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 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. Under this broader definition, the earliest known sharks date back to more than 420 million years ago.
Since then, sharks have diversified into over 505 species. They range in size from the small dwarf lanternshark, a deep sea species of only 17 centimetres 6,7 inch) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (39 foot) in length.
Until the 16th century, sharks were known to mariners as "sea dogs". The etymology of the word "shark" is uncertain. One theory is that it derives from the Yucatec Maya word "xok", pronounced "shok". Evidence for this etymology comes from the Oxford English Dictionary, which notes shark first came into use after Sir John Hawkins' sailors exhibited one in London in 1569 and posted "sharke" to refer to the large sharks of the Caribbean Sea.
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