Osteichthyes

Guiyu oneiros, the earliest known bony fish, lived during the Late Silurian, 425 million years ago.[1] It has a combination of both ray-finned and lobe-finned features.

Bony fish are characterized by a relatively stable pattern of cranial bones, rooted, medial insertion of mandibular muscle in the lower jaw. The head and pectoral girdles are covered with large dermal bones. The eyeball is supported by a sclerotic ring of four small bones, but this characteristic has been lost or modified in many modern species. The labyrinth in the inner ear contains large otoliths. The braincase, or neurocranium, is frequently divided into anterior and posterior sections divided by a fissure.

Early bony fish had simple lungs (a pouch on either side of the esophagus) which helped them breathe in low-oxygen water. In the Actinopteri these have evolved into a swim bladder, which help the body create a neutral balance between sinking and floating. (The lungs of amphibians, reptiles, birds, and mammals were inherited from their bony fish ancestors.)[8][9][10] They do not have fin spines, but instead support the fin with lepidotrichia (bone fin rays). They also have an operculum, which helps them breathe without having to swim.

Bony fish do not produce placoid scales, instead they consist of three types of scales that do not penetrate the epidermis in the process. The three categories of scales for Osteichthyes which are cosmoid scales, ganoid scales, teleost scales. The teleost scales are also then divided into two subgroups which are the cycloid scales, and the ctenoid scales. All these scales have a base of bone that they all originate from, the only difference is that the teleost scales only have one layer of bone. Ganoid scales have lamellar bone, and vascular bone that lies on top of the lamellar bone, then enamel lies on top of both layers of bone. Cosmoid scales have the same two layers of bone that ganoid scales have expect they gave dentin in-between the enamel and vascular bone and lamellar (vascular and lamellar two subcategories for bone found in scales). All these scales are found underneath the epidermis and do not break the epidermis of the fish. Unlike the placoid scales that poke through the epidermis of the fish.

Traditionally, Osteichthyes was considered a class, recognised on the presence of a swim bladder, only three pairs of gill arches hidden behind a bony operculum, and a predominantly bony skeleton.[11] Under this classification system, Osteichthyes was considered paraphyletic with regard to land vertebrates, as the common ancestor of all osteichthyans includes tetrapods amongst its descendants. While the largest subclass, Actinopterygii (ray-finned fish), is monophyletic, with the inclusion of the smaller sub-class Sarcopterygii, Osteichthyes was regarded as paraphyletic.

This has led to the current cladistic classification which splits the Osteichthyes into two full classes. Under this scheme Osteichthyes is monophyletic, as it includes the tetrapods making it a synonym of the clade Euteleostomi. Most bony fish belong to the ray-finned fish (Actinopterygii).

Actinopterygii	ray-finned fish	Actinopterygii, members of which are known as ray-finned fishes, is a class or subclass of the bony fishes. The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines ("rays"), as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii which also possess lepidotrichia. These actinopterygian fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the link or connection between these fins and the internal skeleton (e.g., pelvic and pectoral girdles). In terms of numbers, actinopterygians are the dominant class of vertebrates, comprising nearly 99% of the over 30,000 species of fish (Davis, Brian 2010). They are ubiquitous throughout freshwater and marine environments from the deep sea to the highest mountain streams. Extant species can range in size from Paedocypris, at 8 mm (0.3 in), to the massive ocean sunfish, at 2,300 kg (5,070 lb), and the long-bodied oarfish, to at least 11 m (36 ft).
Sarcopterygii	lobe-finned fish	Sarcopterygii (fleshy fin), members of which are known as lobe-finned fish, is a group of the bony fishes. Traditionally, it is a class or subclass that excludes Tetrapoda, a group of typically terrestrial vertebrates that descends from lobe-finned fish. However, under modern cladistic classification schemes, Sarcopterygii is a clade that includes the tetrapods. The living sarcopterygians are the coelacanths, lungfish, and the tetrapods. Early lobe-finned fishes had fleshy, lobed, paired fins, joined to the body by a single bone.[12] Their fins differ from those of all other fish in that each is borne on a fleshy, lobelike, scaly stalk extending from the body. Pectoral and pelvic fins have articulations resembling those of tetrapod limbs. These fins evolved into 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 actinopterygians (ray-finned fish). The braincase of sarcoptergygians primitively has a hinge line, but this is lost in tetrapods and lungfish. Many early lobe-finned fishes have a symmetrical tail. All lobe-finned fishes possess teeth covered with true enamel.

A phylogeny of living Osteichthyes, including the tetrapods, is shown in the cladogram below.[5][13][14][15] Whole-genome duplication took place in the ancestral Osteichthyes.[16]

All bony fish possess gills. For the majority this is their sole or main means of respiration. Lungfish and other osteichthyan species are capable of respiration through lungs or vascularized swim bladders. Other species can respire through their skin, intestines, and/or stomach.[17]

Osteichthyes are primitively ectothermic (cold blooded), meaning that their body temperature is dependent on that of the water. But some of the larger marine osteichthyids, such as the opah,[18][19] swordfish[20][21] and tuna[22] have independently evolved various levels of endothermy. Bony fish can be any type of heterotroph: numerous species of omnivore, carnivore, herbivore, filter-feeder or detritivore are documented.

Some bony fish are hermaphrodites, and a number of species exhibit parthenogenesis. Fertilization is usually external, but can be internal. Development is usually oviparous (egg-laying) but can be ovoviviparous, or viviparous. Although there is usually no parental care after birth, before birth parents may scatter, hide, guard or brood eggs, with sea horses being notable in that the males undergo a form of "pregnancy", brooding eggs deposited in a ventral pouch by a female.

Ocean sunfish is the heaviest bony fish in the world.

The ocean sunfish is the heaviest bony fish in the world,[23] in late 2021, Portuguese fishermen found a dead sunfish near the coast of Faial Island, Azores, with a weight of 2,744 kilograms (6,049 lb) and 3.6 metres (12 ft) tall and 3.5 metres (11 ft) long established the biggest ocean sunfish ever captured.[24]

The longest is the king of herrings, a type of oarfish. Other very large bony fish include the Atlantic blue marlin, some specimens of which have been recorded as in excess of 820 kilograms (1,810 lb), the black marlin, some sturgeon species, and the giant and goliath grouper, which both can exceed 300 kilograms (660 lb) in weight. In contrast, Paedocypris progenetica and the stout infantfish can measure less than 8 millimetres (0.31 in).[25][26] The Beluga sturgeon is the largest species of freshwater bony fish extant today, and Arapaima gigas is among the largest of the freshwater fish. The largest bony fish ever was Leedsichthys, which dwarfed the beluga sturgeon as well as the ocean sunfish, giant grouper and all the other giant bony fishes alive today.

Cartilaginous fishes can be further divided into sharks, rays and chimaeras. In the table below, the comparison is made between sharks and bony fishes. For the further differences with rays, see sharks versus rays.

• Ostracoderm - armoured jawless fish.
• Prehistoric fish