Коровы-акулы: древнейшие представители семейства гексанкховых

Family of sharks
Automatic taxobox
| name = Cow sharks
| image = Six gill shark. jpg
| taxon = Hexanchidae
| fossil range = Fossil range|Upper Jurassic|Recent|earliest=Late Permian|ref=

Cow sharks are considered the most primitive of all the sharks, as their skeletons resemble those of ancient extinct forms, with few modern adaptations. Their excretory and digestive systems are also unspecialized, suggesting they may resemble those of primitive shark ancestors. A possible hexanchid tooth is known from the Permian of Japan, making the family a possible extant survivor of the Permian–Triassic extinction. Their most distinctive feature, however, is the presence of a sixth, and, in two genera, a seventh, gill slit, in contrast to the five found in all other sharks. The first pair are not connected across the throat. They range from in adult body length. These cylindrical sharks have a ventral mouth with compressed, comb like teeth in the lower jaw and smaller, pointed teeth in the upper jaw. They have a short, angular and spinless dorsal fin. The pelvic fins are smaller than the angular pectoral fins. The caudal fin has a notch towards the end.

The only fossil records of the cow shark consist of mainly only isolated teeth. Although skeletal remains for this species have been found from the Jurassic time period, these have been very rare and have only been found in the "Late Jurassic lithographic limestones of South Germany, Nusplingen, Solnhofen, and late Cretaceous calcareous sediments of Lebanon." Due to these sparse records some scientists conclude that the cow shark is now a more "diverse and numerous species".