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the proportion that perished。 moreover； we are talking about the death ofspecies； not individuals。 for individuals the death toll could be much higher鈥攊n many cases；practically total。 the species that survived to the next phase of life鈥檚 lottery almost certainlyowe their existence to a few scarred and limping survivors。
in between the big kill…offs； there have also been many smaller； less well…known extinctionepisodes鈥攖he hemphillian； frasnian； famennian； rancholabrean； and a dozen or so others鈥攚hich were not so devastating to total species numbers； but often critically hit certainpopulations。 grazing animals； including horses； were nearly wiped out in the hemphillianevent about five million years ago。 horses declined to a single species； which appears sosporadically in the fossil record as to suggest that for a time it teetered on the brink ofoblivion。 imagine a human history without horses； without grazing animals。
in nearly every case； for both big extinctions and more modest ones； we have bewilderinglylittle idea of what the cause was。 even after stripping out the more crackpot notions there arestill more theories for what caused the extinction events than there have been events。 at leasttwo dozen potential culprits have been identified as causes or prime contributors： globalwarming； global cooling； changing sea levels； oxygen depletion of the seas （a conditionknown as anoxia）； epidemics； giant leaks of methane gas from the seafloor； meteor and etimpacts； runaway hurricanes of a type known as hypercanes； huge volcanic upwellings；catastrophic solar flares。
this last is a particularly intriguing possibility。 nobody knows how big solar flares can getbecause we have only been watching them since the beginning of the space age； but the sun isa mighty engine and its storms are mensurately enormous。 a typical solar flare鈥攕omething we wouldn鈥檛 even notice on earth鈥攚ill release the energy equivalent of a billionhydrogen bombs and fling into space a hundred billion tons or so of murderous high…energyparticles。 the magnetosphere and atmosphere between them normally swat these back intospace or steer them safely toward the poles （where they produce the earth鈥檚 ely auroras）；but it is thought that an unusually big blast； say a hundred times the typical flare； couldoverwhelm our ethereal defenses。 the light show would be a glorious one； but it would almostcertainly kill a very high proportion of all that basked in its glow。 moreover； and ratherchillingly； according to bruce tsurutani of the nasa jet propulsion laboratory； 鈥渋t wouldleave no trace in history。鈥�
what all this leaves us with； as one researcher has put it； is 鈥渢ons of conjecture and verylittle evidence。鈥潯ooling seems to be associated with at least three of the big extinctionevents鈥攖he ordovician； devonian； and permian鈥攂ut beyond that little is agreed； includingwhether a particular episode happened swiftly or slowly。 scientists can鈥檛 agree； for instance；whether the late devonian extinction鈥攖he event that was followed by vertebrates movingonto the land鈥攈appened over millions of years or thousands of years or in one lively day。
one of the reasons it is so hard to produce convincing explanations for extinctions is that itis so very hard to exterminate life on a grand scale。 as we have seen from the manson impact；you can receive a ferocious blow and still stage a full； if presumably somewhat wobbly；recovery。 so why； out of all the thousands of impacts earth has endured； was the kt event sosingularly devastating？ well； first itwas positively enormous。 it struck with the force of 100million megatons。 such an outburst is not easily imagined； but as james lawrence powell haspointed out； if you exploded one hiroshima…sized bomb for every person alive on earth todayyou would still be about a billion bombs short of the size of the kt impact。 but even thatalone may not have been enough to wipe out 70 percent of earth鈥檚 life； dinosaurs included。
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sks 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nd leave it at that。鈥潯f； 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ortey 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