The Shark Order Hexanchiformes
Hexanchiformes is the order consisting of the most primitive types of sharks, and numbering just five extant species (still in existence; not extinct). Fossil sharks that were apparently very similar to modern seven-gill species are known from Jurassic specimens. Their ancient lineage goes back 150 million years. The hexanchiform fossil record indicates that they were never very diverse but more so than the current period, as there are only five living species. The five extant species of hexanchiforms are divided into two families: Chlamydoselachidae and Hexanchidae. The latter family also is known as "cowsharks". There has been different opinions as to whether the frilled shark is actually part of the Hexanchiformes or belongs in an order of its own; however, derived features shared with other hexanchiforms support its placement within the order.
The Hexanchiform families are very distinct in their physical shape and size. Chlamydoselachus is a modified and unique shark. It has an eel like body, an enlarged mouth, and well-delimited rows of teeth (teeth unlike any other shark). Like the hexanchids it has a single, posteriorly located dorsal fin and a long caudal fin, an extra gill arch (hexanchiforms have either six or seven gill arches with gill slits on each side), small spiracles, a demarcated lateral line along the trunk, precaudal tail regions, and a mouth extending posteriorly behind the level of the eyes. Hexanchids have unique teeth that are differentiated between upper and lower jaws and also along either jaw.
Hexanchiforms are unusual for having mostly uncalcified vertebrae, which is a primitive vertebra characteristic, and notochords with little constriction. Notochords are a flexible rod-like structure that forms the main support of the body.
Chlamydoselachus is a uniform dark brown to grayish in color, the hexanchids are mostly gray without strong color patterns. A specific difference is Notorynchus, which has darker spots. Notorynchus and Hexanchiform sharks have only one dorsal fin, either six or seven gill slits and o nictitating membrane in the eyes, a thin protective membrane located at the inner corner of the eye, beneath the lower eyelid, also called the "third eyelid."
The frilled shark, Chlamydoselachus anguineus,, is very different from the cow sharks, and has been proposed it be moved to its own order Chlamydoselachiformes.
They are found worldwide in tropical and temperate waters, they are known without records from many isolated regions since most hexanchiform sharks occur along the continental slopes their scattered distribution maybe only a sampling. However, most species are deepwater along the continental slopes and somethimes venturing into mor shallow pelagic, open sea, away from the sea bottom, or inshore waters. The exception is Notorynchus cepedianus, that is a coastal species. There are shallow water records for Heptranchias perlo and Hexanchus griseus also, however these species are more common in waters deeper than 328 feet. Some species have been recorded from oceanic islands.
Six-gill and seven-gill sharks feed on a variety of bony fishes, sharks, rays and invertebrates. Some hexanchiform species are known to have attacked individuals of the same species that have been hooked, biting off pieces while they were being reeled or towed in. At least one species may hunt in packs. Hexanchiform sharks are thouhgt consumed by larger sharks, including their own species.
All species are ovoviviparous. Litters can be very large in the two largest species (more than 100 pups may be born at once to the bluntnose six-gill shark. The amount of pups vary in other species Females of the two largest species give birth in shallow water nurseries. Almost nothing is known concerning gestation periods.
The Order Hexanchiformes has two families
Family Chlamydoselachus anguineus - Frilled Sharks
|species in the family Chlamydoselachus|
|Chlamydoselachus anguineus||Frilled shark|
Family Hexanchidae - Sixgill and Sevengill Sharks
|species in the family Heptranchias perlo - Sharpnose sevengill shark|
|Hexanchus griseus||Bluntnose sixgill shark|
|Hexanchus vitulus||Bigeye sixgill shark|
|Notorynchus cepedianus||Broadnose or spotted sevengill shark|