Xenacanthiformes

Xenacanthiformes (or Xenacanthida) is an order or superorder of extinct shark-like chondrichthyans (cartilaginous fish) known from the Carboniferous to the Late Triassic. They were native to freshwater, marginal marine, and shallow marine habitats. Some xenacanths may have grown to lengths of . Most xenacanths died out at the end of the Permian in the End-Permian Mass Extinction, with only a few forms surviving into the Triassic.

== Description == left|thumb|Teeth of Triodus teberdaensis left|thumb|Skull of Orthacanthus The foundation of the tooth is prolonged lingually with a circlet button and a basal tubercle on the oral and aboral surfaces individually. The family Xenacanthidae consists of five genera: Xenacanthus, Triodus, Plicatodus, Mooreodontus and Wurdigneria; all of these are distinguished by cross sections of the points, crown center, length of the median edge, type of vertical cristae, and microscopic anatomy.