The Carboniferous is a geologic period and system that extends from the end of the Devonian period, about 359.2 ± 2.5 Ma (million years ago), to the beginning of the Permian period, about 299.0 ± 0.8 Ma (ICS, 2004).
The Carboniferous was a time of glaciation, low sea level and mountain building; a minor marine extinction event occurred in the middle of the period. The name comes from the Latin word for coal, carbo. Carboniferous means "coal-bearing." Many beds of coal were laid down all over the world during this period, hence the name.
Many fish inhabited the Carboniferous seas; predominantly Elasmobranchs (sharks and their relatives). These included some, like Psammodus, with crushing pavement-like teeth adapted for grinding the shells of brachiopods, crustaceans, and other marine organisms. Other sharks had piercing teeth, such as the Symmoriida; some, the petalodonts, had peculiar cycloid cutting teeth. Most of the sharks were marine, but the Xenacanthus invaded fresh waters of the coal swamps. Among the bony fish, the Palaeonisciformes found in coastal waters also appear to have migrated to rivers. Sarcopterygian fish were also prominent.
Most species of Carboniferous marine fish have been described largely from teeth, fin spines and dermal ossicles, with smaller freshwater fish preserved whole.
Sharks (especially the Stethacanthids) underwent a major evolutionary radiation during the Carboniferous. It is believed that this evolutionary radiation occurred because the decline of the placoderms at the end of the Devonian period caused many environmental niches to become unoccupied and allowed new organisms to evolve and fill these niches. As a result of the evolutionary radiation carboniferous sharks assumed a wide variety of bizarre shapes including Stethacanthus who possessed a flat brush-like dorsal fin with a patch of denticles on its top. Stethacanthus unusual fin may have been used in mating rituals.