Sharks are a group of 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 (orSelachii) and are the sister group to therays. However, the term "shark" has also been used for extinct members of the subclass Elasmobranchii outside the Selachimorpha, such as Cladoselacheand Xenacanthus, as well as otherChondrichthyes such as theholocephalid eugenedontidans. Under this broader definition, the earliest known sharks date back to more than 420 million years ago.[1] Acanthodiansare 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 intoover 500 species. They range in size from the small dwarf lanternshark(Etmopterus perryi), a deep sea species of only 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (39 ft) in length. Sharks are found in all seas and are common to depths of 2,000 metres (6,600 ft). They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can survive and be found in both seawater and freshwater.[2] They breathe through five to seven gill slits. Sharks have a covering of dermal denticles that protects their skin from damage andparasites in addition to improving theirfluid dynamics. They have several sets of replaceable teeth.[3]
