Xenacanthida

Xenacanthida; Temporal range: Mississippian–Norian PreꞒ Ꞓ O S D C P T J K Pg N
	Life restoration of Xenacanthus
	Fossil of Lebachacanthus senckenbergianus
Scientific 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 eel-like.^[4] 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.^[5] The spine is usually thought to have acted as a defense against attackers.^[2] They also bore two anal fins, with the tail (caudal) fin being pseudo-diphycercal.^[4] They were probably slow swimmers that swam using side to side undulations of the body (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 euryhaline and to have migrated between freshwater and marine environments. Orthacanthus platypternus from the Early Permian of North America is suggested 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 Bransonella, which is thought to have had a seafloor dwelling (benthic) ecology similar to that of a modern catshark.^[11]

Most xenacanths are thought to have been ambush predators.^[12] The diet of freshwater xenacanths is known to have included temnospondyl amphibians^[7] as well as palaeoniscid fish,^[13] 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] The large Barbclabornia, which has proportionally tiny teeth, has been suggested to be a filter feeder by some authors, different from the lifestyle inferred for other xenacanths.^[14]

Taxonomy

Xenacanths are typically placed as stem-group elasmobranchs, more closely related to modern sharks and rays than to Holocephali, which includes chimaeras.^[15]^[16]^[17]

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^[18])
  - Genus: Desinia Ivanov, 2022^[18] (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

^ ^a ^b 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. Bibcode:2017JSAES..80..482P. doi:10.1016/j.jsames.2017.09.007.
^ ^a ^b ^c ^d ^e 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.
^ 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
^ ^a ^b Soler-Gijón, Rodrigo; Ruiz, Antonio Díez (2023-06-20). ""Carbonífero de Puertollano" Natural Monument (Puertollano basin, Spain): a window for the knowledge of Early Vertebrates". Spanish Journal of Palaeontology. 38 (1): 81–94. doi:10.7203/sjp.26788. ISSN 2660-9568.
^ ^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). doi:10.3897/zookeys.1188.108571. PMC 10790574. PMID 38230382.
^ ^a ^b Schneider, J. (1993). "Environment, biotas and taphonomy of the Lower Permian lacustrine Niederhäslich limestone, Döhlen basin, Germany". Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 84 (3–4): 453–464. Bibcode:1993EESTR..84..453S. doi:10.1017/S0263593300006258. ISSN 1755-6910.
^ ^a ^b ^c 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.
^ 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. Bibcode:2014HBio...26..710F. doi:10.1080/08912963.2013.838953. ISSN 0891-2963. S2CID 128991144.
^ 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. Bibcode:2013ChGeo.342...44F. doi:10.1016/j.chemgeo.2013.01.022. ISSN 0009-2541.
^ Luccisano, Vincent; Cuny, Gilles; Pradel, Alan; Fourel, François; Lécuyer, Christophe; Pouillon, Jean-Marc; Lachat, Kathleen; Amiot, Romain (2023-10-15). "Palaeoenvironmental and palaeoecological reconstructions based on oxygen, carbon and sulfur isotopes of Early Permian shark spines from the French Massif central". Palaeogeography, Palaeoclimatology, Palaeoecology. 628: 111760. Bibcode:2023PPP...62811760L. doi:10.1016/j.palaeo.2023.111760.
^ ^a ^b ^c Elliott, David K.; Hodnett, John-Paul M. (November 2013). "A new species of Bransonella (Chondrichthyes, Xenacanthimorpha, Bransonelliformes) from the Middle Permian Kaibab Formation of northern Arizona". Journal of Paleontology. 87 (6): 1136–1142. Bibcode:2013JPal...87.1136E. doi:10.1666/12-099. ISSN 0022-3360.
^ Luccisano, Vincent; Pradel, Alan; Amiot, Romain; Gand, Georges; Steyer, J.-Sebastien; Cuny, Gilles (2021-03-04). "A new Triodus shark species (Xenacanthidae, Xenacanthiformes) from the lowermost Permian of France and its paleobiogeographic implications". Journal of Vertebrate Paleontology. 41 (2). Bibcode:2021JVPal..41E6470L. doi:10.1080/02724634.2021.1926470. ISSN 0272-4634.
^ Greb, Stephen F.; Storrs, Glenn W.; Garcia, William J.; Eble, Cortland F. (April 2016). "Late Mississippian vertebrate palaeoecology and taphonomy, Buffalo Wallow Formation, western Kentucky, USA". Lethaia. 49 (2): 199–218. Bibcode:2016Letha..49..199G. doi:10.1111/let.12138. ISSN 0024-1164.
^ Zidek, J., Johnson, G. D., May, W. & Claborn, A. 2003. New specimens of xenacanth and hybodont sharks from the Lower Permian of Southwestern Oklahoma. –– Oklahoma Geology Notes 63 (4): 136––147.
^ Coates, Michael I.; Gess, Robert W.; Finarelli, John A.; Criswell, Katharine E.; Tietjen, Kristen (January 2017). "A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes". Nature. 541 (7636): 208–211. Bibcode:2017Natur.541..208C. doi:10.1038/nature20806. ISSN 0028-0836. PMID 28052054.
^ Luccisano, Vincent; Rambert-Natsuaki, Mizuki; Cuny, Gilles; Amiot, Romain; Pouillon, Jean-Marc; Pradel, Alan (2021-12-02). "Phylogenetic implications of the systematic reassessment of Xenacanthiformes and 'Ctenacanthiformes' (Chondrichthyes) neurocrania from the Carboniferous–Permian Autun Basin (France)". Journal of Systematic Palaeontology. 19 (23): 1623–1642. Bibcode:2021JSPal..19.1623L. doi:10.1080/14772019.2022.2073279. ISSN 1477-2019.
^ Frey, Linda; Coates, Michael; Ginter, Michał; Hairapetian, Vachik; Rücklin, Martin; Jerjen, Iwan; Klug, Christian (2019-10-09). "The early elasmobranch Phoebodus : phylogenetic relationships, ecomorphology and a new time-scale for shark evolution". Proceedings of the Royal Society B: Biological Sciences. 286 (1912): 20191336. doi:10.1098/rspb.2019.1336. ISSN 0962-8452. PMC 6790773. PMID 31575362. S2CID 203619135.
^ ^a ^b Ivanov, A. O.; Kovalenko, E. S.; Murashev, M. M.; Podurets, K. M. (December 2022). "Euselachian Sharks (Elasmobranchii, Chondrichthyes) from the Middle and Late Permian of European Russia". Paleontological Journal. 56 (11): 1372–1384. Bibcode:2022PalJ...56.1372I. doi:10.1134/S0031030122110065. ISSN 0031-0301.