The Actinopterygii constitute the class or sub-class of the ray-finned fishes.
The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines ("rays"), as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii which also, however, possess lepidotrichia. These actinopterygian fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the link or connection between these fins and the internal skeleton (e.g., pelvic and pectoral girdles).
In terms of numbers, actinopterygians are the dominant class of vertebrates, comprising nearly 95% of the 25,000 species of fish. They are ubiquitous throughout fresh water and marine environments from the deep sea to the highest mountain streams. Extant species can range in size from Paedocypris, at 8 millimetres (0.31 in), to the massive Ocean Sunfish, at 2,300 kilograms (5,100 lb), and the long-bodied Oarfish, to at least 11 metres (36 ft).
The earliest known fossil Actinopterygiian is Andreolepis hedei, dating back 420 million years (Late Silurian). This microvertebrate has been uncovered in Russia, Sweden, and Estonia.
Traditionally three grades of actinopterygians have been recognised: the Chondrostei, Holostei, and Teleostei. Some morphological evidence suggests that the second is paraphyletic and should be abandoned; however, recent work based on more complete sampling of fossil taxa, and also an analysis of DNA sequence data from the complete mitochondrial genome, supports its recognition. Nearly all living bony fishes are teleosts.
A listing of the different groups is given below, down to the level of orders, arranged in what has been suggested to represent the evolutionary sequence down to the level of order based primarily on the long history of morphological studies. This classification, like any other taxonomy based on phylogenetic research is in a state of flux. Recent morphological and molecular has shown that several of these ordinal and higher-level groupings represent evolutionary grades rather than clades. Examples of demonstrably paraphyletic groups include the Paracanthopterygii, Scorpaeniformes, and Perciformes. The listing follows FishBase with notes when this differs from Nelson and ITIS.
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