osteichthyans

Osteichthyans, or bony fishes, are a major lineage of jawed vertebrates characterized by an endoskeleton largely composed of bone tissue. The living osteichthyans are divided into two main lineages: the ray-finned fishes (Actinopterygii) and the lobe-finned fishes (Sarcopterygii). Tetrapods (amphibians, reptiles, birds, and mammals) are descended from sarcopterygian ancestors, making them part of the wider osteichthyan lineage in cladistic terms.

A defining feature of osteichthyans is a bony endoskeleton, often accompanied by dermal scales and a bony operculum that covers the gills. Most have a swim bladder or lung for buoyancy regulation. Fins are supported by internal skeletal elements; in ray-finned fishes, the fin rays (lepidotrichia) support a mostly webs-like fin structure, while sarcopterygians possess fleshy, lobed fins with a robust inner bone framework that in some lineages evolved into weight-bearing limbs.

The fossil record shows osteichthyans first appearing in the Silurian period, with early forms generally small and fishes-like. From the sarcopterygian lineage, fins evolved into limbs, giving rise to tetrapods. Actinopterygians underwent extensive diversification, becoming the most species-rich and ecologically varied group of living fishes, including most familiar teleosts such as tuna, salmon, and goldfish.

Osteichthyans inhabit a wide range of freshwater and marine environments. Reproduction varies, with external fertilization common in many species and some lineages showing internal fertilization or viviparity. The group displays extensive ecological and morphological diversity, making it the dominant vertebrate lineage in modern aquatic ecosystems.