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 tissue. ...

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 Sawfish, also known as carpenter sharks, are a family of rays characterized by a long, narrow, flattened rostrum, or nose extension, lined with sharp transverse teeth, arranged in a way that resembles a saw. They are among the largest fish ...

(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 scale A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as ...

s 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

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

s. The pelvic fins in males are modified to create

clasper In biology, a clasper is a male anatomical structure found in some groups of animals, used in mating. Male cartilaginous fish have claspers formed from the posterior portion of their pelvic fin which serve to channel semen into the female's c ...

s 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 primitiv ...

(the clade containing

chimaera Chimaeras are cartilaginous fish in the order Chimaeriformes , known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Opisthoproctidae, or Siganidae, respectively. At ...

s 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, wh ...

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 Paleo ...

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 in ...

during the

Jurassic The Jurassic ( ) is a geologic period and 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 Jurassic constitutes the middle period of ...

. 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 pe ...

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

(

s). Members of the elasmobranchii subclass have no swim bladders, five to seven pairs of

clefts opening individually to the exterior, rigid

s, and small

s. 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 ''tapetum lucidum'' ( ; ; ) is a layer of tissue in the eye of many vertebrates and some other animals. Lying immediately behind the retina, it is a retroreflector. It reflects visible light back through the retina, increasing the light a ...

. The inner margin of each pelvic fin in the male fish is grooved to constitute a

for the transmission of

sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, ...

. 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 ...

waters. Many fish maintain buoyancy with

swim bladder 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 wit ...

s. 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 The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...

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 Cladoselachidae is an extinct family of holocephalian cartilaginous fish Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes o ...

**†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 ...

***†Division Hybodonta ****†Order Hybodontiformes ***Division Neoselachii ****†Order

Synechodontiformes Synechodontiformes is an extinct order of prehistoric sharks, known from the Permian to the Paleogene. They are considered to be members of Neoselachii, the group that contains modern sharks and rays. Their placement in the group is uncertain, s ...

****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 Selachimor ...

(Selachimorpha) (modern sharks) ******Order Echinorhiniformes (bramble sharks) *****Superorder

Galeomorphii Galeomorphii is a superorder of cartilaginous fishes which includes all modern sharks except the dogfish and its relatives. They are sometimes called galea or galean sharks. There are about 300 living species in 23 families. Galean sharks are d ...

******Order Heterodontiformes (bullhead sharks) ******Order Orectolobiformes (carpet sharks) ******Order

Lamniformes The Lamniformes (, from Greek ''lamna'' "fish of prey") are an order of sharks commonly known as mackerel sharks (which may also refer specifically to the family Lamnidae). It includes some of the most familiar species of sharks, such as the gre ...

(mackerel sharks) ******Order

Carcharhiniformes Carcharhiniformes , the ground sharks, are the largest order of sharks, with over 270 species. They include a number of common types, such as catsharks, swellsharks, and the sandbar shark. Members of this order are characterized by the presenc ...

(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 living ...

******Order

Hexanchiformes The Hexanchiformes are the order consisting of the most primitive types of sharks, and numbering just seven extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens. Hexanch ...

(frilled and cow sharks) ******Order

Squaliformes The Squaliformes are an order of sharks that includes about 126 species in seven families. Members of the order have two dorsal fins, which usually possess spines, they usually have a sharp head, no anal fin or nictitating membrane, and fiv ...

(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 Sawfish, also known as carpenter sharks, are a Family (biology), family of batoidea, rays characterized by a long, narrow, flattened rostrum (anatomy), rostrum, or nose extension, lined with sharp transverse plane, transverse teeth, arranged i ...

(sawfishes) ******Order Rajiformes (skates and relatives) ******Order

Myliobatiformes Myliobatiformes () is one of the four orders of batoids, cartilaginous fishes related to sharks. They were formerly included in the order Rajiformes, but more recent phylogenetic studies have shown the myliobatiforms to be a monophyletic grou ...

(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