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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 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rote the paleobiologist steven m。 stanley； 鈥渂ut it often takes a long timeto fill it。鈥潯or 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