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A taxonomy rank below Gnasthostomata (and thus Vertebrata).It includes the Euteleostomi rank. The teleostomi are jawed vertebrates that include all jawed vertebrates except the chondrichthyes, and the placoderms. They appeared in the Latest Ordovician period. The distant ancestor of teleostomes that lived in sea of Latest Ordovician, whatever it was, its autapomorphies were enough to eventually out-compete every other vertebrate group. Living teleostomes are Osteichthyes ("bony fishes), a group that was ancestral to the tetrapods - a lineage that includes land vertebrates. Other extinct teleostomes are Acanthodii, extinct relatives of Osteichthyes. The Acanthodii were successful teleostomes, but they went extinct by Permian Period, leaving the Teleostomi's daughter clade, Osteichthyes, to remain the dominant clade and continued to evolve. Even now, Osteichthyes (including the ray-finned fishes, the lobe-fins and, ALL tetrapods) outnumber every other vertebrate groups.
Teleostome Physiology
Teleostomes have two major adaptations that relate to 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 adaption, the teleostomes also developed a swim bladder and the ability to use some atmospheric oxygen, if primarilly for bouyancy, very early on. The primary function of the bladder is keeping the fish at neutral buoyancy. Later these swim bladders will evolve and modify into lungs, as in tetrapods.
See also
"Teleostomi"