Elasmobranchii

Elasmobranchii

Overview
Elasmobranchii is a subclass of Chondrichthyes
Chondrichthyes
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...

 or cartilaginous fish, that includes the shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s (Selachii) and the rays and skates
Batoidea
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...

 (Batoidea).


Fossilised shark teeth are known from the early Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

, around 400 million years ago. During the following Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 period, the sharks underwent a period of diversification, with many new forms evolving. Many of these became extinct during the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

, but the remaining sharks underwent a second burst of adaptive radiation
Adaptive radiation
In evolutionary biology, adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different...

 during the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

, around which time the skates and rays first appeared.
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Encyclopedia
Elasmobranchii is a subclass of Chondrichthyes
Chondrichthyes
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...

 or cartilaginous fish, that includes the shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s (Selachii) and the rays and skates
Batoidea
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...

 (Batoidea).

Evolution



Fossilised shark teeth are known from the early Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

, around 400 million years ago. During the following Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 period, the sharks underwent a period of diversification, with many new forms evolving. Many of these became extinct during the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

, but the remaining sharks underwent a second burst of adaptive radiation
Adaptive radiation
In evolutionary biology, adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different...

 during the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

, around which time the skates and rays first appeared. Many surviving orders of elasmobranch date back to the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

, or earlier.

Description


Elasmobranchii is one of the two subclasses of cartilaginous fish in the class Chondrichthyes
Chondrichthyes
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...

, the other being Holocephali
Holocephali
The subclass Holocephali is a taxon of cartilaginous fish, of which the order Chimaeriformes is the only surviving group.Holocephali has an extensive fossil record that starts during the Devonian period. However, most fossils are teeth, and the body forms of numerous species are not known, or, at...

 (chimaera
Chimaera
Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, ratfish , spookfish , or rabbitfishes...

s).

Members of the elasmobranchii subclass have no swim bladders, five to seven pairs of gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

 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
Tapetum lucidum
The tapetum lucidum is a layer of tissue in the eye of many vertebrate animals....

. The inner margin of each pelvic fin in the male fish is grooved to constitute a clasper for the transmission of sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...

. These fish are widely distributed in tropical and temperate
Temperate
In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...

 waters.

Taxonomy


Nelson's 2006 Fishes of the World arranges the class as follows:
  • Subclass Elasmobranchii
    • Plesioselachus
    • †Order Squatinactiformes
    • †Order Protacrodontiformes
    • †Infraclass Cladoselachimorpha
      • †Order Cladoselachiformes
        Cladoselachiformes
        Cladoselachidae is an extinct family of cartilaginous fishes and among the earliest predecessors of modern sharks. They are the only members of the order Cladoselachiformes and were characterized by having an elongated body with a spine in each of the two dorsal fins.-References:*, Dictionary of...

    • †Infraclass Xenacanthimorpha
      • †Order Xenacanthiformes
    • Infraclass Euselachii (sharks and rays)
      • †Order Ctenacanthiformes
      • †Division Hybodonta
        • †Order Hybodontiformes
          Hybodontiformes
          Hybodontiformes was an order of sharks, they were characterised by possessing 1-2 pairs of hooked protrusions on their heads.-External links:* http://www.palaeos.com/Vertebrates/Units/070Chondrichthyes/070.600.html#Hybodontiformes...

      • Division Neoselachii
        • Subdivision Selachii
          Shark
          Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

           (modern sharks)
          • Superorder Galeomorphi
            • Order Heterodontiformes (bullhead sharks)
            • Order Orectolobiformes (carpet sharks)
            • Order Lamniformes
              Lamniformes
              Lamniformes is an order of sharks commonly known as mackerel sharks . It includes some of the most familiar species of sharks, such as the great white shark, as well as more unusual representatives, such as the goblin shark and the megamouth shark.Members of the order are distinguished by...

               (mackerel sharks)
            • Order Carcharhiniformes
              Carcharhiniformes
              The ground sharks, order Carcharhiniformes, are the largest order of sharks. With over 270 species, carcharhiniforms include a number of common types, such as the blue shark, catsharks, swellsharks, and sandbar shark....

               (ground sharks)
          • Superorder Squalomorphi
            • Order Hexanchiformes
              Hexanchiformes
              Hexanchiformes is the order consisting of the most primitive types of sharks, and numbering just five extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens....

               (frilled and cow sharks)
            • Order Echinorhiniformes (bramble sharks)
            • Order Squaliformes
              Squaliformes
              Squaliformes is an order of sharks that includes about 97 species in seven families.Members of the order have two dorsal fins, which usually possess spines, no anal fin or nictitating membrane, and five gill slits. In most other respects, however, they are quite variable in form and size...

               (dogfish sharks)
            • †Order Protospinaciformes
            • Order Squatiniformes (angel sharks)
            • Order Pristiophoriformes (sawsharks)
        • Subdivision Batoidea
          Batoidea
          Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...

           (rays)
            • Order Torpediniformes (electric rays)
            • Order Pristiformes (sawfishes)
            • Order Rajiformes
              Rajiformes
              Rajiformes is one of the four orders of batoids, flattened cartilaginous fishes related to sharks.Rajiforms are distinguished by the presence of greatly enlarged pectoral fins, which reach as far forward as the sides of the head, with a generally flattened body. The undulatory pectoral fin motion...

               (skates and relatives)
            • Order Myliobatiformes
              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 that the myliobatiforms are a monophyletic group, and that its more derived members evolved their...

              (stingrays and relatives)


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.