page_title: Shark
section_title: Life history
hierarchical_section_title: Shark / Life history
caption_attribution_description: English: A foetus of a small-spotted catshark (Scyliorhinus canicula). In the photo the umbilical cord and food supply are clearly visible (see image notes).
context_page_description: 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 Selachimorpha and are the sister group to the rays. However, the term "shark" has also been used for extinct members of the subclass Elasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthus, as well as other Chondrichthyes such as the holocephalid eugenedontidans.
  Under this broader definition, the earliest known sharks date back to more than 420 million years ago. Acanthodians are often referred to as "spiny sharks"; though they are not part of Chondrichthyes proper, they are a paraphyletic assemblage leading to cartilaginous fish as a whole. Since then, sharks have diversified into over 500 species. They range in size from the small dwarf lanternshark, a deep sea species of only 17 centimetres in length, to the whale shark, the largest fish in the world, which reaches approximately 12 metres in length. Sharks are found in all seas and are common to depths of 2,000 metres.
context_section_description: Shark lifespans vary by species. Most live 20 to 30 years. The spiny dogfish has one of the longest lifespans at more than 100 years. Whale sharks (Rhincodon typus) may also live over 100 years. Earlier estimates suggested the Greenland shark (Somniosus microcephalus) could reach about 200 years, but a recent study found that a 5.02-metre-long (16.5 ft) specimen was 392 ± 120 years old (i.e., at least 272 years old), making it the longest-lived vertebrate known.
caption: Shark egg
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