The following classification has been derived primarily from the works of British ichthyologists C. Patterson, R. Miles, P.H. Greenwood, and K.S. Thomson and American ichthyologist D.E. Rosen, with extensive modifications from American ichthyologists G.D. Johnson, W.N. Eschmeyer, M.L.J. Stiassny, L.R. Parenti, S.V. Frank, and W.L. Fink and Canadian ichthyologist J.S. Nelson, among others. Fishes are typically divided into three groups: superclass Agnatha (jawless fishes), class Chondrichthyes (cartilaginous fishes), and superclass Osteichthyes (bony fishes). The latter two groups are included within the infraphylum Gnathostomata, a category containing all jawed vertebrates.

• Superclass Agnatha (jawless fishes)

  Vertebrates with a suctorial or filter-feeding mouth; no true jaws; 2 (possibly 1 sometimes) semicircular canals; pelvic fins lacking; pectoral finlike structures, when present, lacking fin rays; persistent notochord, without bone or cartilage; bony skeleton, when present, formed in skin; true gill arches absent, gill basket present. Habitat of fossil groups uncertain; earliest probably in fresh water. About 113 living species.

  • Class Myxini

    • Order Myxiniformes (hagfishes)

      Without dermal ossification of any sort; pectoral appendages absent; eyes poorly developed; 1–16 pairs of external gill openings; tail more or less diphycercal. Primarily bottom-dwelling fishes, but suctorial, rasping and feeding on flesh of dead or dying fishes; horny teeth present. Length about 15–110 cm (roughly 6–43 inches). About 70 species. Marine. Pennsylvanian to present.

  • Class Cephalaspidomorphi (Monorhina)

    • Order Petromyzontiformes (lampreys)

      Without dermal ossification of any sort; pectoral appendages absent; eyes more or less lateral or dorsal; 7 pairs of external gill openings; tail more or less diphycercal. Primarily bottom-dwelling fishes, but suctorial, feeding on blood and juices of live fishes; horny teeth present. Species are either parasitic or nonparasitic. Length about 15–100 cm (roughly 6–39 inches). About 43 species. Freshwater and marine, breeding in fresh water. Pennsylvanian and present.

• Class Chondrichthyes (or Selachii) (cartilaginous fishes)

  Cartilaginous fishes. Lacks true bone (except in the roots of teeth). Gill clefts: 5–7 in Selachii, 5 gill openings occur on the lower surface of Batoidei, and only 1 in Chimaeriformes. Dorsal fin, fins, and fin spines rigid, not erectile, if present. Approximately 940 species.

  • Subclass Elasmobranchii

    Chondrichthians with 5–7 pairs of gill clefts not covered by a fold of skin, opening separately to the exterior.

    • Order Selachii (sharks)

      Elasmobranchs with gill clefts opening at least partly on the side of the body. More than 400 species.

    • Order Batoidei (rays, sawfishes, guitarfishes, skates, and stingrays)

      5 gill openings, wholly on ventral surface; pectoral fins united with sides of head forward past the gill opening. Differ from all sharks in lacking upper free eyelid. More than 500 species. Jurassic to present.

  • Subclass Holocephali

    Jaws holostylic (the palatoquadrate) supporting the upper jaw completely fused to cranium; hyoid arch complete, unmodified; branchial arches below cranium; internal skeleton of cartilage, often calcified but never of bone; dermal skeleton of dentine or dentinelike tissue (placoid scales), never with true bone; scales do not continue to grow once fully formed; pelvic and cephalic claspers in males of some groups. About 40 species.

    • Order Chimaeriformes (chimaeras)

      Teeth in a single series of a few tooth plates along each jaw ramus (half); pectoral with 2, and pelvic fins with 1 basal element; pelvic fin claspers present; dermal armour frequently present on head; primitive forms with placoid scales covering body, lost in certain advanced forms; scales specialized in some; dorsal fin spine present or absent; cephalic clasper present in some. Marine. Late Devonian to present.

• Superclass Osteichthyes (bony fishes)

  • Class Actinopterygii (ray-finned fishes)

    Fins supported by rays of dermal bone rather than by cartilage. A group of jawed fishes so diverse that no single definition for them can be derived; better understood by determining the distinctive characters of the primitive members and then tracing their various lines of evolution. Primitive actinopterygians can be separated from the sarcopterygians by the following characteristics: scales ganoid; single dorsal fin; pectoral fins with a series of thin radial bones, rather than basal plates and fleshy lobes; no internal nares. Other important characters: skeleton usually well ossified; scales grow throughout life; swim bladder present (occasionally modified to a lunglike structure). More than 27,000 living species.

    • Subclass Chondrostei

      A mixed group that has undergone many evolutionary diversifications. The remaining orders of the Chondrostei are specialized, often for special habitats and ways of life, but many of the groups show trends toward the holostean level of organization, especially in median fin structure and the development of hemiheterocercal tail, in which externally at least the tail appears nearly homocercal. About 40 living species.

      • Order Acipenseriformes (sturgeons and paddlefishes)

        Almost no internal ossification; scales as large scutes in isolated rows (Acipenseridae); snout enlarged and tactile (Polyodontidae); median fins chondrostean in having more fin rays than basal elements; tail heterocercal. Length (sturgeons) up to 6 metres (roughly 20 feet), weight to 3,200 kg (roughly 7,000 pounds). Marine and freshwater, bottom suctorial feeders (sturgeons, Acipenseridae; Europe, Asia, North America) carnivores and plankton feeders (paddlefishes, Polyodontidae; China and North America). Middle Jurassic to present.

      • Order Polypteriformes (bichirs and reedfish)

        Relationships controversial, placed in own subclass by some and thought related to crossopterygians by others. Typical chondrostean characters, such as ganoid scales and a paleoniscoid type of preopercle. Fins modified into long continuous dorsal, tail diphycercal. Freshwater, Africa. Late Cretaceous to present.

    • Infraclass Holostei

      Paraphyletic group. Tail hemiheterocercal; maxillary scale free of preopercle; rays of median fins about equal basal elements in number; spiracle lost; vertebral column tended to increasing ossification; trend toward thinning scales and loss of ganoid layer. Preoperculum intimately bound to and supporting the posterior border of the palate. About 8 species.

      • Order Amiiformes (bowfins and fossil relatives)

        Relatively conservative holosteans with typical holostean characters as given above; some specialized in body shape (elongate); most typical fusiform holosteans. 1 living member of the family Amiidae, with 1 species, Amia calva (bowfin), of North America. Marine and freshwater, almost worldwide. Middle Jurassic to present.

      • Order Semionotiformes (gar and fossil relatives)

        3 (2 extinct and 1 living) families of widely divergent fishes; probably independent of the Amiiformes but with typical holostean characters; length to about 3 metres (roughly 10 feet). Late Permian to present.

    • Infraclass Teleostei (advanced bony fishes)

      Tail homocercal; caudal skeleton with perichordally (around the spinal cord) ossified centra; neural arches modified into elongate uroneurals extending forward onto the preural centra, “stiffening” the joints between the terminal 4 or 5 vertebrae. 2 hypural bones supporting the lower caudal fin lobe. Teleosts never have ganoid scales; typically, their scales when present are thin, overlapping plates of bone that continue to grow throughout life; their lower jaws lack certain bones found in many chondrosteans or at least have some of these bones fused to single elements. 26,840 living species.

      • Superorder Osteoglossomorpha

        • Order Osteoglossiformes (bonytongues, freshwater butterfly fishes, mooneyes, knife fishes, mormyrs)

          A diverse group of freshwater fishes with a relatively primitive jaw suspension and shoulder girdle. The primary bite of the mouth between parasphenoid and tongue (basihyal and glossohyal); paired rods present, usually bony, at the base of the 2nd gill arch; no bony ethmoid commissure; no leptocephalus larvae. Some with electricity-producing organs; circumorbital bones well-developed or reduced; scales with an irregular reticulated pattern (except Pantodontidae). 6 living families. About 220 species. Freshwater, almost worldwide except extremely cold regions. Middle Cretaceous to present.

      • Superorder Elopomorpha

        A diverse group including very primitive fishes and specialized fishes such as eels and therefore difficult to define. Some primitive members with a gular plate (absent in eels), ethmoid commissure present in some forms in a dermal rostral bone (absent in many eels); a leptocephalus larva; no bone cells in scales of primitive members; pelvic fins abdominal when present. More than 740 living species.

        • Order Elopiformes (tarpons and ten-pounders)

          Body fusiform, typical fishlike shape; bone-enclosed ethmoid commissure present; roofed post-temporal fossae; primary bite a tongue-parasphenoid type. 2 families, 2 genera, and 8 species. Marine; worldwide in temperate and tropical zones. Late Jurassic to present.

        • Order Albuliformes (bonefishes, halosaurs, and deep-sea spiny eels)

          Snout enlarged; mouth small and underslung; crushing teeth on palate; single supramaxillary bone; gular plate small or absent; 6 hypural bones. Length to 70 cm (28 inches), weight to about 6.5 kg (15 pounds). 3 families (Albulidae, Halosauridae, and Notocanthidae), 8 genera, and about 30 species. Middle Cretaceous to present.

        • Order Anguilliformes (eels)

          Body elongate; fins reduced and gill chamber modified; displaced posterior to much of head; opercular apparatus reduced; pectoral girdle free of skull; caudal and other fins often greatly reduced; bony ethmoid commissure sometimes present. Length about 15–300 cm (roughly 6–120 inches). 15 families, about 141 genera, and more than 790 species. Marine and freshwater, worldwide in temperate and tropical regions. Cretaceous to present.

        • Order Saccopharyngiformes (gulper eels)

          Jaws and hyomandibular greatly elongate; caudal fin absentor rudimentary; no opercular bones, branchiostegal rays, scales, pelvic fins or ribs. 4 families, 5 genera, and about 28 species. Marine (some bathypelagic), tropical and temperate Atlantic, Indian and Pacific oceans.

      • Superorder Clupeomorpha

        Special type of ear–swim bladder connection present, consisting of a diverticulum of the swim bladder, forming bulla (cavity) within the ear capsule; head lateral line canals on operculum. A diverse group of mostly oceanic, silvery, compressed fishes, many of great commercial importance.

        • Order Clupeiformes (herrings, anchovies, and allies)

          Characters of the superorder. More than 400 species, worldwide. Marine and freshwater, some anadromous. Early Cretaceous to present.

      • Superorder Ostariophysi

        A group of some 8,000 species, including the majority of known freshwater fishes.

        • Series Anotophysi

          • Order Gonorynchiformes (milkfish, beaked sandfishes, snake mudheads, and relatives)

            Toothless; with epibranchial organs and a characteristic caudal skeleton. The anterior ribs and vertebrae show affinities with the superorder Ostariophysi, and the group may belong with the ostariophysans rather than with the Protacanthopterygii. Length about 10–150 cm (roughly 4–60 inches). 4 families, about 37 species. Marine of Indo-Pacific and freshwater of Africa. Cretaceous to present.

        • Series Otophysi

          Characterized by possession of a complex Weberian apparatus (a swim bladder–internal ear connection with 4 movable bones).

          • Order Characiformes

            Mouth not protractile; jaws toothed. Characidae most generalized; other families have specialized skeletal structures, jaws, and teeth. North, Central, and South America, and Africa. 18 families with about 270 genera and nearly 1,700 species. Cretaceous (about 112 million years ago) to present.

          • Order Cypriniformes (carps and minnows)

            Pharyngeal teeth, mouth toothless, protractile. Adipose fin rarely present. About 3,270 species. A few North Asian forms enter the sea. Worldwide in fresh water except South America, Antarctica and Australia. Paleocene to present.

          • Order Siluriformes (catfishes)

            Parietal, symplectic, suboperculum, and true scales absent; often with dermal plates or little bony spines in the skin. Fusion of the supportive parts of the Weberian apparatus extensive. About 30 families. Distribution of the superorder primarily freshwater but some families marine, with the majority of the 2,867 species in Africa and South America. Paleocene to present.

          • Order Gymnotiformes (knifefishes, gymnotid and electric eels)

            Body elongated; anal fin very long; electric organs present, some extraordinarily powerful. Size to 2.2 metres (about 7 feet), weight to 22 kg (48 pounds). About 5 families, 30 genera, and about 134 species. Fossils discovered from Upper Miocene.

      • Superorder Protacanthopterygii

        Epicentral cartilages, absence of proximal forking in the intermuscular bones. Vertebrae usually more than 24; adipose fin present in many members; mesocoracoid bone usually present; glossohyal teeth usually prominent (lost in some); upper jaw usually not protrusible; proethmoid and a series of several perichondral ethmoid commissures; 1 supraorbital bone; no gular plate.

        • Order Esociformes (pikes and pickerels)

          Maxilla toothless, but in gape of mouth; no adipose fin; paired, elongate proethmoids; basibranchial tooth plate in 2 sections; single postcleithrum; cheek and operculum scaled. 2 families, 4 living genera, 10 species. Freshwater, Northern Hemisphere. Late Cretaceous to present.

        • Order Osmeriformes (argentines and smelts)

          Complex posterior branchial structure, the crumenal organ; adipose fin present in many forms. 6 families, 57 genera, and about 202 species. Marine, all oceans.

        • Order Salmoniformes (salmons, trouts, and allies)

          Cartilaginous epicentrals; absence of ossified epipleurals; separate dermethmoid and supraethmoid; scales without radii. Length about 10–150 cm (roughly 4–60 inches); weight to about 50 kg (roughly 110 pounds). 1 family, 11 genera, and about 66 species. Marine and freshwater, worldwide. Cretaceous to present.

      • Superorder Stenopterygii

        • Order Stomiiformes

          Adipose fin present or absent, some species with both a dorsal and a ventral adipose fin; swim bladder without duct or absent entirely; maxilla the dominant bone of the upper jaw; some species with greatly enlarged, depressable teeth; anterior vertebrae sometimes unossified; light organs present in most families; members of some families with chin barbel, which may be a highly elaborate structure; tail support on single vertebral centrum. Order contains 4 families: Stomiidae, barbeled dragonfishes; Gonostomatidae, bristlemouths; Sternoptychidae, marine hatchetfishes; and Phosichthyidae, lightfishes. About 391 species; 2.5–45 cm (1–18 inches) long. Marine, worldwide.

        • Order Ateleopodiformes (highfin tadpole fish)

          Snout bulbous, caudal fin reduced; all genera except Guentherus have a caudal fin united with a long anal fin; pelvic fin of adults with 1 ray on throat; skeleton largely cartilaginous. 1 family, Ateleopodidae, jellynose fishes. 4 genera with about 12 species. Marine, Caribbean, eastern Atlantic, Indo-West Pacific, and eastern Pacific.

      • Superorder Cyclosquamata

        • Order Aulopiformes (barracudinas, lizardfishes, greeneyes, pearleyes, and relatives)

          3rd pharyngobranchial without a cartilaginous condyle for articulation of the 2nd epibranchial. Benthic fishes, or bottom dwellers (such as Aulopididae), tropical inshore fishes (Synodontidae, or lizardfishes), and bottom-dwelling deep-sea fishes (such as the spiderfishes [Ipnopidae]). 15 families, 44 genera, and about 236 species. Marine, worldwide.

      • Superorder Scopelomorpha

        • Order Myctophiformes (lantern fishes)

          Head and body compressed, adipose fin present, mouth usually large and terminal. Mostly small fishes 10–15 cm (roughly 4–6 inches). 2 families of deep-sea pelagic and bathypelagic fishes, the Myctophidae, or lantern fishes (about 32 genera and 235 species)—with bioluminescent organs—and the Neoscopelidae (3 genera with 6 species). Marine, worldwide. Cretaceous to present.

      • Superorder Lampridiomorpha

        • Order Lampriformes (opahs, oarfishes, and relatives)

          No subocular shelf and pelvic spine; some have a peculiar condition (hypurostegy) in which caudal rays are expanded. Medium to large size; to about 2 metres (about 7 feet) and 300 kg (660 pounds) in the opah (Lampridae) and about 10 metres (about 33 feet)—but far less weight—in the more-slender oarfishes, Regalecidae. 7 families, 12 genera, and about 21 species. Marine, oceanic, tropic, and temperate regions. Paleocene to present.

      • Superorder Polymixiomorpha

        • Order Polymixiiformes (barbudos or beardfishes)

          Barbels suspended from the hypohyal bones (anterior part of the gill arches); spines on the dorsal and anal fins; pelvic fins subthoracic. Retain some primitive characters, such as an antorbital bone, a free 2nd ural centrum, 6 autogenous hypurals, 2 uroneurals, and Baudelot’s ligament to the 1st vertebra. Adipose fin lacking. Adult length about 30 cm (roughly 12 inches). 1 family, 10 species. Deep-water marine fishes. Middle Cretaceous to present.

      • Superorder Paracanthopterygii

        Most with a distinctive type of jaw musculature (involving levator maxillae superioris muscle and associated structures); caudal vertebrae with 2nd ural centrum fused with the upper hypural, 2 or fewer epurals and a full neural spine on 2nd preural centrum; pelvic fins usually placed anteriorly, thoracic (midbody) or even farther forward. In general, these fishes have tended to lose primitive acanthopterygian characters. 5 included orders.

        • Order Percopsiformes (trout-perches, pirate perches, and cave fishes)

          Mouth gape and buccal dentition reduced; median fin spines reduced or lost; head with spine ornamentation; scale covering of the adipose fin lost. Length 8–13 cm (roughly 3–5 inches). 3 extant families, 1 fossil family, about 9 living species. All living species freshwater, North America. Eocene to present.

        • Order Gadiformes (cods and allies)

          Early gadiforms were similar in structure to early percopsiforms, but almost all remained marine and subsequently specialized into a variety of environments. Reduced caudal skeleton; elongate body; altered head and jaw structure. Primitive gadiforms have 7 branchiostegal rays, primitive percopsiforms 6. All with very reduced fin spines. Order includes cods, hakes, grenadiers, and rattails. Length 7 to about 200 cm (about 3 to 79 inches). About 9 families, with about 75 genera and 555 species. Marine, worldwide. Early Eocene to present.

        • Order Batrachoidiformes (toadfishes)

          Bottom fishes with short, small, spinous dorsal fins; long soft-rayed dorsal fins; flat heads. Length to about 40 cm (about 16 inches). 1 family, Batrachoididae, 22 genera and 78 species. Marine, occasionally freshwater, worldwide shore fishes. Miocene to present.

        • Order Lophiiformes (goosefishes, anglerfishes, frogfishes, and batfishes)

          Spinous dorsal fin modified as a movable lure. Some deep-sea forms with light organs and males parasitic on females. Length to about 130 cm (51 inches). About 18 families, 66 genera, and 313 species. Marine, widespread; in shallow-water and deep-sea habitats. Eocene to present.

        • Order Ophidiiformes (cusk eels, brotulas, and pearlfishes)

          Pelvic fins, when present, anterior, with 1 or 2 soft rays in each fin, some with a spine, some species live-bearing. 5 families, 100 genera and about 385 species. All marine except for about 5 species in the family Bythitidae, which are limited to fresh or brackish water.

      • Superorder Acanthopterygii (spiny-rayed fishes)

        Spiny fins usually emphasized, rather than reduced (as in paracanthopterygians). Mobile, protractile mouth owing to the almost universal lack of the levator mandibula superioris muscle; pectoral fin relatively higher on side of body; Baudelot’s ligament almost always attached to basicranium. The 13 orders of the superorder Acanthopterygii may be divided into 3 categories (sometimes called series) on the basis of the number of vertebrae, the condition of the fin spines, the position of the pelvic fins, and the presence or absence of ctenoid scales. The series Atherinomopha contains 3 orders, the Atheriniformes, silversides and relatives, Cyprinodontiformes, killifishes and relatives, and Beloniformes, needlefishes and relatives. The series Mugilomorpha contains 1 order, Mugiliformes, the mullets. The series Percomorpha contains the remaining 9 acanthopterygian orders.

        • Series Atherinomorpha

          Testes are a restricted spermatogonial type; egg demersal, with chorionic filaments; fin spines present or absent but frequently weak when present; vertebral number higher than 24; ctenoid scales rare; pelvic fins abdominal, subabdominal, or thoracic in position; pelvic fins may be connected to pleural rib via a ligament. Mostly small fishes (2–10 cm [roughly 1–4 inches]), but some needlefishes (Belonidae) grow to about 2 metres (about 7 feet). 3 included orders. Eocene to present.

          • Order Atheriniformes (silversides)

            Short preanal length of flexion larvae; single row of melanophores on dorsal margin of larvae; ventral face of vomer concave; long A1 muscle tendon to lacrimal; pelvic medial plate not extended to anterior end; and 2nd dorsal-fin spine flexible. 6 families, 48 genera and about 312 species. Marine and freshwater.

          • Order Cyprinodontiformes (killifishes and live-bearers)

            Symmetrical caudal skeleton with single epural mirroring autogenous parhypural; 1st pleural rib on the 2nd, rather than the 3rd vertebra. 9 families, with about 109 genera and at least 1,000 species. Freshwater and coastal marine.

          • Order Beloniformes (medakas, needlefishes, halfbeaks, and allies)

            Absence of the interhyal bone; reduction or loss of the interarcual cartilage; a single, ventral hypohyal bone; distinctive caudal skeleton characterized by the lower caudal lobe with more principal rays than in the upper caudal lobe. 5 families, with about 36 genera and about 227 species. Marine and freshwater, worldwide.

        • Series Mugilomorpha

          Oral and branchial filter-feeding mechanism; intestines muscular and extremely long; lateral line absent or highly reduced; 2 dorsal fins, the 1st spinous; pelvic fin with 1 spine and 5 rays; pelvic fin connected to postcleithrum via a ligament; ctenoid scales; 24 to 26 vertebrae.

          • Order Mugiliformes (mullets)

            Definition as for the Series. 1 family, Mugilidae, with about 17 genera and as many as 80 species. Coastal marine and brackish or freshwater; tropical and temperate seas.

        • Series Percomorpha (perches and perchlike fishes)

          Pelvic fin with 1 spine and 5 rays (or number of rays reduced); pelvic fin connected to postcleithrum or coracoid via a ligament; anterior pelvic process displaced ventrally; ctenoid scales. 9 included orders.

          • Order Stephanoberyciformes (whalefishes, bigscale fishes, and allies)

            Body roundish, skull bones extremely thin, subocular shelf absent; supramaxilla reduced or absent; uniquely modified extrascapular bone. 9 families, 28 genera, and about 75 species. Marine.

          • Order Beryciformes (squirrelfishes and several deep-sea fishes)

            Subocular shelf present or reduced; orbitosphenoid present; 5 or more soft rays in pelvic fins; Jaukbowski’s organ present (modification of the anterior portion of infraorbital and supraorbital sensory canal system). Length 5–60 cm (roughly 2–24 inches). 7 families of small to medium-sized fishes, about 30 genera with about 150 species. Marine, worldwide in tropical and temperate regions. Cretaceous to present.

          • Order Zeiformes (dories, boarfishes, and relatives)

            Anal fin with 1–4 spines; pelvic fin with 1 spine and 5–9 branched rays; caudal fin with less than 15 principal rays. Length to about 1 metre (3 feet). 6–7 families, of which the dories, Zeidae, are best known. About 17 genera and about 38 species. Marine, deep-sea, widespread. Early Eocene to present.

          • Order Gasterosteiformes (sticklebacks, tubesnout fish, and seahorses)

            Frequently with strong spines in dorsal and pelvic fins, spines absent in some; snout often elongated; body often with dermal plates. 11 families. Length about 2–200 cm (0.8–79 inches). 11 families, 374 species. Marine and freshwater, widely distributed. Eocene to present.

          • Order Synbranchiformes (swamp eels and spiny eels)

            Fins reduced, fin spines absent, pharynx modified for breathing air in swamp eels. Length 20 to about 70 cm (roughly 8 to 28 inches). 3 families, about 100 species. Largely restricted to fresh water, in tropics. No fossil record.

          • Order Scorpaeniformes (scorpionfishes, sculpins, flying gurnards, and relatives)

            A complex group of widely divergent fishes that may be polyphyletic and is difficult to characterize. 3 groups may be recognized: scorpaenoid, hexagramoid-cottoid, and anoplopomatoid. United as an order because of a distinctive caudal skeleton and a bony process connecting the 3rd orbital with the preoperculum in most members. Some members with external bony plates. About 26 families, nearly 1,500 species. Primarily marine, some freshwater, in tropical and temperate regions. Eocene to present.

          • Order Perciformes

            Fins usually with spines; pelvic fin with 1 spine and not more than 5 rays, usually below pectoral fins; caudal fin with 15 rays; no orbitosphenoid, mesocoracoid, or intermuscular bones. An extremely varied assemblage of fishes, with a variety of body plans and other adaptations. Size shows broad range; adult length from about 1 cm (less than 0.5 inch) in certain gobies to about 4.8 metres (16 feet) in swordfish); weight to about 900 kg (roughly 2000 pounds). About 10,000 species in some 160 families in 20 suborders. Mostly marine, worldwide. Late Cretaceous to present.

          • Order Pleuronectiformes (flounders, soles, halibuts, and other flatfishes)

            Both eyes on same side of head, skull twisted and asymmetrical, fins usually without spines. 7 families include flounders, soles, and halibuts. 14 families with about 678 species. Mostly marine; bottom fishes, worldwide in tropical and temperate regions. Eocene to present.

          • Order Tetraodontiformes (pufferfishes)

            With a beaklike snout, gill opening restricted to a small opening. 9 families, about 357 species. Marine, occasionally freshwater, worldwide in tropics and subtropics. Eocene to present.

  • Class Sarcopterygii (fleshy-finned fishes)

    Primitive members have heterocercal tail fin with a small amount of fin development above the vertebral column at the posterior end of the tail fin; 2 dorsal fins present; pectoral fins an archipterygium of variable form (an axial median support with side branches); cosmoid scales present, similar to that in acanthodians. Modern freshwater forms have lungs; presumably present in fossil freshwater forms also. Scales grow throughout life of the individual. The internal nares of the Crossopterygii and the Dipnoi may or may not have the same origin. 2 orders.

    • Subclass Coelacanthimorpha (Crossopterygii)

      • Order Coelacanthiformes (coelacanths and fossil relatives)

        Cranium divided into 2 parts (anterior and posterior) at region for exit of the 5th cranial nerve, these parts movable on each other; choanae (internal nares) present (lost in coelacanths); teeth labyrinthodont (that is, with complicated unfoldings of the enamel surface). Length of coelacanths to about 2 metres (roughly 7 feet). Mostly marine, including the so-called living fossil, Latimeria chalumnae, from South Africa, and L. menadoensis, from Indonesia, both of which lack lungs. Early forms emerged from the Early Devonian to Early Permian in mostly shallow freshwater areas and are thought to have given rise to terrestrial vertebrates during the Devonian.

    • Subclass Dipnoi (lungfishes)

      Cranium not divided into movable parts; teeth on upper jaw early, reduced and lost in later members; pterygoid bones with fused teeth in plates modified for eating mollusks and other organisms. Length 60–200 cm (roughly 24–80 inches). 3 surviving types, 1 each in Australia, Africa, and South America. Early Devonian to present.

      • Order Ceratodontiformes (Australian lungfishes)

        1 family, 1 genus, and 1 species.

      • Order Lepidosireniformes (South American and African lungfishes)

        2 families, 2 genera, and 5 species.

Critical appraisal

Fish classification has undergone major revisions in recent years, and further modifications can be expected in the future. Ichthyologists frequently disagree on major as well as minor concepts of phyletic relationships. There remains much to learn about both living and fossil fishes. The geographical distribution given for a poorly known fossil group usually represents only the location of fossil finds, not necessarily the true distribution of the group.