The Hexanchiformes /hɛkˈsæŋkɪfɔːrmiːz/ are a primitive

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

s, numbering just five extant

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

in two

families Family (from ) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictability, structure, and safety as ...

Chlamydoselachidae Chlamydoselachidae is a family of primitive deep-sea sharks in the order Hexanchiformes. They are one of only two extant families in the order alongside the cow sharks in the family Hexanchidae, and the only members of the suborder Chlamydosela ...

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 Squalomorphi is a superorder of sharks, generally characterized by lacking traits such as an anal fin, nictitating membrane, or suborbital shelves in the cranium. Squalomorphs are also called squalea, or squaleans. There are about 163 living spec ...

, which also contains dogfishes,

angelshark Angel sharks are sharks belonging to the genus ''Squatina''. They are the only living members of the family Squatinidae and order Squatiniformes. They commonly inhabit sandy seabeds close to in depth. ''Squatina'' and other Squatiniformes d ...

s, and

sawshark A sawshark or saw shark is a member of a shark order (Pristiophoriformes ) bearing a unique long, saw-like rostrum (snout or bill) edged with sharp teeth, which they use to slash and disable their prey. There are eight species within the Pristi ...

s, although they are thought to be the most basal member of the group.

Description

Hexanchiform sharks have one spineless

dorsal fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...

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 slit Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In c ...

s, located in front of the pectoral fins. They have a large mouth, with eyes on either side of the head. The spiraclesare small and located well above and behind the eyes. The eyes have no

nictitating membrane The nictitating membrane (from Latin '' nictare'', to blink) is a transparent or translucent third eyelid present in some animals that can be drawn across the eye from the medial canthus to protect and moisten it while maintaining vision. Most ...

. 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. Shark teeth similar to modern hexanchids and echinorhinids are known from

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

deposits in

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...

and

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

, as well as

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...

deposits in

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

. If these are in fact hexanchids, this may be the only

extant Extant or Least-concern species, least concern is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Exta ...

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 ''Mcmurdodus'' is an extinct genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification ...

, 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

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 Constantine Samuel Rafinesque-Schmaltz (; 22 October 178318 September 1840) was a French early 19th-century polymath born near Constantinople in the Ottoman Empire and self-educated in France. He traveled as a young man in the United States, ult ...

, 1810 *** '' Heptranchias perlo'' ( Bonnaterre, 1788) (sharpnose sevengill shark) ** '' Hexanchus''

, 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

***'' 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 ''Rolfodon'' is an extinct genus of shark in the family Chlamydoselachidae. It is closely related to the extant frilled sharks in the genus '' Chlamydoselachus'', which it can be differentiated from by tooth morphology. It is named after late C ...

'' 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 Crassodontidanidae is a family of extinct cow sharks that lived from the Early Jurassic to the Early Cretaceous. It contains three genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological class ...

***'' Crassodontidanus'' Kriwet & Klug, 2011 ****''Crassodontidanus serratus'' Fraas, 1855 **** ''Crassodontidanus wiedenrothi'' Thies, 1983 *** ''Notidanoides'' Maisey, 1986 ****''Notidanoides muensteri''

Agassiz Jean Louis Rodolphe Agassiz ( ; ) FRS (For) FRSE (May 28, 1807 – December 14, 1873) was a Swiss-born American biologist and geologist who is recognized as a scholar of Earth's natural history. Spending his early life in Switzerland, he recei ...

, 1843 *** ''Notidanus''

Cuvier Jean Léopold Nicolas Frédéric, baron Cuvier (23 August 1769 – 13 May 1832), known as Georges Cuvier (; ), was a French natural history, naturalist and zoology, zoologist, sometimes referred to as the "founding father of paleontology". Cuv ...

, 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''

, 1810 ****''Heptranchias ezoensis'' Applegate & Uyeno, 1968 **** ''Heptranchias howellii'' Reed, 1946 **** ''Heptranchias karagalensis'' Kozlov in Zhelezko & Kozlov, 1999 **** ''Heptranchias tenuidens'' Leriche, 1938 *** '' Hexanchus''

, 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''

, 1843 **** ''Hexanchus tusbairicus'' Kozlov in Zhelezko & Kozlov, 1999 *** '' Notidanodon'' Cappetta, 1975 **** ''Notidanodon lanceolatus'' Woodward, 1886 **** ''Notidanodon pectinatus''

, 1843 *** '' Notorynchus'' Ayres, 1855 ****''Notorynchus borealus'' Jordan & Hannibal, 1923 **** ''Notorynchus kempi'' Ward, 1979 **** ''Notorynchus lawleyi'' Cigala Fulgosi, 1983 **** ''Notorynchus primigenius''

, 1843 **** ''Notorynchus serratissimus''

, 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 ''Welcommia'' is an extinct genus of shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the ...

'' 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''

, 1843 ****''Sphenodus alpinus'' Gümbel, 1861 **** ''Sphenodus longidens''

, 1843 **** ''Sphenodus lundgreni'' Davis, 1890 **** ''Sphenodus macer'' Quenstedt, 1852 **** ''Sphenodus nitidus'' Wagner, 1862 **** ''Sphenodus longidens''

, 1843 **** ''Sphenodus planus''

, 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

Footnotes

References

External links

* (Fish Base family reference) * (Fish Base family reference) {{Authority control Extant Middle Jurassic first appearances Cartilaginous fish orders Taxa named by Fernando de Buen y Lozano