Ray-finned fishes get their name from the ray-like arrangement of bones, covered with skin, that form their fins. The fin design may be one feature that contributes to the extraordinary success of ray-finned fishes. Numerically, they are the dominant back-boned animals (vertebrates) on Earth, and include more than 95 percent of all fish species. Ray-finned fishes were not always so common. They appear first in the fossil record during the Devonian (about 400 million years ago), but only in freshwater. Later, during the Carboniferous, they spread from freshwater into the sea, radiating into a variety of habitats. While other groups of fishes went extinct (e.g. spiny sharks), or declined dramatically at the end of the Permian (e.g. elasmobranch sharks and rays), ray-finned fishes continued strong through the Triassic and to the present. Scientists have recorded about 31, 000 living species, with some species going extinct even as we discover more.

The earliest evidence of bony fishes is from fossils found in China dated to the early Cambrian (about 530 million years ago). The fossils are fragments of the head and gill covering of jawless fish (Agnatha), of which a few species persist today. The jawless bony fishes are not only the earliest bony fishes, but also among the earliest known animals with backbones (vertebrates) on Earth. Lacking jaws for chewing, they were filter feeders, straining small organisms from the water. By the early Devonian (about 400 million years ago), various groups of bony fishes with jaws had arisen. The earliest group of jawed fishes was the small, freshwater Ancanthodii, but they went extinct less than 150 million years later. Today's wildly successful group, the ray-finned bony fishes (Actinopterygii) began to diversify during the Devonian. Their earliest representatives had heavy scales, large eyes, and wide mouths.