Xenacanthida (or Xenacanthiforms) is a super-order of extinct

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...

-like elasmobranchs that appeared during the Lower

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...

period. The order includes the families Xenacanthidae, Sphenacanthidae, Diplodoselachidae, and Orthacanthidae. The most notable members of the group are the genera '' Xenacanthus'' and '' Orthacanthus''. Some xenacanths may have grown to lengths of .Beck, Kimberley G.; oler-Gijón, Rodrigo; Carlucci, Jesse R.; Willis, Ray E. (December 2014). "Morphology and Histology of Dorsal Spines of the Xenacanthid Shark Orthacanthus platypternus from the Lower Permian of Texas, USA: Palaeobiological and Palaeoenvironmental Implications". ''Acta Palaeontologica Polonica''. 61 (1): 97–117. doi:10.4202/app.00126.2014 Most forms had large serrated spines extending backwards from the neck. ''Xenacanthus'' had characteristic teeth. Most xenacanths died out at the end of the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...

in the

Permian Mass Extinction The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleozo ...

, with only a few forms surviving into the

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...

period. They were native to freshwater, marginal marine and shallow marine habitats.

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. 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.

Ecology

Based on isotope analysis of teeth, some xenacanths are likely to have lived permanently in freshwater environments. The diet of freshwater xenacanths is known to have included temnospondyls as well as palaeoniscid fish. In the Early Permian freshwater lakes of the Saar–Nahe Basin in southern Germany, large xenacanths are suggested to have acted as the apex predators of this ecosystem.

Taxonomy

* Order: Bransonelliformes Hampe & Ivanov, 2007 *** Genus: '' Barbclabornia'' Johnson, 2003 *** Genus: '' Bransonella'' Harlton, 1933 * Order: Xenacanthiformes Berg, 1955 ** Family: Diplodoselachidae Dick, 1981 *** Genus: '' Diplodoselache'' Dick, 1981 *** Genus: '' Dicentrodus'' Traquair, 1888 *** Genus: '' Hagenoselache'' Hampe & Heidkte, 1997 *** Genus: '' Hokomata'' Hodnett & Elliott, 2018 ***Genus: '' Lebachacanthus'' Soler-Gijon, 1997 ** Family: Sphenacanthidae Heyler & Poplin 1989 *** Genus: '' Sphenacanthus'' Agassiz, 1837 *** Genus: '' Xenosynechodus'' Agassiz, 1980 ** Family: Orthacanthidae Heyler & Poplin 1989 *** Genus: '' Orthacanthus'' Agassiz, 1843 ** Family: Xenacanthidae Fritsch, 1889 *** Genus: '' Mooreodontus'' Ginter et al., 2010 North America, Europe, South America, Australia, India, Triassic *** Genus: '' Plicatodus'' Hampe, 1995 *** Genus: '' Triodus'' Jordan, 1849 *** Genus: '' Xenacanthus'' Beyrich, 1848 *** Genus: '' Wurdigneria'' Richter, 2005 ** ''incertae sedis'' *** Genus: '' Anodontacanthus'' Davis, 1881 *** Genus: '' Tikiodontus'' Bhat, Ray & Datta, 2018. Tiki Formation, India, Late Triassic

Description

Ecology

Taxonomy

References

Further reading