<p>Actinopterygians, or ‘ray-finned fishes,’ are the largest and most successful group of fishes and make up half of all living vertebrates. While actinopterygians appeared in the fossil record during the Devonian period, between 400-350 million years ago (Ma), it was not until the Carboniferous period (360 Ma) that they had become dominant in freshwaters and started to invade the seas. At present, approximately 42 orders, 431 families, and nearly 24,000 species are recognized within this class but there are bound to be taxonomic revisions as research progresses. <span class="taxon">Teleostei</span> comprise approximately 23,000 of the 24,000 species within the actinopterygians, and 96 percent of all living fish species (see Systematic/Taxonomic History). The latter estimates, however, will probably never be accurate because actinopterygian species are becoming extinct faster than they can be discovered in some areas, such as the Amazon and Congo Basins. Unfortunately, habitat destruction, pollution and international trade, among other human impacts, have contributed to the endangerment of many actinopterygians (see Conservation Status).<span> (Grande, 1998; Helfman, Collete, and Facey, 1997; Moyle and Cech, 2004)</span></p>  <p>Clearly, given the enormous diversity of this class, entire books could be (and are) written for each of the categories below, so this account does not attempt an exhaustive summary of the diversity of habitats, body forms, behaviors, reproductive habits, etc. of actinopterygians. Instead, each section introduces important <a href="845109596" target="_gloss">ichthyological</a> concepts and terminology, as well as numerous examples from a diverse range of ray-finned fish families. A section of particular interest is <strong>Systematic/Taxonomic History</strong> because salient features of the evolutionary history of actinopterygians are discussed. The <a href="20020904145837" target="_gloss">phylogenetic</a> trends within early actinopterygians provide a basis for understanding why this group has been so successful, as more derived forms (i.e. <span class="taxon">Neopterygii</span> and <span class="taxon">Teleostei</span>), which make up nearly all existing ray-finned fishes, have repeated and extended early trends. Many of the sections, such as Physical Description, Reproduction, Behavior and Ecosystem Roles merely scratch the surface, but there are numerous links to family-level ray-finned fish accounts. (<strong>‘Fishes’ is used interchangeably with ‘ray-finned fishes’ and 'actinopterygians' from this point forward</strong>).<span> (Helfman, Collete, and Facey, 1997; Liem, 1998; Moyle and Cech, 2004; Nelson, 1994; Wheeler, 1985)</span></p>