A family of Paleozoic elasmobranch sharks that are characterized by teeth with large divergent lateral cusps (diplodont crowns), an elongate dorsal fin preceded by a distinctive spine, and paired fins with an unusually well-developed skeletal axis. The xenacanths are one of the most clearly defined and stable groups to emerge from the early radiation of cartilaginous fishes (Chondrichthyes), with body fossils known from the early Carboniferous (Diplodoselache) through the late Triassic (Xenacanthus) (see illustration). Numerous articulated specimens have been discovered, predominantly from the upper Carboniferous and Permian of Europe and North America, wherein xenacanths are a classic feature of faunas associated with brackish and freshwater lagoons and coal swamps. These xenacanths include the most frequently depicted genera, such as Orthacanthus and Pleuracanthus, with eellike body forms of up to 3 m (10 ft) or more in length.