Elasmobranchii () is a subclass of

Chondrichthyes Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissu ...

or cartilaginous fish, including

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...

s (superorder Selachii), rays, skates, and sawfish (superorder Batoidea). Members of this subclass are characterised by having five to seven pairs of

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...

clefts opening individually to the exterior, rigid

dorsal fin A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through c ...

s and small placoid scales on the skin. The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper. The details of this jaw anatomy vary between species, and help distinguish the different elasmobranch clades. The pelvic fins in males are modified to create claspers for the transfer of sperm. There is no swim bladder; instead, these fish maintain buoyancy with large livers rich in oil. The definition of the clade is unclear with respect to fossil chondrichthyans. It has been used by different authors as equivalent to Neoselachii (the clade including modern sharks and rays and their last common ancestor) or for all chondrichthyans more closely related to modern sharks and rays than to

Holocephali Holocephali ("complete heads"), sometimes given the term Euchondrocephali, is a subclass of cartilaginous fish in the class Chondrichthyes. The earliest fossils are of teeth and come from the Devonian period. Little is known about these primit ...

(the clade containing chimaeras and their extinct relatives). The earliest elasmobranch fossils came from the

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, w ...

and many surviving

orders Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

date back to the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

, or even earlier. Many species became extinct during the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Pale ...

and there was a burst of

adaptive radiation In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic int ...

during the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

. The name Elasmobranchii comes from the

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

words ''elasmo-'' ("plate") and ''bránchia'' ("gill"), referring to the broad, flattened gills which are characteristic of these fishes.

Description

Elasmobranchii is one of the two subclasses of cartilaginous fish in the class

, the other being

( chimaeras). Members of the elasmobranchii subclass have no

swim bladders The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wi ...

, five to seven pairs of

clefts opening individually to the exterior, rigid

s, and small placoid scales. The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper. Extant elasmobranchs exhibit several archetypal jaw suspensions: amphistyly, orbitostyly, hyostyly, and euhyostyly. In amphistyly, the palatoquadrate has a postorbital articulation with the chondrocranium from which ligaments primarily suspend it anteriorly. The hyoid articulates with the mandibular arch posteriorly, but it appears to provide little support to the upper and lower jaws. In orbitostyly, the orbital process hinges with the orbital wall and the hyoid provides the majority of suspensory support. In contrast, hyostyly involves an ethmoid articulation between the upper jaw and the cranium, while the hyoid most likely provides vastly more jaw support compared to the anterior ligaments. Finally, in euhyostyly, also known as true hyostyly, the mandibular cartilages lack a ligamentous connection to the cranium. Instead, the hyomandibular cartilages provide the only means of jaw support, while the ceratohyal and basihyal elements articulate with the lower jaw, but are disconnected from the rest of the hyoid. The eyes have a tapetum lucidum. The inner margin of each pelvic fin in the male fish is grooved to constitute a clasper for the transmission of sperm. These fish are widely distributed in

tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

and

temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...

waters. Many fish maintain buoyancy with swim bladders. However elasmobranchs lack swim bladders, and maintain buoyancy instead with large livers that are full of oil. This stored oil may also function as a nutrient when food is scarce.Hoenig, J.M. and Gruber, S.H. (1990
"Life-history patterns in the elasmobranchs: implications for fisheries management"
In: ''Elasmobranchs as living resources: advances in the biology, ecology, systematics and the status of the fisheries'', eds. J. H. L. Pratt, S. H. Gruber and T. Taniuchi, US Department of Commerce, NOAA technical report NMFS 90, pp.1–16. Deep sea sharks are usually targeted for their oil, because the livers of these species can weigh up to 20% of their total weight.Vannuccini, Stefania (2002

In: ''Shark Utilization, Marketing and Trade'', Fisheries Technical paper 389, FAO, Rome. .

Evolution

Fossilised shark teeth are known from the early

, around 400 million years ago. During the following Carboniferous period, the sharks underwent a period of diversification, with many new forms evolving. Many of these became extinct during the

, but the remaining sharks underwent a second burst of

during the

, around which time the skates and rays first appeared. Many surviving orders of elasmobranch date back to the

, or earlier.

Habitats

Elasmobranchs are mostly a marine taxon, but we know several species that live in freshwater environment (approximately 60 species which represent only 5% of the 1154 described species). They can be divided into two groups: The euryhaline elasmobranchs, which are marine species that may survive and reproduce in freshwater environments, and the obligated freshwater elasmobranchs, which only lives in freshwater environment their entire life. This group contains only one clade: the subfamily ''Potamotrygoninae''. This clade is endemic (i.e. exclusive) to one specific region: tropical, subtropical water and wetland of South America. Recent research in Paraná river have shown that obligated freshwater elasmobranchs were more susceptible to anthropogenic threats as overfishing and destruction of habitats due to the very small areas they live in compared to the marine species. New research has highlighted the importance of coastal wetlands, like mangroves and seagrasses, as habitats for many species of elasmobranch

Taxonomy

Compagno's 2005 ''Sharks of the World'' arranges the class as follows: *Subclass Elasmobranchii **†''Plesioselachus'' **†Order Squatinactiformes **†Order Protacrodontiformes **†Infraclass Cladoselachimorpha ***†Order Cladoselachiformes **†Infraclass Xenacanthimorpha ***†Order Xenacanthiformes **Infraclass Euselachii (sharks and rays) ***†Order

Ctenacanthiformes Ctenacanthiformes is an extinct order of chondrichthyan fish. They possessed ornamented fin spines and cladodont dentition. Members of the family Ctenacanthidae may have survived into the Cretaceous based on teeth found in deep water deposits of ...

***†Division Hybodonta ****†Order Hybodontiformes ***Division Neoselachii ****†Order Synechodontiformes ****Subdivision

Selachii Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorp ...

(Selachimorpha) (modern sharks) ******Order

Echinorhiniformes ''Echinorhinus'' is the only extant genus in the family Echinorhinidae. Taxonomy Echinorhinidae are traditionally classified in the order Squaliformes, together with kitefin and gulper sharks.Compagno, 2005. "Sharks of the World". However, a ...

(bramble sharks) *****Superorder Galeomorphii ******Order Heterodontiformes (bullhead sharks) ******Order

Orectolobiformes Carpet sharks are sharks classified in the order Orectolobiformes . Sometimes the common name "carpet shark" (named so because many species resemble ornately patterned carpets) is used interchangeably with "wobbegong", which is the common name ...

(carpet sharks) ******Order Lamniformes (mackerel sharks) ******Order Carcharhiniformes (ground sharks) *****Superorder

Squalomorphii Squalomorphii is a superorder of cartilaginous fishes, generally characterized by lacking traits such as an anal fin, nictitating membrane, or suborbital shelves in the cranium. Also called squalea, or squalean sharks. There are about 163 livi ...

******Order Hexanchiformes (frilled and cow sharks) ******Order Squaliformes (dogfish sharks) ******†Order Protospinaciformes ******Order Squatiniformes (angel sharks) ******Order Pristiophoriformes (sawsharks) ****Subdivision

Batoidea Batoidea is a superorder of cartilaginous fishes, commonly known as rays. They and their close relatives, the sharks, comprise the subclass Elasmobranchii. Rays are the largest group of cartilaginous fishes, with well over 600 species in 26 fa ...

(rays, skates, and sawfish) ******Order Torpediniformes (electric rays) ******Order Pristiformes (sawfishes) ******Order Rajiformes (skates and relatives) ******Order Myliobatiformes (stingrays and relatives) ****''Incertae sedis'' ***** Ptychodontidae Recent molecular studies suggest the Batoidea are not derived selachians as previously thought. Instead, skates and rays are a monophyletic superorder within Elasmobranchii that shares a common ancestor with the selachians.

References

External links

* Skaphandrus.co
Elasmobranchii
{{Taxonbar, from=Q194257 Wenlock first appearances Vertebrate subclasses Taxa named by Charles Lucien Bonaparte