TheHexanchiformes/hɛkˈsæŋkɪfɔːrmiːz/ are a primitiveorder ofsharks, numbering just five extantspecies in twofamilies,Chlamydoselachidae andHexanchidae. 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.[2][3]
Due to their primitive anatomy, hexanchiforms were previously considered the mostbasal group of sharks. However, more recent phylogenetic studies indicate that while primitive, they in fact belong to the superorderSqualomorphi, which also containsdogfishes,angelsharks, andsawsharks, although they are thought to be the most basal member of the group.[4][5]
Hexanchiform sharks have one spinelessdorsal 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 sevengill slits, located in front of the pectoral fins. They have a large mouth, with eyes on either side of the head. Thespiracles are small and located well above and behind the eyes.[6] The eyes have nonictitating membrane.
Thefrilled sharks of the genusChlamydoselachus are very different from thecow 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.[4][5]
Shark teeth similar to modern hexanchids andechinorhinids are known fromDevonian deposits inAntarctica andAustralia, as well asPermian deposits inJapan. If these are in fact hexanchids, this may be the onlyextant order ofelasmobranchs to have survived thePermian extinction (and by extension, the oldest extant order of elasmobranchs). However, the Australian/Antarctic shark teeth, from the familyMcmurdodontidae, have also been found to lack a multilayerenameloid 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.[7][8][9]
It is debated whether the extinct familiesOrthacodontidae and Paraorthacodontidae belong to the Hexanchiformes or the extinctSynechodontiformes. However, the Shark-References database currently lists them as members of the Hexanchiformes.[10][11][12]
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.[6]
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.[2]
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 thefrilled shark, known as a living fossil, along with theSouthern African frilled shark, found along coastal areas of South Africa. Several extinct species are known.
Cow sharks are considered the most primitive of all the sharks, because their skeletons resemble those of ancient extinct forms, with few modern adaptations. Theirexcretory anddigestive 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.[15] They range from 1.4 metres (4.6 ft) to over 5.5 metres (18 ft) in adult body length.
^abCompagno, 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.ISBN978-0-00-713610-0.
^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.ISSN0195-6671.
^abCappetta, 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.ISSN0891-2963.
