The Hexanchiformes /hɛkˈsæŋkɪfɔːrmiːz/ 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.
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.
- Notorynchus <small>Ayres, 1855</small>
- Notorynchus cepedianus <small>(Péron, 1807)</small> (broadnose sevengill shark)
Extinct species
thumb|Notidanodon sp. fossil at the Geological Museum, Copenhagen
- Suborder Chlamydoselachoidi
- Family Chlamydoselachidae
- Chlamydoselachus <small>Garman, 1884</small>
- Chlamydoselachus balli <small>Cappetta, Morrison & Adnet, 2019</small>
- Chlamydoselachus gracilis <small>Antunes & Cappetta, 2002</small>
- Chlamydoselachus lawleyi <small>Davis, 1887</small>
- Chlamydoselachus tobleri <small>Leriche, 1929</small>
- Dykeius <small>Cappetta, Morrison & Adnet, 2019</small>
- Dykeius garethi <small>Cappetta, Morrison & Adnet, 2019</small>
- Rolfodon <small>Cappetta, Morrison & Adnet, 2019</small>
- Rolfodon bracheri <small>(Pfeil, 1983)</small>
- Rolfodon fiedleri <small>(Pfeil, 1983)</small>
- Rolfodon goliath <small>(Antunes & Cappetta, 2002)</small>
- Rolfodon keyesi <small>(Mannering & Hiller 2008)</small>
- Rolfodon landinii <small>(Carrillo-Briceño et al. 2014)</small>
- Rolfodon ludvigseni <small>Cappetta, Morrison & Adnet, 2019</small>
- Rolfodon tatere <small>(Consoli, 2008)</small>
- Rolfodon thomsoni <small>(Richter & Ward, 1990)</small>
- Suborder Hexanchoidei
- Family Crassodontidanidae
- Crassodontidanus <small>Kriwet & Klug, 2011</small>
- Crassodontidanus serratus <small>Fraas, 1855</small>
- Crassodontidanus wiedenrothi <small>Thies, 1983</small>
- Notidanoides <small>Maisey, 1986</small>
- Notidanoides muensteri <small>Agassiz, 1843</small>
- Notidanus <small>Cuvier, 1816</small>
- Notidanus amalthei <small>Oppel, 1854</small>
- Notidanus atrox <small>Ameghino, 1899 </small>
- Notidanus intermedius <small>Wagner, 1862</small>
- Notidanus nikitini <small>Chabakov & Zonov, 1935</small>
- Pachyhexanchus <small>Cappetta, 1990</small>
- Pachyhexanchus pockrandti <small>Ward & Thies, 1987</small>
- Family Hexanchidae
- Gladioserratus <small>Underwood, Goswami, Prasad, Verma & Flynn, 2011</small>
- Gladioserratus aptiensis <small>Pictet, 1864</small>
- Gladioserratus dentatus <small>Guinot, Cappetta & Adnet, 2014</small>
- Gladioserratus magnus <small>Underwood, Goswami, Prasad, Verma & Flynn, 2011</small>
- Heptranchias <small>Rafinesque, 1810</small>
- Heptranchias ezoensis <small>Applegate & Uyeno, 1968</small>
- Heptranchias howellii <small>Reed, 1946</small>
- Heptranchias karagalensis <small>Kozlov in Zhelezko & Kozlov, 1999</small>
- Heptranchias tenuidens <small>Leriche, 1938</small>
- Hexanchus <small>Rafinesque, 1810</small>
- Hexanchus agassizi <small>Cappetta, 1976</small>
- Hexanchus andersoni <small>Jordan, 1907</small>
- Hexanchus casieri <small>Kozlov, 1999</small>
- Hexanchus collinsonae <small>Ward, 1979</small>
- Hexanchus gracilis <small>Davis, 1887</small>
- Hexanchus hookeri <small>Ward, 1979</small>
- Hexanchus microdon <small>Agassiz, 1843</small>
- Hexanchus tusbairicus <small>Kozlov in Zhelezko & Kozlov, 1999</small>
- Notidanodon <small>Cappetta, 1975</small>
- Notidanodon lanceolatus <small>Woodward, 1886</small>
- Notidanodon pectinatus <small>Agassiz, 1843</small>
- Notorynchus <small>Ayres, 1855</small>
- Notorynchus borealus <small>Jordan & Hannibal, 1923</small>
- Notorynchus kempi <small>Ward, 1979</small>
- Notorynchus lawleyi <small>Cigala Fulgosi, 1983</small>
- Notorynchus primigenius <small>Agassiz, 1843</small>
- Notorynchus serratissimus <small>Agassiz, 1843</small>
- Notorynchus subrecurvus <small>Oppenheimer, 1907</small>
- Pachyhexanchus <small>Cappetta, 1990</small>
- Pachyhexanchus pockrandti <small>Ward & Thies, 1987</small>
- Paraheptranchias <small>Pfeil, 1981</small>
- Paraheptranchias repens <small>Probst, 1879</small>
- Pseudonotidanus <small>Underwood & Ward, 2004</small>
- Pseudonotidanus semirugosus <small>Underwood & Ward, 2004</small>
- Welcommia <small>Cappetta, 1990</small>
- Welcommia bodeuri <small>Cappetta, 1990</small>
- Welcommia cappettai <small>Klug & Kriwet, 2010</small>
- Weltonia <small>Ward, 1979</small>
- Weltonia ancistrodon <small>Arambourg, 1952</small>
- Weltonia burnhamensis <small>Ward, 1979</small>
- Xampylodon <small>Cappetta, Morrison & Adnet, 2019</small>
- Xampylodon brotzeni (<small>Siverson, 1995)</small>
- Xampylodon dentatus (<small>Woodward, 1886)</small>
- Xampylodon loozi (<small>Vincent, 1876)</small>
- ?Family Orthacodontidae
- Occitanodus <small>Guinot, Cappetta & Adnet, 2014</small>
- Occitanodus sudrei <small>Guinot, Cappetta & Adnet, 2014</small>
- Sphenodus <small>Agassiz, 1843</small>
- Sphenodus alpinus <small>Gümbel, 1861</small>
- Sphenodus longidens <small>Agassiz, 1843</small>
- Sphenodus lundgreni <small>Davis, 1890</small>
- Sphenodus macer <small>Quenstedt, 1852</small>
- Sphenodus nitidus <small>Wagner, 1862</small>
- Sphenodus longidens <small>Agassiz, 1843</small>
- Sphenodus planus <small>Agassiz, 1843</small>
- Sphenodus rectidens <small>Emmons, 1858</small>
- Sphenodus robustidens <small>Seguenza, 1900</small>
- Sphenodus tithonius <small>Gemmellaro, 1871</small>
- Sphenodus virgai <small>Gemmellaro, 1871</small>
- ?Family Komoksodontidae <small>Cappetta, Morrison & Adnet, 2019</small> They range from to over in adult body length.
|-
! style="background:rgb(150,80,150)" | <span style="color:white;">†Komoksodontidae</span>?
|
| align=center | †Komoksodontidae?
| align=center | 1
| align=center | 1
| valign=top | Extinct
|-
! style="background:rgb(150,80,150)" | <span style="color:white;">†Orthacodontidae?</span>
| frameless|138x138px
| align=center | †Orthacodontidae?
| align=center | 2
| align=center | 12
| valign=top | Extinct
|-
! style="background:rgb(150,80,150)" | <span style="color:white;">†Paraorthacodontidae?</span>
| frameless|139x139px
| align=center | †Paraorthacodontidae?
| align=center | 2
| align=center | 11
| valign=top | Extinct
|-
|}
See also
- Time range of Hexanchiformes species