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" is also used to refer to extinct shark-like members of the subclass Elasmobranchii, such as hybodonts, that lie outside the modern group. Modern sharks first appeared and diversified during the Jurassic period. Since then, sharks have diversified into over 500 species. They range in size from the small dwarf lanternshark, a deep sea species that is 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 up to 2,000 metres.