Superregnum: Eukaryota
Cladus: Unikonta
Cladus: Opisthokonta
Cladus: Holozoa
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Superphylum: Deuterostomia
Phylum: Chordata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Classis: Chondrichthyes
Subclassis: Elasmobranchii
Infraclassis: Euselachii
Division/Cohort: Neoselachii
Subdivision/Subcohort: Selachimorpha
Superordo: Squalomorphii
Ordo: Hexanchiformes
Subordines: ...Hexanchoidei...
Familiae: Chlamydoselachidae - Hexanchidae - ?†Mcmurdodontidae - †Orthacodontidae
Name
Hexanchiformes F. de Buen, 1926
Type family: Hexanchidae Gray, 1851
References
Hexanchiformes in FishBase,
Froese, R. & Pauly, D. (eds.) 2024. FishBase. World Wide Web electronic publication, www.fishbase.org, version 02/2024.
Vernacular names
čeština: Šedouni
Deutsch: Grauhaie
suomi: Kaulushaikalat
magyar: Szürkecápa-alakúak
italiano: Exanchiformi
日本語: カグラザメ目
Nederlands: Grauwe haaien
The Hexanchiformes /hɛkˈsæŋkɪfɔːrmiːz/ are a primitive order of sharks,[a] numbering just seven extant species in two families. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens.[2]
Taxonomy
Due to their primitive anatomy, hexanchiforms were previously considered the most basal group of sharks. However, more recent phylogenetic studies indicate that while primitive, they in fact belong to the superorder Squalomorphii, which also contains dogfishes, angelsharks, and sawsharks, although they are thought to be the most basal member of the group.[3][4]
Description
Hexanchiform sharks have one spineless dorsal fin located over or behind the pelvic fins and one anal fin. The vertebral column extends into the long dorsal lobe of the caudal fin, while the ventral lobe is either small or absent. They have either six or seven gill slits, located in front of the pectoral fins. They have a large mouth, with eyes on either side of the head. The spiracles are small and located well above and behind the eyes.[5] The eyes have no nictitating membrane.
The frilled sharks of the genus Chlamydoselachus are very different from the cow sharks, and have been proposed to be moved to a distinct order, Chlamydoselachiformes. However, genetic studies have found them to be each others' closest relatives, and they share certain derived features supporting them both being in the same order.[3][4]
Shark teeth similar to modern hexanchids and echinorhinids are known from Devonian deposits in Antarctica and Australia, as well as Permian deposits in Japan. If these are in fact hexanchids, this may be the only extant order of elasmobranchs to have survived the Permian extinction (and by extension, the oldest extant order of elasmobranchs). However, the Australian/Antarctic shark teeth, from the family Mcmurdodontidae, have also been found to lack a multilayer enameloid layer covering the tooth crown, something found in all modern sharks and most Devonian sharks, indicating that they are neoselachians of uncertain affinity or even indeterminate chondrichthyans. The occurrence of derived sharks in the Devonian is also irreconcilable with the results of all phylogenetic estimates in the group.[6][7][8]
It is debated whether the extinct families Orthacodontidae and Paraorthacodontidae belong to the Hexanchiformes or the extinct Synechodontiformes. However, the Shark-References database currently lists them as members of the Hexanchiformes.[9][10][11]
Distribution
Species are widespread and found across most of the world. They are most common in cold deep water in the tropics, but are also found closer to the shore in more temperate regions.[5]
Classification
Living species
Family Chlamydoselachidae Garman 1884 (frilled sharks)
Chlamydoselachus Garman, 1884
Chlamydoselachus africana Ebert & Compagno, 2009 (Southern African frilled shark)
Chlamydoselachus anguineus Garman, 1884 (frilled shark)
Family Hexanchidae J. E. Gray 1851 (cow sharks)
Heptranchias Rafinesque, 1810
Heptranchias perlo (Bonnaterre, 1788) (sharpnose sevengill shark)
Hexanchus Rafinesque, 1810
Hexanchus griseus (Bonnaterre, 1788) (bluntnose sixgill shark)
Hexanchus nakamurai Teng, 1962 (bigeyed sixgill shark)
Hexanchus vitulus Daly-Engel, 2018 (Atlantic sixgill shark)[12][13]
Notorynchus Ayres, 1855
Notorynchus cepedianus (Péron, 1807) (broadnose sevengill shark)
Extinct species
Notidanodon sp. fossil at the Geological Museum, Copenhagen
Suborder Chlamydoselachoidi
Family Chlamydoselachidae
Chlamydoselachus Garman, 1884
Chlamydoselachus balli Cappetta, Morrison & Adnet, 2019
Chlamydoselachus gracilis Antunes & Cappetta, 2002
Chlamydoselachus lawleyi Davis, 1887
Chlamydoselachus tobleri Leriche, 1929
Dykeius Cappetta, Morrison & Adnet, 2019
Dykeius garethi Cappetta, Morrison & Adnet, 2019
Rolfodon Cappetta, Morrison & Adnet, 2019
Rolfodon bracheri (Pfeil, 1983)
Rolfodon fiedleri (Pfeil, 1983)
Rolfodon goliath (Antunes & Cappetta, 2002)
Rolfodon keyesi (Mannering & Hiller 2008)
Rolfodon landinii (Carrillo-Briceño et al. 2014)
Rolfodon ludvigseni Cappetta, Morrison & Adnet, 2019
Rolfodon tatere (Consoli, 2008)
Rolfodon thomsoni (Richter & Ward, 1990)
Suborder Hexanchoidei
Family Crassodontidanidae
Crassodontidanus Kriwet & Klug, 2011
Crassodontidanus serratus Fraas, 1855
Crassodontidanus wiedenrothi Thies, 1983
Notidanoides Maisey, 1986
Notidanoides muensteri Agassiz, 1843
Notidanus Cuvier, 1816
Notidanus amalthei Oppel, 1854
Notidanus atrox Ameghino, 1899
Notidanus intermedius Wagner, 1862
Notidanus nikitini Chabakov & Zonov, 1935
Pachyhexanchus Cappetta, 1990
Pachyhexanchus pockrandti Ward & Thies, 1987
Family Hexanchidae
Gladioserratus Underwood, Goswami, Prasad, Verma & Flynn, 2011
Gladioserratus aptiensis Pictet, 1864
Gladioserratus dentatus Guinot, Cappetta & Adnet, 2014
Gladioserratus magnus Underwood, Goswami, Prasad, Verma & Flynn, 2011
Heptranchias Rafinesque, 1810
Heptranchias ezoensis Applegate & Uyeno, 1968
Heptranchias howellii Reed, 1946
Heptranchias karagalensis Kozlov in Zhelezko & Kozlov, 1999
Heptranchias tenuidens Leriche, 1938
Hexanchus Rafinesque, 1810
Hexanchus agassizi Cappetta, 1976
Hexanchus andersoni Jordan, 1907
Hexanchus casieri Kozlov, 1999
Hexanchus collinsonae Ward, 1979
Hexanchus gracilis Davis, 1887
Hexanchus hookeri Ward, 1979
Hexanchus microdon Agassiz, 1843
Hexanchus tusbairicus Kozlov in Zhelezko & Kozlov, 1999
Notidanodon Cappetta, 1975
Notidanodon lanceolatus Woodward, 1886
Notidanodon pectinatus Agassiz, 1843
Notorynchus Ayres, 1855
Notorynchus borealus Jordan & Hannibal, 1923
Notorynchus kempi Ward, 1979
Notorynchus lawleyi Cigala Fulgosi, 1983
Notorynchus primigenius Agassiz, 1843
Notorynchus serratissimus Agassiz, 1843
Notorynchus subrecurvus Oppenheimer, 1907
Pachyhexanchus Cappetta, 1990
Pachyhexanchus pockrandti Ward & Thies, 1987
Paraheptranchias Pfeil, 1981
Paraheptranchias repens Probst, 1879
Pseudonotidanus Underwood & Ward, 2004
Pseudonotidanus semirugosus Underwood & Ward, 2004
Welcommia Cappetta, 1990
Welcommia bodeuri Cappetta, 1990
Welcommia cappettai Klug & Kriwet, 2010
Weltonia Ward, 1979
Weltonia ancistrodon Arambourg, 1952
Weltonia burnhamensis Ward, 1979
Xampylodon Cappetta, Morrison & Adnet, 2019
Xampylodon brotzeni (Siverson, 1995)
Xampylodon dentatus (Woodward, 1886)
Xampylodon loozi (Vincent, 1876)
?Family Orthacodontidae
Occitanodus Guinot, Cappetta & Adnet, 2014
Occitanodus sudrei Guinot, Cappetta & Adnet, 2014
Sphenodus Agassiz, 1843
Sphenodus alpinus Gümbel, 1861
Sphenodus longidens Agassiz, 1843
Sphenodus lundgreni Davis, 1890
Sphenodus macer Quenstedt, 1852
Sphenodus nitidus Wagner, 1862
Sphenodus longidens Agassiz, 1843
Sphenodus planus Agassiz, 1843
Sphenodus rectidens Emmons, 1858
Sphenodus robustidens Seguenza, 1900
Sphenodus tithonius Gemmellaro, 1871
Sphenodus virgai Gemmellaro, 1871
?Family Komoksodontidae Cappetta, Morrison & Adnet, 2019[14]
Komoksodon Cappetta, Morrison & Adnet, 2019
Komoksodon kwutchakutch Cappetta, Morrison & Adnet, 2019
?Family Paraorthacodontidae
Macrourogaleus Fowler, 1947
Macrourogaleus hassei
Paraorthacodus Glückman, 1957
Paraorthacodus andersoni (Case, 1978)
Paraorthacodus antarcticus Klug, Kriwet, Lirio & Nuñez, 2008
Paraorthacodus arduennae Delsate, 2001
Paraorthacodus clarkii (Eastman, 1901)
Paraorthacodus conicus (Davis, 1890)
Paraorthacodus eocaenus (Leriche, 1902)
Paraorthacodus jurensis (Schweizer, 1964)
Paraorthacodus recurvus (Trautschold, 1877)
Paraorthacodus rossi Cappetta, Morrison & Adnet, 2019
Paraorthacodus turgaicus Glikman, 1964
Species
| Family | Image | Common name | Genera | Species | Description |
|---|---|---|---|---|---|
| Chlamydoselachidae |  |
Frilled sharks | 1 extant 1 extinct |
2 extant 12 extinct |
Frilled sharks contain only two extant species of deepsea creatures which are typically weakened in areas closer to the surface. The most widely known species still surviving is the frilled shark, known as a living fossil, along with the Southern African frilled shark, found along coastal areas of South Africa. Several extinct species are known. |
| †Crassodontidanidae | †Crassodontidanidae | 4 | 8 | Extinct | |
| Hexanchidae |  |
Cow sharks | 3 extant 5 extinct |
5 extant 31 extinct |
Cow sharks are considered the most primitive of all the sharks, because their skeletons resemble those of ancient extinct forms, with few modern adaptations. Their excretory and digestive systems are also unspecialised, suggesting that they may also resemble those of their primitive shark ancestors. Their most distinctive feature, however, is the presence of a sixth, and, in two genera, a seventh, gill slit, in addition to the five found in all other sharks.[15] They range from 1.4 metres (4.6 ft) to over 5.5 metres (18 ft) in adult body length. |
| †Komoksodontidae? | †Komoksodontidae? | 1 | 1 | Extinct | |
| †Orthacodontidae? |  |
†Orthacodontidae? | 2 | 12 | Extinct |
| †Paraorthacodontidae? |  |
†Paraorthacodontidae? | 2 | 11 | Extinct |
See also
Time range of Hexanchiformes species
Footnotes
The Chimaeras (Holocephali) are arguably more primitive than the Hexanchiformes, but arguably may not be sharks, depending on whether "sharks" are taken to be superorder Selachimorpha or instead class Chondrichthyes (all cartilaginous fish).
References
Froese, Rainer; Pauly, Daniel (eds.). "Order Hexanchiformes". FishBase. January 2009 version.
Allen, Thomas B. (1999). The Shark Almanac. New York: The Lyons Press. p. 45. ISBN 1-55821-582-4.
Vélez-Zuazo, Ximena; Agnarsson, Ingi (2011-02-01). "Shark tales: A molecular species-level phylogeny of sharks (Selachimorpha, Chondrichthyes)". Molecular Phylogenetics and Evolution. 58 (2): 207–217. doi:10.1016/j.ympev.2010.11.018. ISSN 1055-7903.
Tanaka, Keiko; Shiina, Takashi; Tomita, Taketeru; Suzuki, Shingo; Hosomichi, Kazuyoshi; Sano, Kazumi; Doi, Hiroyuki; Kono, Azumi; Komiyama, Tomoyoshi; Inoko, Hidetoshi; Kulski, Jerzy K.; Tanaka, Sho (2013-09-05). "Evolutionary Relations of Hexanchiformes Deep-Sea Sharks Elucidated by Whole Mitochondrial Genome Sequences". BioMed Research International. 2013: e147064. doi:10.1155/2013/147064. ISSN 2314-6133. PMC 3780621.
Compagno, Leonard J. V.; Dando, Marc; Fowler, Sarah L.; Compagno, Leonard; Fowler, Sarah (2005). A field guide to the Sharks of the world. Collins field guide. London: Collins. ISBN 978-0-00-713610-0.
Adnet, S.; Guinot, G.; Cappetta, H.; Welcomme, J.-L. (2012). "Oldest evidence of bramble sharks (Elasmobranchii, Echinorhinidae) in the Lower Cretaceous of southeast France and the evolutionary history of orbitostylic sharks". Cretaceous Research. 35: 81–87. doi:10.1016/j.cretres.2011.11.021. ISSN 0195-6671.
Klug, Stefanie; Kriwet, Jürgen (2013). "Node age estimations and the origin of angel sharks, Squatiniformes (Neoselachii, Squalomorphii)". Journal of Systematic Palaeontology. 11 (1): 91–110. doi:10.1080/14772019.2012.674066. ISSN 1477-2019.
Long, John; Thomson, Victoria; Burrow, Carole; Turner, Susan (2021-10-28), Pradel, Alan; Denton, John S.S.; Janvier, Philippe (eds.), "Fossil chondrichthyan remains from the Middle Devonian Kevington Creek Formation, South Blue Range, Victoria" (PDF), Ancient Fishes and their Living Relatives, Munich, Germany: Verlag, Dr Friedrich Pfeil, pp. 239–245, ISBN 978-3-89937-269-4, retrieved 2023-11-30
Cappetta, Henri; Morrison, Kurt; Adnet, Sylvain (2019-12-10). "A shark fauna from the Campanian of Hornby Island, British Columbia, Canada: an insight into the diversity of Cretaceous deep-water assemblages". Historical Biology. 33 (8): 1121–1182. doi:10.1080/08912963.2019.1681421. ISSN 0891-2963.
"Sphenodus hybodoides | Shark-References". shark-references.com. Retrieved 2023-12-01.
"Paraorthacodus andersoni | Shark-References". shark-references.com. Retrieved 2023-12-01.
"New shark species confirmed: Genetic testing finds a different sixgill shark". ScienceDaily. Retrieved 2020-08-21.
"New species of shark discovered through genetic testing". phys.org. Retrieved 2020-08-21.
Cappetta, Henri; Morrison, Kurt; Adnet, Sylvain (2019-12-10). "A shark fauna from the Campanian of Hornby Island, British Columbia, Canada: an insight into the diversity of Cretaceous deep-water assemblages". Historical Biology. 33 (8): 1121–1182. doi:10.1080/08912963.2019.1681421. ISSN 0891-2963.
Matt's, J. & Last P.R. (1998). Paxton, J.R. & Eschmeyer, W.N. (eds.). Encyclopedia of Fishes. San Diego: Academic Press. p. 61. ISBN 0-12-547665-5.
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