The Cretaceous-Tertiary Extinction

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    Sixty-five million years ago a minor vagary of celestial mechanics struck a mighty blow against our planet in a major event that marked the temporal boundary between the Cretaceous and Tertiary Periods of geological history. We know that event primarily as the event that led to the extinction of the dinosaurs. Indeed, for a long time paleontologists only knew that such an event occurred because they knew of the mass extinction of the great reptiles, though they didnít know what constituted the event itself. Only in the 1980's did sufficient evidence come to light to prove and verify the hypothesis that the event involved a large asteroid striking Earth.

    It came without warning and took as little as three seconds to punch through Earthís atmosphere and slam into the Caribbean Sea just off the coast of the Yucatan, near a place now occupied by the Mexican fishing village of Chicxulub (pronounced, more or less, as chick-shoe-LUBE). The blast accompanying the impact ripped a crater spanning 180 kilometers (and perhaps as much as 300 kilometers) in Earthís crust.

    The Chicxulub impact also led to the cracking of India and the flood vulcanism that created the Deccan traps (from the Dutch word trapper, which means steps) and the Deccan plateau. India lay almost exactly on the opposite side of the planet from Chicxulub at the time, so the shock waves from the impact, traveling through Earth as earthquake vibrations do, came to a focus there. Just as the impact that created the Caloris Basin on Mercury also created the wrinkled "weird terrain" on the opposite side of the planet, the rippling, wrinkling effect of the Chicxulub impact cracked India and released vast floods of molten basalt onto the surface along with massive clouds of sulphur oxides.

    Vast quantities of dust and sulphur oxides high in the atmosphere prevented sunlight from reaching the ground and warming the world. Those plants that survived the initial shock of the impact and the fires that were ignited by incandescent debris falling from the sky would have suffered from the cold and the diminished light. In 1816 we got a hint of what happened.

    The Year Without a Summer (1816) occurred due to a series of volcanic eruptions in previous years, which series climaxed in the explosion of Tambora in April 1815. That explosion blasted 160 cubic kilometers of tephra dust and sulphur oxides into the stratosphere, much less than came from the Chicxulub-Deccan Event. The column rose 43 kilometers into the sky and the shading effect of the dust shroud lowered average Northern Hemisphere temperatures in the summer by Ĺ degree Celsius. The growth of plants faltered and harvests diminished. Famine followed and that in a creature that had developed agriculture.

    The dinosaurs did not have agriculture. Opportunistic feeders, they could only exploit plants that had been seeded haphazardly. As a consequence the great herbivores, such as Apatosaurus and Triceratops, starved. A Great Famine swept across the world. But starvation alone did not kill off the dinosaurs: famine has companions.

    Acid rain was one companion, though not likely a significant one. Combined with water, the sulphur oxides in the atmosphere became sulphurous and sulphuric acids. But tree frogs in South America survived the Great Die-off, which fact tells us that acid rain did not contribute greatly to the death toll. Indeed, it tells us that the sulphur oxides, and by implication the dust, came out of the stratosphere only slowly, prolonging the shrouding effect.

    Cold was another companion, though cold weather likely did not do the deed itself: turtles survived and if you canít freeze a turtle, as Robert Bakker once said, you cannot freeze a tyrannosaur. But you donít need to freeze the tyrannosaur to kill it. The turtle is cold-blooded and thus needs little food to survive and the amount of food it needs doesnít depend strongly on ambient temperature. The tyrannosaur, like other theropods, was warm-blooded and thus needed enough food to fuel the cellular processes that generated its body heat. As the temperature dropped, the tyrannosaur needed more food to compensate the increased rate of losing body heat and thus it would starve more quickly in a cold climate.

    Disease also attended famine. Contemplate the Great Plague that swept across Asia and Europe in the late 1340's: by some estimates it wiped out 50% of the population of Europe and extinguished completely small villages in Mongolia. And this happened to a sentient species that could, however imperfectly, defend itself with treatment and quarantine. One of the major factors in the death toll in Fourteenth-Century Europe was poor nutrition: the people were not as healthy as they could have been and were, thus, more susceptible to the disease. Starving dinosaurs would have been easy prey for whatever diseases existed in their world.

    And then we have sheer desperation. A starving animal will eat just about anything and will take risks it would never take when well fed. In the 1920's Roy Chapman Andrews, the original Indiana Jones, discovered the fossilized eggs of dinosaurs in the Gobi Desert. Once people understood that dinosaurs reproduced by laying eggs, it didnít take long for some of them to infer that the big extinction was caused by little, egg-sucking mammals. But eating a dinosaurís eggs was not a safe business. With the dinosaurs distracted by other concerns, though, and weakened by hunger, the hungry mammals found more opportunities and in feeding themselves interfered with dinosaur reproduction.

    That interference acted on an already debilitated dinosaur population. Some paleontologists, noting that dinosaurs diminish in the fossil record before they reach the K-T boundary, believe that the Chicxulub-Deccan Event merely delivered the final death blow to Order Dinosauria. The great reptiles had been caught at a bad moment in their evolutionary history, one from which they likely would have recovered through the usual interaction between mutation and natural selection. Had the asteroid not fallen from the sky the descendants of the dinosaurs would likely dominate the world today.

    They fit well into their environment, well enough to dominate life on Earth for 185 million years, but their environment changed and did so too rapidly for mutation and selection to follow. Instead of adapting, species went extinct. Such things happen in the history of life: evolution does not always proceed smoothly and continuously. An asteroid slamming into Earth produced one such discontinuity.

    Under the pall of dust and smoke, photosynthesis slowed to a glacial creep and plant growth effectively stopped. In the oceans as well as on the land, herbivores effectively finished off the plants that the initial impact had not wiped out. Then the big herbivores died out, killed off by both starvation and predation.

    One day the last triceratops died; one day the last hadrosaur died; one day doom came to the great predators Ė to the tyrannosaurs, to the troodons, to the velociraptors. The biggest fell first. Weakened by starvation, a tyrannosaur would collapse, but would not die in peace. Desperate with hunger, packs of velociraptors came to attack twenty-five tons of still-breathing meat. All too soon the big predators ceased to exist and the velociraptors had only one recourse left to them: in a grotesque parody of the Social Darwinistsí dog-eat-dog world, Earth became a velociraptor-eat-velociraptor world. And then ... silence ... for a time.

    Imagine wandering through the wreckage of a Late Cretaceous forest. Detritus of all kinds litters the ground: dried leaves and twigs, bones, eggshells, and so on. You wade through the dead remains of once-living things. But life still exists here. Insects hum all around you, feeding on the detritus. You hear more insects than you have ever heard before. The world seems to teem with them in their writhing masses and they crunch under your feet. And mixed among the insects worms crawl, recycling the once-living matter surrounding them.

    You see a dragonfly hover near a branch and suddenly, appearing as if from nowhere, a mouth clamps down on it. The mouthís owner, a small feather-covered theropod dinosaur, hops up onto a branch and stares at you through slit-pupiled eyes. Then it flaps its forearms and lifts itself into the air. Such small hunters seek small prey and small prey abound in this new world. Thus one tiny group of small dinosaurs survives the Great Extinction and becomes birds.

    Underfoot you hear the sounds of small creatures scurrying about. The rustle of dead leaves marks the progress of a lizard, a snake, or a mammal, out hunting and eating insects and worms. Of all the animals that existed on Earth in the Mesozoic Era, the main survivors consist of the insects, the worms, and the predators that eat them Ė mammals, lizards, and birds. Those privileged few species, the ones that eat insects and/or worms, have survived the cataclysm that took down the giant reptiles that reigned over Earth for 185 million years.

    Now imagine that your stroll through the ruined forest has brought you to a river. Through clear water you see a few small fish, maybe a frog or two. Cold-blooded, they donít need a lot of food, so they survived the Great Famine, as did other cold-blooded creatures, like the crocodile that lunges at you as you approach the water, like the snake that you glimpse slithering into the bushes growing above the water, like the turtle that slips off a log and into the water to swim away from the crocodile. But you notice, with a sigh of gratitude, that these specimens havenít grown as large as did their ancestors only a few centuries earlier. For all their cold-bloodedness, they still need food and an environment deficient in food favors the smaller members of a population. The reptiles have, in fact, become smaller as a consequence of the extinction that went on around them.

    Climbing back up the bank, you walk along the river, following its course downstream. At last you come out onto a bluff overlooking the ocean. Through your binoculars you search in vain for any of the big sea-going reptiles. No more do ichthyosaurs cavort among the swells; no more do plesiosaurs row themselves through the sea with their great paddle-shaped fins. The giant mosasaurs have vanished.

    As on land, so in the sea, the Great Famine has killed off the big reptiles. It didnít matter whether they were cold-blooded or warm-blooded; they all starved to death. In the cooling seas the cold-blooded ones remained sluggish, less able to hunt the increasingly rare prey that they needed to survive. The warm-blooded ones could still hunt efficiently, but they needed more food than normal and were trying to obtain it from an increasingly empty pantry. As on land, so in the sea, the great predators began to feed on each other and, in time, extinguished their species.

    Standing on the bluff, you gaze out over a world empty of all but the smallest dinosaurs. You may feel a certain sadness, but it is mitigated by your knowledge of what is yet to come.

    Over time the skies cleared again and photosynthesis sped up to its normal rates. The plants grew again and slowly filled in the areas where they had died out. Gaiaís tattered cloak rewove itself and elaborated anew the swirling patterns of life.

    Again predator and prey enacted their steps in the grand waltz of mutation and selection. Life revisited Darwinís tangled bank. Ecological niches emptied out by the Great Die-off became opportunities that promoted certain mutations and thereby drew life into occupying them once again, creating new niches as it did so. Briefly Earth hosted an Age of Giant Birds. Then faintly, quietly dawn broke on the Age of Mammals.

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