Chondrostei

Chondrostei is a small, early-diverging group of ray-finned fishes within Actinopterygii that includes the living sturgeons and paddlefishes, as well as several extinct relatives. In many taxonomic schemes, it comprises two major extant lineages: the sturgeons (Acipenseridae) and the paddlefishes (Polyodontidae). Chondrosteans are notable for retaining several primitive features of early actinopterygian fish.

Anatomy and biology: The endoskeleton of chondrosteans is largely cartilaginous, with limited bone formation compared with more derived ray-finned fishes. Sturgeons typically have elongated bodies with rows of bony scutes along the sides and a subterminal mouth equipped with barbels for sensing prey; they are mainly bottom feeders. Paddlefishes have an extremely elongated rostrum (snout) that is rich in electroreceptors and used to detect prey, and they are primarily filter feeders of zooplankton. Both groups show relatively primitive fin and skull features compared with more derived actinopterygians, reflecting their early place in ray-finned fish evolution.

Distribution and ecology: Living chondrosteans are found in temperate regions of the Northern Hemisphere, occupying freshwater rivers and lakes, estuaries, and occasionally coastal zones. Some sturgeon species undertake long migrations between riverine and marine habitats to spawn. They tend to be long-lived and slow to mature, making them sensitive to environmental changes.

Fossil record and conservation: Chondrostei appear in the fossil record from deep geological time, illustrating early diversification of ray-finned fishes. Many species are threatened by overfishing, habitat degradation, damming of rivers, and pollution; several are protected by conservation laws and international agreements.