Xenacanthida

Xenacanthida (or Xenacanthiforms) is a super-order of extinct shark-like chondrichthyans known from the Carboniferous to 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 
The foundation of the tooth is prolonged lingually with a circlet button and a basal tubercle on the oral and aboral surfaces individually. Xenacanthida's teeth are famed by articulated bones, cephalic vertebrae and isolated teeth and found global in each aquatic and clean environment. The family Xenacanthidae consist of five genera that are 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. These kinds of fishes are largely marked from Paleozoic remains and their diversity cut drastically throughout the period of their extinction.

Xenacanths are divided into two groups based on dental characteristics. Group one has tricuspid crown containing two stout, slightly diverging lateral cusps pointing in the same direction, a high median cusp, with a crown-base angle almost at 90 degrees, a large, rounded, apical button with several foramina and multiple, 8-9 coarse vertical cristae on all the cusps. Group two has bicuspid crowns with two upright, asymmetric cusps, where the medial cusp is thicker than the distal one, and consistently lacks a median cusp.

The bodies of xenacanths are elongate and eel-like. Xenacanths had long dorsal fins, as well as a large spine projecting from the top of the head, which was a modified dorsal-fin spine. The spine is usually thought to have acted as a defense against attackers. They also bore two anal fins, with the tail (caudal) fin being pseudo-diphycercal.

Some xenacanths like Barbclabornia, are thought to have reached lengths of . While others such as Triodus were only around  long.

Ecology 
Many xenacanths are thought to have been euryhaline and to have migrated between freshwater and marine environments. Orthacanthus platypternus is suggested to have been catadromous, migrating into freshwater environments as a juvenile before returning to the sea as an adult. Based on isotope analysis of teeth, some xenacanths have been suggested to have lived permanently in freshwater environments. However, this proposal has been criticised, 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.  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. 

The diet of freshwater xenacanths is known to have included temnospondyl amphibians as well as palaeoniscid fish, acanthodians, and other xenacanths. Large xenacanths are suggested to have acted as the apex predators of late Paleozoic freshwater ecosystems, such as the Early Permian freshwater lakes of the Saar–Nahe Basin in southern Germany.

Taxonomy
Xenacanths are typically placed as stem-group elasmobranchs, more closely related to modern sharks and rays than to Holocephali, which includes chimaeras.

Subdivisions 
 Order: Bransonelliformes Hampe & Ivanov, 2007
 Genus: Barbclabornia Johnson, 2003 (Early Permian, possibly also Late Carboniferous, North America)
 Genus: Bransonella Harlton, 1933 (Early Carboniferous-Middle Permian, Worldwide)
 Order: Xenacanthiformes Berg, 1955
 Family: Diplodoselachidae Dick, 1981
 Genus: Diplodoselache Dick, 1981 (Early Carboniferous, Europe)
 Genus: Dicentrodus Traquair, 1888 (Early Carboniferous, Europe, North America)
 Genus: Hagenoselache Hampe & Heidkte, 1997 (mid-Carboniferous, Europe)
 Genus: Hokomata Hodnett & Elliott, 2018  (mid-Carboniferous, North America)
 Genus: Lebachacanthus Soler-Gijon, 1997 (Late Carboniferous-Early Permian, Europe)
 Genus: Reginaselache Turner & Burrow, 2011 (Early Carboniferous, Australia)
 Family: Sphenacanthidae Maisey, 1982
 Genus: Sphenacanthus Agassiz, 1837 (Early Carboniferous-Late Permian, Worldwide)
 Genus: Xenosynechodus Agassiz, 1980 (Middle-Late Permian, Europe, later authors have rejected its placement as a xenacanth) 
 Genus: Desinia Ivanov, 2022 (Middle-Late Permian, Europe)
 Family: Orthacanthidae Heyler & Poplin 1989
 Genus: Orthacanthus Agassiz, 1843 (Late Carboniferous-Early Permian, Europe, North America)
 Family: Xenacanthidae Fritsch, 1889
 Genus: Mooreodontus Ginter et al., 2010 (Middle-Late Triassic, Worldwide)
 Genus: Plicatodus Hampe, 1995 (Late Carboniferous-Early Permian, Europe)
 Genus: Triodus Jordan, 1849 (Late Carboniferous-Middle Permian, Europe, North America, South America)
 Genus: Xenacanthus Beyrich, 1848 (Carboniferous-Permian, Worldwide)
 Genus: Wurdigneria Richter, 2005 (Middle-Late Permian, South America)
 incertae sedis
 Genus: Tikiodontus Bhat, Ray & Datta, 2018 (Late Triassic, India)

References

Further reading

Xenacanths
Prehistoric cartilaginous fish orders
Carboniferous sharks
Permian sharks
Triassic sharks
Mississippian first appearances
Mississippian taxonomic orders
Pennsylvanian taxonomic orders
Cisuralian taxonomic orders
Guadalupian taxonomic orders
Lopingian taxonomic orders
Early Triassic taxonomic orders
Middle Triassic taxonomic orders
Middle Triassic extinctions