<P> The etymology of the word "shark" is uncertain, the most likely etymology states that the original sense of the word was that of "predator, one who preys on others" from the Dutch schurk, meaning "villain, scoundrel" (cf . card shark, loan shark, etc .), which was later applied to the fish due to its predatory behaviour . </P> <P> A now disproven theory is that it derives from the Yucatec Maya word xok, pronounced' shok' . Evidence for this etymology came 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 . However, the Middle English Dictionary records an isolated occurrence of the word shark (referring to a sea fish) in a letter written by Thomas Beckington in 1442, which rules out a New World etymology . </P> <P> Evidence for the existence of sharks dates from the Ordovician period, 450--420 million years ago, before land vertebrates existed and before many plants had colonized the continents . Only scales have been recovered from the first sharks and not all paleontologists agree that these are from true sharks, suspecting that these scales are actually those of thelodont agnathans . The oldest generally accepted shark scales are from about 420 million years ago, in the Silurian period . The first sharks looked very different from modern sharks . The majority of modern sharks can be traced back to around 100 million years ago . Most fossils are of teeth, often in large numbers . Partial skeletons and even complete fossilized remains have been discovered . Estimates suggest that sharks grow tens of thousands of teeth over a lifetime, which explains the abundant fossils . The teeth consist of easily fossilized calcium phosphate, an apatite . When a shark dies, the decomposing skeleton breaks up, scattering the apatite prisms . Preservation requires rapid burial in bottom sediments . </P> <P> Among the most ancient and primitive sharks is Cladoselache, from about 370 million years ago, which has been found within Paleozoic strata in Ohio, Kentucky, and Tennessee . At that point in Earth's history these rocks made up the soft bottom sediments of a large, shallow ocean, which stretched across much of North America . Cladoselache was only about 1 metre (3.3 ft) long with stiff triangular fins and slender jaws . Its teeth had several pointed cusps, which wore down from use . From the small number of teeth found together, it is most likely that Cladoselache did not replace its teeth as regularly as modern sharks . Its caudal fins had a similar shape to the great white sharks and the pelagic shortfin and longfin makos . The presence of whole fish arranged tail - first in their stomachs suggest that they were fast swimmers with great agility . </P>

What part of ocean do sharks live in