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]
^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.
^ a bVélez-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.
^ a bTanaka, 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.
^ a bCompagno, 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.
^"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.
External links
Froese, Rainer; Pauly, Daniel (eds.). "Family Chlamydoselachidae". FishBase. February 2011 version. (Fish Base family reference)
Froese, Rainer; Pauly, Daniel (eds.). "Family Hexanchidae". FishBase. February 2011 version. (Fish Base family reference)
