Xenacanthida

Xenacanthida; Temporal range: Mississippian–Norian PreꞒ Ꞓ O S D C P T J K Pg N
	Life restoration of Xenacanthus
	Fossil of Orthacanthus senckenbergianus
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Elasmobranchii
Order:	†Xenacanthida; Glikman, 1964
Families and genera
	See text

Xenacanthida (or Xenacanthiforms) is a super-order of extinct shark-like elasmobranchs that appeared during the Lower Carboniferous 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 Xenacanthida may have grown to lengths of 5 m (16 ft).^[1] 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 in the Permian Mass Extinction, with only a few forms surviving into the Triassic period. They were native to freshwater, marginal marine and shallow marine habitats.^[2] Xenacanthids likely acted as apex predators of late-Paleozoic freshwater ecosystems.^[3]

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.^[4]

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

References

^ 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
^ Pauliv, Victor E.; Martinelli, Agustín G.; Francischini, Heitor; Dentzien-Dias, Paula; Soares, Marina B.; Schultz, Cesar L.; Ribeiro, Ana M. (December 2017). "The first Western Gondwanan species of Triodus Jordan 1849: A new Xenacanthiformes (Chondrichthyes) from the late Paleozoic of Southern Brazil". Journal of South American Earth Sciences. 80: 482–493. doi:10.1016/j.jsames.2017.09.007.
^ Kriwet, Jürgen; Witzmann, Florian; Klug, Stefanie; Heidtke, Ulrich H.J (2008-01-22). "First direct evidence of a vertebrate three-level trophic chain in the fossil record". Proceedings of the Royal Society B: Biological Sciences. 275 (1631): 181–186. doi:10.1098/rspb.2007.1170. ISSN 0962-8452. PMC 2596183. PMID 17971323.
^ Bhat, M. S., Ray, S., & Datta, P. (2018). A new assemblage of freshwater sharks (Chondrichthyes: Elasmobranchii) from the Upper Triassic of India. Geobios, 51(4), 269-283. doi:10.1016/j.geobios.2018.06.004

Taxonomy

References

Further reading