Шарки отряда Гексанкхообразные: особенности и эволюция.

Order of sharks

The Hexanchiformes /h//E//k//'//s//ae//N//k//ᵻ//f//ɔr//m//iː//z/ are a primitive order of sharks, numbering just seven extant species in two families. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens.

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

Hexanchiform sharks have one spineless dorsal 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 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 others' 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.

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