A short history of nearly everything - Bill Bryson [173]
In a sense, an even greater question than that of what wiped out 70 percent of the species that were existing at the time is how did the remaining 30 percent survive? Why was the event so irremediably devastating to every single dinosaur that existed, while other reptiles, like snakes 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. “Why should these delicate creatures have emerged unscathed from such an unparalleled disaster?” asks Tim Flannery in his fascinating prehistory of America, Eternal Frontier.
In the seas it was much the same story. All the ammonites vanished, but their cousins the nautiloids, who lived similar lifestyles, swam on. Among plankton, some species were practically wiped out—92 percent of foraminiferans, for instance—while other organisms like diatoms, designed to a similar plan and living alongside, were comparatively unscathed.
These are difficult inconsistencies. As Richard Fortey observes: “Somehow it does not seem satisfying just to call them ‘lucky ones' and leave it at that.” If, as seems entirely likely, the event was followed by months of dark and choking smoke, then many of the insect survivors become difficult to account for. “Some insects, like beetles,” Fortey notes, “could live on wood or other things lying around. But what about those like bees that navigate by sunlight and need pollen? Explaining their survival isn't so easy.”
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 the last few years to corals dying from changes in sea temperature of only a degree or so. If they are 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 far less severe in the southern hemisphere than the northern. New Zealand in particular appears to have come through largely unscathed even though it had almost no burrowing creatures. Even its vegetation was overwhelmingly spared, and yet the scale of conflagration elsewhere suggests that devastation was global. In short, there is just a great deal we don't know.
Some animals absolutely prospered—including, a little surprisingly, the turtles once again. As Flannery notes, the period immediately after the dinosaur extinction could well be known as the Age of Turtles. Sixteen species survived in North America and three more came into existence soon after.
Clearly it helped to be at home in water. The KT impact wiped out almost 90 percent of land-based species but only 10 percent of those living in fresh water. Water obviously offered protection against heat and flame, but also presumably provided more sustenance in the lean period that followed. All the land-based animals that survived had a habit of retreating to a safer environment during times of danger—into water or underground—either of which would have provided considerable shelter against the ravages without. Animals that scavenged for a living would also have enjoyed an advantage. Lizards were, and are, largely impervious 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 the survivors 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 the world dark and hostile, it was a perfect time to be small, warm-blooded, nocturnal, flexible in diet, and cautious by nature—the very qualities that distinguished our mammalian forebears. Had our evolution been more advanced,