actinopterygians

Actinopterygians, or ray-finned fishes, form the largest and most diverse clade of vertebrates. They are defined by fins that are webs of skin supported by numerous slender internal elements called lepidotrichia. Their skeleton is predominantly bone, and most have a swim bladder for buoyancy. Gills are protected by an operculum. The group first appears in the fossil record in the late Silurian to early Devonian, about 420–435 million years ago, and today encompasses well over 30,000 described species, far more than any other vertebrate group.

Taxonomy and major lineages: Actinopterygians include early-diverging chondrosteans such as sturgeons and paddlefishes, which retain partly cartilaginous skeletons. The Neopterygii include gars and bowfin (Holosteii) and, most prominently, the Teleostei. Teleosts comprise the vast majority of living actinopterygians, including perches, herrings, tuna, eels, and countless other forms, many of which exhibit highly specialized body plans and ecologies.

Anatomy, ecology, and diversity: Actinopterygians inhabit virtually all aquatic environments, from fast rivers to the open ocean and deep sea. They display a wide range of feeding strategies, reproductive modes, and life histories. Scales evolved from heavy ganoid types in early lineages to lighter cycloid and ctenoid types in many modern teleosts. Most actinopterygians are free-swimming and occupy roles from top predators to planktivores, making them key components of aquatic ecosystems.

Evolutionary significance: The rise and diversification of teleosts during the Mesozoic era are linked to jaw mechanics, buoyancy innovations, and sensory adaptations that underlie their extraordinary species richness today.