See text Xenacanthida (or Xenacanthiformes) is an order or superorder of extinct shark-like chondrichthyans (cartilaginous fish) known from the Carboniferous to Triassic.

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.

Orthacanthus platypternus from the Early Permian of North America is suggested to have been catadromous, migrating into freshwater environments as a juvenile before returning to the sea as an adult.

[8][9] However, this proposal has been criticised by some authors, as the mineralization window of individual teeth only spans a short interval of time of days to weeks, and may not be reflective of long term behaviour.

[2] However isotopic analysis of fin spines of Orthacanthus and Triodus from the Early Permian of France, suggests that at least for these species, growth exclusively occurred in freshwater environments.

[6] A number of xenacanths are likely to have been fully marine, such as the small primitive genus Bransonella, which is thought to have had a seafloor dwelling (benthic) ecology similar to that of a modern catshark.