Xenacanthida

Xenacanthida (or Xenacanthiforms) is a super-order of extinct sharks known from the Carboniferous to Triassic. They were native to freshwater, marginal marine and shallow marine habitats.[1] Some xenacanths may have grown to lengths of 5 m (16 ft).[2] Most forms had large serrated spines extending backwards from the neck. 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.

Xenacanthida
Temporal range:
Life restoration of Xenacanthus
Fossil of Orthacanthus senckenbergianus
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Order: †Xenacanthida
Glikman, 1964
Families and genera

See text

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.[3]

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.[2]

Ecology

Based on isotope analysis of teeth, some xenacanths are likely to have lived permanently in freshwater environments.[4][5] The diet of freshwater xenacanths is known to have included temnospondyls[6] as well as palaeoniscid fish.[7] 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.[6]

Taxonomy
  • Order: Bransonelliformes Hampe & Ivanov, 2007
  • 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
      • Genus: Reginaselache Turner & Burrow, 2011
    • Family: Sphenacanthidae Heyler & Poplin 1989
      • Genus: Sphenacanthus Agassiz, 1837
      • Genus: Xenosynechodus Agassiz, 1980
    • Family: Orthacanthidae Heyler & Poplin 1989
    • Family: Xenacanthidae Fritsch, 1889
      • Genus: Mooreodontus Ginter et al., 2010
      • 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

References
  1. 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.
  2. 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
  3. 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
  4. Fischer, Jan; Schneider, Jörg W.; Hodnett, John-Paul M.; Elliott, David K.; Johnson, Gary D.; Voigt, Silke; Joachimski, Michael M.; Tichomirowa, Marion; Götze, Jens (2014-11-02). "Stable and radiogenic isotope analyses on shark teeth from the Early to the Middle Permian (Sakmarian–Roadian) of the southwestern USA". Historical Biology. 26 (6): 710–727. doi:10.1080/08912963.2013.838953. ISSN 0891-2963. S2CID 128991144.
  5. Fischer, Jan; Schneider, Jörg W.; Voigt, Silke; Joachimski, Michael M.; Tichomirowa, Marion; Tütken, Thomas; Götze, Jens; Berner, Ulrich (2013-03-29). "Oxygen and strontium isotopes from fossil shark teeth: Environmental and ecological implications for Late Palaeozoic European basins". Chemical Geology. 342: 44–62. doi:10.1016/j.chemgeo.2013.01.022. ISSN 0009-2541.
  6. 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.
  7. Greb, Stephen F.; Storrs, Glenn W.; Garcia, William J.; Eble, Cortland F. (April 2016). "Late M ississippian vertebrate palaeoecology and taphonomy, B uffalo W allow F ormation, western K entucky, USA". Lethaia. 49 (2): 199–218. doi:10.1111/let.12138. ISSN 0024-1164.

Further reading
  • Bhat, Mohd Shafi; Ray, Sanghamitra; Datta, P.M. (September 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. S2CID 134435176.
  • Huttenlocker, Adam K.; Henrici, Amy; John Nelson, W.; Elrick, Scott; Berman, David S; Schlotterbeck, Tyler; Sumida, Stuart S. (June 2018). "A multitaxic bonebed near the Carboniferous–Permian boundary (Halgaito Formation, Cutler Group) in Valley of the Gods, Utah, USA: Vertebrate paleontology and taphonomy". Palaeogeography, Palaeoclimatology, Palaeoecology. 499: 72–92. Bibcode:2018PPP...499...72H. doi:10.1016/j.palaeo.2018.03.017. S2CID 135112608.
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