Osteichthyes

Osteichthyes (/ˌɒstiːˈɪkθiːz/ ost-ee-IK-theez; from Ancient Greek ὀστέον (ostéon) 'bone' and ἰχθύς (ikhthús) 'fish'), also known as osteichthyans or commonly referred to as the bony fish, is a diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes (cartilaginous fish) and the extinct placoderms and acanthodians, which have endoskeletons primarily composed of cartilage. The vast majority of extant fish are members of Osteichthyes, being an extremely diverse and abundant group consisting of 45 orders, over 435 families and 28,000 species.

The group is divided into two main clades, the ray-finned fish (Actinopterygii, which makes up the vast majority of extant fish) and the lobe-finned fish (Sarcopterygii, which gave rise to all land vertebrates, i.e. tetrapods). The oldest known fossils of bony fish are about 425 million years old from the late Silurian, which are also transitional fossils showing a tooth pattern that is in between the tooth rows of sharks and true bony fishes. Despite the name, these early basal bony fish had not yet evolved ossification and their skeletons were still mostly cartilaginous, and the main distinguishing feature that set them apart from other fish clades were the development of foregut pouches that eventually evolved into the swim bladders and lungs, respectively.

Osteichthyes is broadly equivalent to Euteleostomi. In paleontology the terms are synonymous. In ichthyology the difference is that Euteleostomi presents a cladistic view which includes the terrestrial tetrapods that evolved from lobe-finned fish. Until recently, the view of most ichthyologists has been that Osteichthyes were paraphyletic and include only fishes. However, since 2013 widely cited ichthyology papers have been published with phylogenetic trees that treat the Osteichthyes as a clade including tetrapods.

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 respiratory diverticula (an outpouching on either side of the esophagus) which helped them breathe air in low-oxygen water as a form of supplementary enteral respiration. In ray-finned fish these have evolved into swim bladders, the changing sizes of which help to alter the body's specific density and buoyancy. In elpistostegalians, a crown group of lobe-finned fish that gave rise to the land-dwelling tetrapods, these respiratory diverticula became further specialized for obligated air breathing and evolved into the modern amphibian, reptilian, avian and mammalian lungs. Early bony fish did not have fin spines like most modern fish, but instead had the fleshy paddle-like fins similar to other non-bony clades of fish, although the lobe-finned fish evolved articulated appendicular skeletons within their paired fins, which gave rise to tetrapods' limbs. They also evolved a pair of opercula (gill covers), which can actively draw water across the gills so they can breathe without having to swim.

Bony fish do not have placoid scales like cartilaginous fish, but instead have scales that lie underneath the epidermis and do not penetrate it. The three categories of scales in Osteichthyes are cosmoid scales, ganoid scales, and teleost scales. Teleost scales are then divided into two subgroups which are cycloid scales and ctenoid scales. All of these scales have a base of bone that they all originate from; the main difference is that teleost scales have only one layer of bone. Ganoid scales have lamellar bone, and vascular bone that lies on top of the lamellar bone, then enamel that lies on top of both layers of bone. Cosmoid scales have the same two layers of bone that ganoid scales have except that they have dentin in between the enamel and vascular bone.

...it is increasingly widely accepted that tetrapods, including ourselves, are simply modified bony fishes, and so we are comfortable with using the taxon Osteichthyes as a clade, which now includes all tetrapods...

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. 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 subclass Sarcopterygii, Osteichthyes was regarded as paraphyletic.