Teleostomi

Teleostomi is a clade of jawed vertebrates that includes the tetrapods, bony fish, and the wholly extinct acanthodian fish. Key characters of this group include an operculum and a single pair of respiratory openings, features which were lost or modified in some later representatives. The teleostomes include all jawed vertebrates except the chondrichthyans and the placodermi.

The clade Teleostomi should not be confused with the similar-sounding fish clade Teleostei.

Taxonomy and phylogeny
Subphylum Vertebrata ├─(unranked) Gnathostomatomorpha └─Infraphylum Gnathostomata ├─Class Placodermi — extinct (armored gnathostomes) └Microphylum Eugnathostomata (true jawed vertebrates) ├─Class Chondrichthyes (cartilaginous fish) └─(unranked) Teleostomi (Acanthodii & Osteichthyes) ├─Class Acanthodii — extinct ("spiny sharks") └Superclass Osteichthyes (bony fish) ├─Class Actinopterygii (ray-finned fish) └─Class Sarcopterygii (lobe-finned fish) └Superclass Tetrapoda ├─Class Amphibia (amphibians) └(unranked) Amniota (amniotic egg) ├─Class Sauropsida (reptiles or sauropsids) │ └─Class Aves (birds) └─Class Synapsida └─Class Mammalia (mammals) Note: lines show evolutionary relationships.

Origins
The origins of the teleostomes are obscure, but their first known fossils are Acanthodians ("spiny sharks") from the Late Ordovician Period. Living teleostomes constitute the clade Euteleostomi, which includes all osteichthyans and tetrapods. Even after the acanthodians perished at the end of the Permian, their euteleostome relatives flourished such that today they comprise 99% of living vertebrate species.

Physical characteristics
Teleostomes have two major adaptations that relate to aquatic respiration. First, the early teleostomes probably had some type of operculum, however, it was not the one-piece affair of living fish. The development of a single respiratory opening seems to have been an important step. The second adaptation, the teleostomes also developed a primitive lung with the ability to use some atmospheric oxygen. This developed, in later species, into the lung and (later) the swim bladder, used to keep the fish at neutral buoyancy.

Acanthodians share with Actinopterygii the characteristic of three otoliths, the sagitta in the sacculus, the asteriscus in the lagena, and the lapillus in the utriculus. In dipnoans there are only two otoliths and in Latimeria there is only one.