万物简史英文版_比尔·布莱森- 第90节

the　kt　meteor　had　the　additional　advantage鈥攁dvantage　if　you　are　a　mammal；　that　is鈥攖hat　it　landed　in　a　shallow　sea　just　ten　meters　deep；　probably　at　just　the　right　angle；　at　a　timewhen　oxygen　levels　were　10　percent　higher　than　at　present　and　so　the　world　was　morebustible。　above　all　the　floor　of　the　sea　where　it　landed　was　made　of　rock　rich　in　sulfur。

the　result　was　an　impact　that　turned　an　area　of　seafloor　the　size　of　belgium　into　aerosols　ofsulfuric　acid。　for　months　afterward；　the　earth　was　subjected　to　rains　acid　enough　to　burn　skin。

in　a　sense；　an　even　greater　question　than　that　of　what　wiped　out　70　percent　of　the　speciesthat　were　existing　at　the　time　is　how　did　the　remaining　30　percent　survive？　why　was　the　eventso　irremediably　devastating　to　every　single　dinosaur　that　existed；　while　other　reptiles；　likesnakes　and　crocodiles；　passed　through　unimpeded？　so　far　as　we　can　tell　no　species　of　toad；newt；　salamander；　or　other　amphibian　went　extinct　in　north　america。　鈥渨hy　should　thesedelicate　creatures　have　emerged　unscathed　from　such　an　unparalleled　disaster？鈥潯�asks　timflannery　in　his　fascinating　prehistory　of　america；　eternal　frontier。

in　the　seas　it　was　much　the　same　story。　all　the　ammonites　vanished；　but　their　cousins　thenautiloids；　who　lived　similar　lifestyles；　swam　on。　among　plankton；　some　species　werepractically　wiped　out鈥�92　percent　of　foraminiferans；　for　instance鈥攚hile　other　organisms　likediatoms；　designed　to　a　similar　plan　and　living　alongside；　were　paratively　unscathed。

these　are　difficult　inconsistencies。　as　richard　fortey　observes：　鈥渟omehow　it　does　notseem　satisfying　just　to　call　them　鈥榣ucky　ones鈥櫋�and　leave　it　at　that。鈥潯�if；　as　seems　entirely　likely；the　event　was　followed　by　months　of　dark　and　choking　smoke；　then　many　of　the　insectsurvivors　bee　difficult　to　account　for。　鈥渟ome　insects；　like　beetles；鈥潯�fortey　notes；　鈥渃ouldlive　on　wood　or　other　things　lying　around。　but　what　about　those　like　bees　that　navigate　bysunlight　and　need　pollen？　explaining　their　survival　isn鈥檛　so　easy。鈥�

above　all；　there　are　the　corals。　corals　require　algae　to　survive　and　algae　require　sunlight；and　both　together　require　steady　minimum　temperatures。　much　publicity　has　been　given　in　thelast　few　years　to　corals　dying　from　changes　in　sea　temperature　of　only　a　degree　or　so。　if　theyare　that　vulnerable　to　small　changes；　how　did　they　survive　the　long　impact　winter？

there　are　also　many　hard…to…explain　regional　variations。　extinctions　seem　to　have　been　farless　severe　in　the　southern　hemisphere　than　the　northern。　new　zealand　in　particular　appears　tohave　e　through　largely　unscathed　even　though　it　had　almost　no　burrowing　creatures。　evenits　vegetation　was　overwhelmingly　spared；　and　yet　the　scale　of　conflagration　elsewheresuggests　that　devastation　was　global。　in　short；　there　is　just　a　great　deal　we　don鈥檛　know。

some　animals　absolutely　prospered鈥攊ncluding；　a　little　surprisingly；　the　turtles　once　again。

as　flannery　notes；　the　period　immediately　after　the　dinosaur　extinction　could　well　be　knownas　the　age　of　turtles。　sixteen　species　survived　in　north　america　and　three　more　came　intoexistence　soon　after。

clearly　it　helped　to　be　at　home　in　water。　the　kt　impact　wiped　out　almost　90　percent　ofland…based　species　but　only　10　percent　of　those　living　in　fresh　water。　water　obviously　offeredprotection　against　heat　and　flame；　but　also　presumably　provided　more　sustenance　in　the　leanperiod　that　followed。　all　the　land…based　animals　that　survived　had　a　habit　of　retreating　to　asafer　environment　during　times　of　danger鈥攊nto　water　or　underground鈥攅ither　of　whichwould　have　provided　considerable　shelter　against　the　ravages　without。　animals　thatscavenged　for　a　living　would　also　have　enjoyed　an　advantage。　lizards　were；　and　are；　largelyimpervious　to　the　bacteria　in　rotting　carcasses。　indeed；　often　they　are　positively　drawn　to　it；and　for　a　long　while　there　were　clearly　a　lot　of　putrid　carcasses　about。

it　is　often　wrongly　stated　that　only　small　animals　survived　the　kt　event。　in　fact；　among　thesurvivors　were　crocodiles；　which　were　not　just　large　but　three　times　larger　than　they　are　today。

but　on　the　whole；　it　is　true；　most　of　the　survivors　were　small　and　furtive。　indeed；　with　theworld　dark　and　hostile；　it　was　a　perfect　time　to　be　small；　warm…blooded；　nocturnal；　flexible　indiet；　and　cautious　by　nature鈥攖he　very　qualities　that　distinguished　our　mammalian　forebears。

had　our　evolution　been　more　advanced；　we　would　probably　have　been　wiped　out。　instead；mammals　found　themselves　in　a　world　to　which　they　were　as　well　suited　as　anything　alive。

however；　it　wasn鈥檛　as　if　mammals　swarmed　forward　to　fill　every　niche。　鈥渆volution　mayabhor　a　vacuum；鈥潯�wrote　the　paleobiologist　steven　m。　stanley；　鈥渂ut　it　often　takes　a　long　timeto　fill　it。鈥潯�for　perhaps　as　many　as　ten　million　years　mammals　remained　cautiously　small。　inthe　early　tertiary；　if　you　were　the　size　of　a　bobcat　you　could　be　king。

but　once　they　got　going；　mammals　expanded　prodigiously鈥攕ometimes　to　an　almostpreposterous　degree。　for　a　time；　there　were　guinea　pigs　the　size　of　rhinos　and　rhinos　the　sizeof　a　two…story　house。　wherever　there　was　a　vacancy　in　the　predatory　chain；　mammals　rose（often　literally）　to　fill　it。　early　members　of　the　raccoon　family　migrated　to　south　america；discovered　a　vacancy；　and　evolved　into　creatures　the　size　and　ferocity　of　bears。　birds；　too；prospered　disproportionately。　for　millions　of　years；　a　gigantic；　flightless；　carnivorous　birdcalled　titanis　was　possibly　the　most　ferocious　creature　in　north　america。　certainly　it　was　themost　daunting　bird　that　ever　lived。　it　stood　ten　feet　high；　weighed　over　eight　hundred　pounds；and　had　a　beak　that　could　tear　the　head　off　pretty　much　anything　that　irked　it。　its　familysurvived　in　formidable　fashion　for　fifty　million　years；　yet　until　a　skeleton　was　discovered　inflorida　in　1963；　we　had　no　idea　that　it　had　ever　existed。

which　brings　us　to　another　reason　for　our　uncertainty　about　extinctions：　the　paltriness　ofthe　fossil　record。　we　have　touched　already　on　the　unlikelihood　of　any　set　of　bones　beingfossilized；　but　the　record　is　actually　worse　than　you　might　think。　consider　dinosaurs。

museums　give　the　impression　that　we　have　a　global　abundance　of　dinosaur　fossils。　in　fact；overwhelmingly　museum　displays　are　artificial。　the　giant　diplodocus　that　dominates　theentrance　hall　of　the　natural　history　museum　in　london　and　has　delighted　and　informedgenerations　of　visitors　is　made　of　plaster鈥攂uilt　in　1903　in　pittsburgh　and　presented　to　themuseum　by　andrew　carnegie。　the　entrance　hall　of　the　american　museum　of　natural　historyin　new　york　is　dominated　by　an　even　grander　tableau：　a　skeleton　of　a　large　barosaurusdefending　her　baby　from　attack　by　a　darting　and　toothy　allosaurus。　it　is　a　wonderfullyimpressive　display鈥攖he　barosaurus　rises　perhaps　thirty　feet　toward　the　high　ceiling鈥攂ut　alsoentirely　fake。　every　one　of　the　several　hundred　bones　in　the　display　is　a　cast。　visit　almost　anylarge　natural　history　museum　in　the　world鈥攊n　paris；　vienna；　frankfurt；　buenos　aires；mexico　city鈥攁nd　what　will　greet　you　are　antique　models；　not　ancient　bones。

the　fact　is；　we　don鈥檛　really　know　a　great　deal　about　the　dinosaurs。　for　the　whole　of　the　ageof　dinosaurs；　fewer　than　a　thousand　species　have　been　identified　（almost　half　of　them　knownfrom　a　single　specimen）；　which　is　about　a　quarter　of　the　number　of　mammal　species　alivenow。　dinosaurs；　bear　in　mind；　ruled　the　earth　for　roughly　three　times　as　long　as　mammalshave；　so　either　dinosaurs　were　remarkably　unproductive　of　species　or　we　have　barelyscratched　the　surface　（to　use　an　irresistibly　apt　clich茅）。

for　millions　of　years　through　the　age　of　dinosaurs　not　a　single　fossil　has　yet　been　found。

even　for　the　period　of　the　late　cretaceous鈥攖he　most　studied　prehistoric　period　there　is；thanks　to　our　long　interest　in　dinosaurs　and　their　extinction鈥攕ome　three　quarters　of　allspecies　that　lived　may　yet　be　undiscovered。　animals　bulkier　than　the　diplodocus　or　moreforbidding　than　tyrannosaurus　may　have　roamed　the　earth　in　the　thousands；　and　we　maynever　know　it。　until　very　recently　everything　known　about　the　dinosaurs　of　this　period　camefrom　only　about　three　hundred　specimens　representing　just　sixteen　sp