Xenacanthida

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Xenacanthida
Temporal range: Mississippian–Norian
Life restoration of Xenacanthus
Fossil of Lebachacanthus senckenbergianus
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Order: †Xenacanthida
Glikman, 1964
Families and genera

See text

Xenacanthida (or Xenacanthiformes) is an order or superorder of extinct shark-like chondrichthyans (cartilaginous fish) 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 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

Teeth of Triodus teberdaensis

The foundation of the tooth is prolonged lingually with a circlet button and a basal tubercle on the oral and aboral surfaces individually. 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.

Xenacanths are divided into two groups based on dental characteristics. Group one has tricuspid crowns 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]

The bodies of xenacanths are elongate and

anguilliform locomotion).[6]

Some xenacanths like Barbclabornia, are thought to have reached lengths of 4.5–5 metres (15–16 ft).[2] While others such as Triodus were only around 0.5 metres (1.6 ft) long.[7]

Ecology

Many xenacanths are thought to have been

catadromous, migrating into freshwater environments as a juvenile before returning to the sea as an adult.[2] Based on isotope analysis of teeth, some xenacanths have been suggested to have lived permanently in freshwater environments.[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.[10]

Fossil egg cases assigned to the genus Fayolia, which were probably produced by xenacanths, have a helically twisted collarette running around them, similar to the eggs of bullhead sharks, and taper towards both ends, with one end having a tendril. These eggs are typically found in freshwater deposits.[6]

A number of xenacanths are likely to have been fully marine, such as the small primitive genus

benthic) ecology similar to that of a modern catshark.[11]

Xenacanths are thought to have been

acanthodians, and other xenacanths.[11] 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.[7][11]

Taxonomy

Xenacanths are typically placed as

stem-group elasmobranchs, more closely related to modern sharks and rays than to Holocephali, which includes chimaeras.[14][15][16]

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: Dicentrodus Traquair, 1888 (Early Carboniferous, Europe, North America)
      • Genus: Diplodoselache Dick, 1981 (Early Carboniferous, Europe)
      • 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[17])
      • Genus: Desinia Ivanov, 2022[17] (Middle-Late Permian, Europe)
    • Family: Orthacanthidae Heyler & Poplin 1990[5]
      • Genus: Orthacanthus Agassiz, 1843 (Late Carboniferous-Early Permian, Europe, North America)[5]
    • 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)[1]
      • 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

  1. ^
    doi:10.1016/j.jsames.2017.09.007
    .
  2. ^
    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. ^
    ISSN 2660-9568
    .
  5. ^ a b c Babcock, L. E. (2024). "Replacement names for two species of Orthacanthus Agassiz, 1843 (Chondrichthyes, Xenacanthiformes), and discussion of Giebelodus Whitley, 1940, replacement name for Chilodus Giebel, 1848 (Chondrichthyes, Xenacanthiformes), preoccupied by Chilodus Müller & Troschel, 1844 (Actinopterygii, Characiformes)". ZooKeys. 1188 (219–226).
  6. ^
    ISSN 1755-6910
    .
  7. ^
    PMID 17971323
    .
  8. S2CID 128991144
    .
  9. ISSN 0009-2541
    .
  10. doi:10.1016/j.palaeo.2023.111760
    .
  11. ^
    ISSN 0022-3360
    .
  12. ISSN 0272-4634
    .
  13. ISSN 0024-1164
    .
  14. ISSN 0028-0836
    .
  15. ISSN 1477-2019
    .
  16. S2CID 203619135
    .
  17. ^
    ISSN 0031-0301
    .

Further reading
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