Hexanchiformes

The Hexanchiformes /hɛkˈsæŋkɪfɔːrmiːz/ are a primitive order of sharks, numbering just seven 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 (Péron, 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-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 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 1extant1extinct 2extant12extinct 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 3extant5extinct 5extant31extinct 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. They range from to over in adult body length. †Komoksodontidae? †Komoksodontidae? 1 1 Extinct †Orthacodontidae? †Orthacodontidae? 2 12 Extinct †Paraorthacodontidae? †Paraorthacodontidae? 2 11 Extinct

Hexanchiformes

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