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.