli, holotype 16 cm
Early Tithonian, Germany (Mörnsheim Fm.)
Marine

Ancashichthys peruensis, holotype est. ~7 cm
Late Tithonian, Peru (Tinajones Fm.)
Probably marine or deltaic

This leaves the Osteoglossomorpha and euteleosts but so far they are not known for certain before the Cretaceous, with only an incomplete Paralycoptera possibly from the Tithonian of Hong Kong.

References for fish:

1. Nybelin (1974). A Revision of the Leptolepid Fishes.
2. Schaeffer & Patterson (1984). Jurassic Fishes from the Western United States, With Comments on Jurassic Fish Distribution.
3. Arratia (1987). Anaethalion and similar teleosts (Actinopterygii, Pisces) from the Late Jurassic (Tithonian) of Southern Germany and their relationships.
4. Frickhinger (1994). Die Fossilien von Solnhofen / The fossils of Solnhofen. [N.B. describes eyes as large in Tharsis and smaller in Anaethalion but compare details in Nybelin and Arratia.]
5. Arratia (1997). Basal Teleosts and Teleostean Phylogeny.
6. Sferco, López-Arbarello, Báez (2015). Anatomical description and taxonomy of †Luisiella feruglioi (Bordas), new combination, a freshwater teleost (Actinopterygii, Teleostei) from the Upper Jurassic of Patagonia.
7. Bean & Arratia (2019). Anatomical revision of the Australian teleosts Cavenderichthys talbragarensis and Waldmanichthys koonwarri impacting on previous phylogenetic interpretations of teleostean relationships.
8. Ebert (2025). New species of the genus Thrissops (Teleostei, Ichthyodectiformes) in the Upper Jurassic of the Solnhofen-Archipelago (Germany) and Kimmeridge Clay (England).
9. Ordoñez, Arratia, Tejada, Chacaltana (2026). Oldest †Ellimmichthyiform Fishes from Peru and the Early Radiation of Clupei.

References for distribution, dates, and classification:

1. Wenz, Bernier, Barale, et al. (1994). L’ichthyofaune des calcaires lithographiques du Kimméridgien supérieur de Cerin (Ain, France).
2. Cúneo, Ramezzani, Scasso, et al. (2013). High-precision U-Pb chronology and a new chronostratigraphy for the Cañadón Asfalto Basin, Chubut, central Patagonia: Implications for terrestrial faunal and floral evolution in Jurassic.
3. Tse, Pittman, Chang (2015). A specimen of Paralycoptera Chang & Chou 1977 (Teleostei: Osteoglossoidei) from Hong Kong (China) with a potential Late Jurassic age that extends the temporal and geographic range of the genus.
4. Betancur-R, Wiley, Arratia, et al. (2017). Phylogenetic classification of bony fishes.
5. Arratia, Schultze, Tischlinger (2019). On a remarkable new species of Tharsis, a Late Jurassic teleostean fish from southern Germany: its morphology and phylogenetic relationships.
6. Alvarado-Ortega & Alves (2022). Nusaviichthys nerivelai gen. et sp. nov., an Albian crossognathiform fish from the Tlayúa lagerstätte, Mexico.
7. Parey, Louis, Montfort, et al. (2023). Genome structures resolve the early diversification of teleost fishes.

The Cretaceous spans about 75 million years, broken down as follows:

Elopomorpha are united by having leptocephalus larvae, which are ribbon-shaped and decrease in length as they change into juveniles. This stage is always marine though some also move into brackish or freshwater as adults. There are four orders:

1. Elopiformes (tenpounder order)
2. Albuliformes (bonefish order)
3. Notacanthiformes (spiny eel order)
4. Anguilliformes (true eels)

The earliest Cretaceous representatives are Baugeichthys from the Albuliformes and Ichthyemidion from the Elopiformes, which are set apart by long jaws with a broad band of needle-like teeth. The latter are already close enough to modern families to make the exact relationships uncertain.

Baugeichthys caeruleus, holotype 27 cm
Hauterivian, France (Massif des Bauges)
Marine

Ichthyemidion vidali, to 50 cm
Early Barremian, Spain (La Pedrera de Meià)
Coastal lake with marine connections

Like osteoglossomorphs these orders typically have palates with parasphenoid teeth, which have been lost in most other Cretaceous groups. In many Albuliformes though they have instead developed further into plates of large round teeth suitable for crushing shelled prey, first known from Brannerion.

Brannerion sp. B, to over 40 cm
Possibly = B. latus described from juvenile
Late Aptian, NE Brazil (Romualdo Fm.)
Marine

True eels appear at the start of the Late Cretaceous with Luenchelys and several other Lebanese genera. These already have long bodies and most have lost pelvic fins. The premaxilla has also lost its mobility and in Libanechelys is partly fused to other bones, along with the tail and anal fins being joined together.

Luenchelys minimus, to 16 cm
Libanechelys bultyncki, holotype 20 cm
Cenomanian, Lebanon (Sannine Fm.: L. minimus at Nammoura, both at Hjoula)
Marine

Later eels have the tail fin joined to both the dorsal and anal fins in a continuous ribbon around the end. In modern kinds the pectoral girdle is also separate from the skull, but this does not appear known before the Cenozoic.

References for fish:

1. Forey (1973). A revision of the elopiform fishes, fossil and recent.
2. Blum (1991). Brannerion. In: Maisey ed. Santana Fossils: An Illustrated Atlas.
3. Poyato-Ariza (1995). Ichthyemidion, a new genus for the elopiform fish “Anaethalion” vidali, from the Early Cretaceous of Spain: phylogenetic comments.
4. Filleul (2001). Baugeichthys caeruleus, gen. et sp. nov., a new albuliform fish from the Hauterivian of the Massif des Bauges (France).
5. Belouze, Gayet, Atallah (2003). Les premiers Anguilliformes: I. Révision des genres cénomaniens Anguillavus Hay, 1903 et Luenchelys nov. gen.
6. Taverne (2004). Libanechelys bultyncki gen. et sp. nov., une nouvelle anguille primitive (Teleostei, Anguilliformes) du Cénomanien marin du Liban.
7. Forey & Maisey (2010). Structure and relationships of †Brannerion (Albuloidei), an Early Cretaceous teleost from Brazil.

References for dates:

1. Arai & Assine (2020). Chronostratigraphic constraints and palaeoenvironmental interpretation of the Romualdo Formation (Santana Group, Araripe Basin, Northeastern Brazil) based on palynology.
2. Gil-Delgado, Delclòs, Sellés, et al. (2023). The Early Cretaceous coastal lake Konservat-Lagerstätte of La Pedrera de Meià (Southern Pyrenees).

Otomorpha include several orders that presumably diverged during the Late Jurassic and Early Cretaceous as the continents were separating. Unfortunately the largely freshwater Otophysi have very few fossils then, though the timing is partly reflected in their later distribution.

1. Clupeomorpha
   1. Ellimmichthyiformes – extinct
   2. Clupeiformes (herring order)
2. Alepocephaliformes (slickhead order)
3. Gonorynchiformes (milkfish order)
4. Otophysi
   1. Cypriniformes (carp order) – Asia and adjacent continents
   2. Gymnotiformes (knifefish) – South America
   3. Characiformes (characins) – mostly South America, Africa
   4. Siluriformes (catfish) – worldwide, greatest diversity in South America

Most kinds have swim bladders that transmit sound to the inner ears through different specialized connections, recognizable in fossils from associated skeletal features. However swim bladders have also been lost in the deep-sea Alepocephaliformes and a few others.

Near the start of the Cretaceous otomorphs are mostly known from freshwater and brackish lakes in Brazil and Spain. Some of these probably tolerated a range of salinity though, and they are still closely related to marine kinds, which become better represented afterward.

Ellimmichthys longicostatus, to ~14 cm
Berriasian/Valanginian, NE Brazil (Candeias Fm.)
Freshwater lake

Rubiesichthys gregalis, to ~4½ cm
Barremian, Spain (La Pedrera de Meià, Las Hoyas)
Freshwater lakes

Pseudoellimma gallae, to ~30 cm
Cynoclupea nelsoni, est. over 25 cm
Barremian, NE Brazil (Morro do Chaves Fm.)
Brackish lake

References for fish:

1. Poyato-Ariza (2005). Palaeoecology of the fishes from the Early Cretaceous lake of Las Hoyas Cuenca, Spain, with a hypothesis of sexual dimorphism for the Chanidae Rubiesichthys.
2. De Figueiredo (2009). A new clupeiform fish from the Lower Cretaceous (Barremian) of Sergipe-Alagoas Basin, northeastern Brazil. [N.B. the scale bar and depth do not agree with the measurements; no orbit is known so the eye for Pseudoellimma here is only a guess.]
3. Malabarba & Di Dario (2017). A new predatory herring-like fish from the early Cretaceous of Brazil, and implications for relationships in the Clupeoidei.
4. De Figueiredo & Gallo (2021). Revision of †Ellimmichthys longicostatus (Clupeomorpha: †Ellimmichthyiformes) from the Lower Cretaceous of Brazil with comments on the taxonomy of related species.

Otophysi

Otophysi have swim bladders with an anterior chamber linked to the inner ears by small bones called Weberian ossicles, forming distinctive sound-transmitting structures on either side of the first few vertebrae. The oldest fish where these have been confirmed are Sooinichthys from the Tethys Ocean.

Sooinichthys varii, to 12 cm
Albian, Mexico (Tlayúa Fm.)
Marine

Acronichthys maccagnoi, holotype 5½ cm
Late Maastrichtian, Alberta (Lower Scollard Fm.)
Freshwater stream

The first complete representative from freshwater is Acronichthys, found in North America but placed closest to the African citharinoids. Others groups are also known during the last part of the Cretaceous but only from fragments, recognized based on features of the different orders.

References for fish:

1. Liu, Brinkman, Murray, et al. (2025). Marine origins and freshwater radiations of the otophysan fishes.
2. Alvarado-Ortega, Otero, Mayrinck (2025). A new otophysan from the Tlayúa Quarry, Mexico: a North American clue to the early diversification of this group of freshwater fishes.