Elasmobranchii

Elasmobranchii is a subclass of the Chondrichthyes or cartilaginous fish. It includes the modern sharks and the rays and skates.

Elasmobranchii
Great white shark (Carcharodon carcharias)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Elasmobranchii Bonaparte, 1838
Superorders
Batoidea Selachimorpha

Elasmobranchs lack swim bladders, and maintain buoyancy with oil that they store in their livers. Some deep sea sharks are targeted by fisheries for this liver oil, including the school, gulper and basking sharks (pictured). All three of these species have been assessed by the IUCN as vulnerable due to overfishing.

Radiation of elasmobranchs, based on Michael Benton, 2005.

From a practical point of view the life-history pattern of elasmobranchs makes this group of animals extremely susceptible to over fishing. It is no coincidence that the commercially exploited marine turtles and baleen whales, which have life-history patterns similar to the sharks, are also in trouble.

Evolution

Fossilised shark teeth are known from the early Devonian, 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 Permian, but the remaining sharks underwent a second burst of adaptive radiation during the Jurassic, around which time the skates and rays first appeared.

Taxonomy

• Subclass Elasmobranchii
  • †Plesioselachus
  • †Order Squatinactiformes
  • †Order Protacrodontiformes
  • †Infraclass Cladoselachimorpha
    • †Order Cladoselachiformes
  • †Infraclass Xenacanthimorpha
    • †Order Xenacanthiformes
  • Infraclass Euselachii (sharks and rays)
    • †Order Ctenacanthiformes
    • †Division Hybodonta
      • †Order Hybodontiformes
    • Division Neoselachii
      • Subdivision Selachii (Selachimorpha) (modern sharks)
        • Superorder Galeomorphi
          • Order Heterodontiformes (bullhead sharks)
          • Order Orectolobiformes (carpet sharks)
          • Order Lamniformes (mackerel sharks)
          • Order Carcharhiniformes (ground sharks)
        • Superorder Squalomorphi
          • Order Hexanchiformes (frilled and cow sharks)
          • Order Squaliformes (dogfish sharks)
          • †Order Protospinaciformes
          • Order Squatiniformes (angel sharks)
          • Order Pristiophoriformes (sawsharks)
      • Subdivision Batoidea (rays)
        • • Order Torpediniformes (electric rays)
          • Order Pristiformes (sawfishes)
          • Order Rajiformes (skates and relatives)
          • Order Myliobatiformes (stingrays and relatives)