Xenacanthida (or Xenacanthiforms) is an order of prehistoric sharks that appeared during the Lower Carboniferous period. The order includes the families Xenacanthidae, Diplodoselachidae, and Orthacanthidae. The most notable members of the group are the genera Xenacanthus and Orthacanthus. Some Xenacanthus may have grown to lengths of 4 m (13 ft). This group of sharks inhabited freshwater environments. Some forms had large serrated spines extending backwards from the neck. Xenacanthus had characteristic teeth. The base or root of each tooth had a pair of hook-like cusps. Most Xenacanthus died out at the end of the Permian in the Permian Mass Extinction, with only a few forms surviving into the Triassic period.