Osteichthyan

Osteichthyan refers to any member of Osteichthyes, the clade of bony fishes and their descendants. Members of this group are characterized by a mineralized endoskeleton made of bone, in contrast to the cartilaginous skeletons of the chondrichthyans.

Living osteichthyans are divided into two main lineages: Actinopterygii, the ray-finned fishes, and Sarcopterygii, the lobe-finned fishes. The sarcopterygians include the coelacanths and lungfishes; the latter give rise to the tetrapods—the land-dwelling vertebrates that include amphibians, reptiles, birds, and mammals.

Fishes in this clade typically possess fins supported by internal bony elements and a swim bladder or lungs derived from the gas bladder, enabling buoyancy control or aerial respiration in some species. They have jaws with teeth and a lateral line system for detecting water movements. Modern osteichthyans generally have scales of bone in a variety of forms, and their skin often bears dermal bones.

Fossil evidence places the first osteichthyans in the late Silurian period, with rapid diversification during the Devonian. Early forms show a transition from heavy, ganoid-scaled bodies to the lighter, thin cycloid or ctenoid scales characteristic of many ray-finned fishes. The sarcopterygian lineage produced the first tetrapods, linking fishes to terrestrial vertebrates.

Today, osteichthyans are the most species-rich group of vertebrates, occupying nearly every aquatic habitat and a broad range of terrestrial environments through their tetrapod descendants. Ray-finned fishes constitute the majority of fish diversity, while lungfishes and coelacanths are represented by a relatively small number of living species.