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高考资源网（ks5u.com）您身边的高考专家-3-版权所有@高考资源网SharksSharksaresurelyoneofthemostfearsomecreaturesonEarth.Buttheyarealsooneofthemostsophisticatedandenduring.Theoldestsharkfossilsdatefrommorethan300millionyearsagobeforetheageofthedinosaurs.Andafewexistingsharkspeciessuchasthehornsharkhavekeptthesamebasicphysicalcharacteristicsformorethan150millionyears.Scientistsattributethisremarkablelongevitytotheshark'ssuperiorphysiologicaldevelopments.Sharksareoutfittedwithseveralspecialcharacteristicsthatmakethemhighlyeffectivebothashuntersandsurvivors.Inthisarticlewe'llexaminetheseuniquequalitiestoseewhythesharkistopdogintheocean.SharkAnatomyWhenpeoplethinkofsharkstheygenerallyimagineananimalliketheoneinthispicture.Thetalldorsalfintorpedo-shapedbodyandgiantteethofthisgreatwhitesharkarefamiliartomosteverybody.Butthereareactuallymorethan400differentsharkspeciesalivetodayandtheyvaryconsiderablyinsizeandappearance.Infactroughly50percentofsharkspeciesarelessthanameterlong.Sowhatmakesasharkashark?Oneofthemostwell-knownsharksthegreatwhiteSharksalongwithraysandchimerasaredistinguishedfromotherfishprimarilybytheirbodycomposition.Mostotherfishhaveskeletonsmadeofbonejustlikemammalsreptilesamphibiansandbirds.Sharksandraysontheotherhandhaveskeletonsmadeentirelyofcartilagethesameflexiblematerialinyournoseandears.Cartilageissturdylikebonebutithasamuchlowerdensity.Thismaterialkeepssharksrelativelylightweightsotheydon'tsinkintheoceanandtheydon'tneedanairbladderlikeotherfish.Sharksalsohaveaveryuniqueskintexture.Theydon'thavethelargeprominentscalesfoundinbonyfish.Insteadthey'recove