A large, extinct assemblage of primitive ray-finned (actinopterygian) fishes known from the Silurian to Cretaceous, which were most numerous in Carboniferous and Permian times. They are known as fossils from all continents except Antarctica. Approximately 40 families are recognized but collectively they are not a natural group; some are genealogically more closely related to derived ray-finned fishes (holosteans and teleosts) than to other palaeonisciforms.