[3] They are so called because of their lightly built fins made of webbings of skin supported by radially extended thin bony spines called lepidotrichia, as opposed to the bulkier, fleshy lobed fins of the sister class Sarcopterygii (lobe-finned fish).

[4] They are the most abundant nektonic aquatic animals and are ubiquitous throughout freshwater and marine environments from the deep sea to subterranean waters to the highest mountain streams.

Extant species can range in size from Paedocypris, at 8 mm (0.3 in); to the massive ocean sunfish, at 2,300 kg (5,070 lb); and to the giant oarfish, at 11 m (36 ft).

[7] In some fish like the arapaima, the swim bladder has been modified for breathing air again,[8] and in other lineages it has been completely lost.

[11] Teleosts and chondrosteans (sturgeons and paddlefish) also differ from the bichirs and holosteans (bowfin and gars) in having gone through a whole-genome duplication (paleopolyploidy).

In most cases this involves protogyny, fish starting life as females and converting to males at some stage, triggered by some internal or external factor.

This mode of reproduction may be related to the fish's habit of spending long periods out of water in the mangrove forests it inhabits.

[27] Jaw-less fishes (hagfish, lampreys) Cartilaginous fishes (sharks, rays, ratfish) Coelacanths Lungfish Amphibians Mammals Sauropsids (reptiles, birds) Polypteriformes (bichirs, reedfishes) Acipenseriformes (sturgeons, paddlefishes) Teleostei Amiiformes (bowfins) Lepisosteiformes (gars) The polypterids (bichirs and reedfish) are the sister lineage of all other actinopterygians, the Acipenseriformes (sturgeons and paddlefishes) are the sister lineage of Neopterygii, and Holostei (bowfin and gars) are the sister lineage of teleosts.

[27] The earliest known fossil actinopterygian is Andreolepis hedei, dating back 420 million years (Late Silurian), remains of which have been found in Russia, Sweden, and Estonia.

[27] The listing below is a summary of all extinct (indicated by a dagger, †) and living groups of Actinopterygii with their respective taxonomic rank.

The taxonomy follows Phylogenetic Classification of Bony Fishes[28][36] with notes when this differs from Nelson,[3] ITIS[37] and FishBase[38] and extinct groups from Van der Laan 2016[39] and Xu 2021.