Hexanchiformes

The Hexanchiformes are a primitive order of sharks, numbering just five extant species in two families, Chlamydoselachidae and Hexanchidae. Chlamydoselachidae are also known as frilled sharks, these sharks are very rare fishes and typically reside in deeper waters. Hexanchidae are also known as cow sharks and are the lesser known of the two types of Hexanchiformes and also reside in deep waters.

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 Squalomorphi, which also contains dogfishes, angelsharks, and sawsharks, although they are thought to be the most basal member of the group.

Description

Hexanchiform sharks have one spineless dorsal fin located over or behind the pelvic fins. Therefore, they do not have a "main dorsal fin", that is, the large dorsal fin located towards the middle of the back, unlike other orders of sharks. 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. 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 other's closest relatives, and they share certain derived features supporting them both being in the same order.

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.

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.

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.

Reproductive biology

Hexanchiforms are viviparous, meaning they give birth to live young. Males have two testes which are capable of producing sperm year-round and females have two ovaries and two uteri. Chlamydoselachus africana Males have two testes which produce sperm and females have two ovaries and ovulate from summer to autumn. Embryos develop only in the right uterus of a female. The research regarding the reproductive Biology of the Hexanchidae family is limited but thought to be similar, as no year-round research has been done regarding female hexanchids.

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)
Notorynchus Ayres, 1855
Notorynchus cepedianus (PÃÂ©ron, 1807) (broadnose sevengill shark)

Extinct species

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-BriceÃÂ±o 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 GÃÂ¼mbel, 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
Komoksodon Cappetta, Morrison & Adnet, 2019
Komoksodon kwutchakutch Cappetta, Morrison & Adnet, 2019
?Family Paraorthacodontidae
Macrourogaleus Fowler, 1947
Macrourogaleus hassei
Paraorthacodus GlÃÂ¼ckman, 1957
Paraorthacodus andersoni (Case, 1978)
Paraorthacodus antarcticus Klug, Kriwet, Lirio & NuÃÂ±ez, 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

Footnotes

References

External links

(Fish Base family reference)
(Fish Base family reference)