Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including sharks (superorder Selachii), rays, skates, and sawfish (superorder Batoidea). Members of this subclass are characterised by having five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins 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

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 term, ...

s. 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 (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, whe ...

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 d ...

date back to the Cretaceous, or even earlier. Many species became extinct during the Permian 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 name Elasmobranchii comes from the Ancient Greek 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 Chondrichthyes, the other being Holocephali (

s). 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 gill clefts opening individually to the exterior, rigid dorsal fins, 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 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, whi ...

. These fish are widely distributed in tropical and temperate waters. Many fish maintain buoyancy with

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

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 ''Carbonifero ...

period, the sharks underwent a period of diversification, with many new forms evolving. Many of these became extinct during the Permian, but the remaining sharks underwent a second burst of

during the Jurassic, around which time the skates and rays first appeared. Many surviving orders of elasmobranch date back to the Cretaceous, 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 The Squaliformes are an order (biology), 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 me ...

(dogfish sharks) ******†Order Protospinaciformes ******Order Squatiniformes (angel sharks) ******Order Pristiophoriformes (sawsharks) ****Subdivision Batoidea (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 (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