Cladogram showing the relationships of the extant actinopterygians as presented here. The Clupeomorpha and Ostariophysi compose the subdivision Ostarioclupeomorpha (= Otocephala), sister to the Euteleostei. See text for the many fossil clades omitted.
The class Actinopterygii, one of the major vertebrate taxa, is not diagnosed by strong derived character sets, but is nevertheless thought to be mono-phyletic. The earliest fossil remains are of scales of the Late Silurian Andreolepis, Ligulalepis, Naxilepis, Lophosteus, and OOrvikuina; in addition, there is Devonian material of, for example, Cheirolepis, Dialipina, Howqualepis, Limnomis, and Moythomasia (including an Early Devonian endocranium of a specimen tentatively assigned to the actinopterygian genus Ligulalepsis, Basden and Young, 2001), and Carboniferous material of, for example, Aesopichthys, Cyranorhis, Discoserra, Guildayichthys, Kalops, Melanecta, Mesopoma, Mimia, Proceramala, Wendyichthys, and Woodichthys (Cloutier and Arratia, 2004). Photographs and descriptions of many fossil taxa are given in Frickhinger (1991).
Actinopterygii are the sister taxon of the Sarcopterygii. We infer that at some time there was a common ancestor of both of these major lineages, and there are some interesting fossils, such as Psarolepis, that show combinations of actinopterygian and sarcopterygian characters (Cloutier and Arratia, 2004; Zhu and Yu, 2004). Genera incertae sedis include the Cretaceous Diplospondichthys, known from the same locality as the acanthomorph Spinocaudichthys (Filleul and Dutheil, 2004).
The early diversification of actinopterygians was reviewed by Cloutier and Arratia (2004). That paper gave a historical review of our phylogenetic hypotheses and general understanding of relationships, and discussed the taxa involved and the many contributions of other workers, past and present. Lauder and Liem (1983) gave an earlier valuable review of the actinopterygians. Springer and Johnson (2004) have produced a valuable monograph with many anatomical drawings giving insights into the relationships of teleostome fishes, with emphasis to the Actinopterygii, and especially to the acanthomorphs. It has not been possible to do justice to this work in this edition.
A major problem in understanding actinopterygian phylogeny is, as noted by Cloutier and Arratia (2004), that we still have much to learn about the homologies of various characters. Much more work is needed in studying fossils in a cladistic context and in knowing more on the origin and development of characters.
Actinopterygians are recognized here with three subclasses, 44 orders, 453 families, 4,289 genera, and 26,891 species. About 44% of the species are known only or almost only from freshwater.
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