My Beloved Brontosaurus Read online

  The asteroid rocks the planet during the climax of the documentary. At first, not very much happens. A few trees fall and a Tyrannosaurus briefly loses its balance, and that’s about all. Death slowly creeps up on the dinosaurs. A dust cloud blocks out the sun and kills off the vegetation, and a mother hadrosaur, alone in a scorched world, mourns over a nest of eggs that have all disintegrated. In time, she also disappears. And from beneath the bleached bones of a Triceratops, a hopelessly cross-eyed opossum waddles out into the sunlight to herald the first day of the Age of Mammals. The destruction of the non-avian dinosaurs wasn’t triggered by a built-in factory flaw or geriatric decadence. Dinosaurs just had one really bad day from which they never recovered.

  The extinction of the non-avian dinosaurs was a cosmic twist of bad luck. (Cartoon by Mike Jacobsen)

  Almost every other documentary I saw during the 1980s and ’90s replayed the same scenario. The dinosaurs were thriving until one fateful day when an asteroid struck the planet and wrapped the world in a cloak of dust, ash, and debris. Mammals, frogs, crocodiles, lizards, turtles, and birds were small enough to hide, but there was no hope for the dinosaurs. The lucky survived by virtue of adaptations they already possessed, and the unfortunate, lacking the variation necessary to survive, perished. The global devastation made beautiful, elegant sense. How could an asteroid approximately six miles across smack into the planet and not cause an environmental Armageddon? The end of the dinosaurs was not a gradual decline, winnowing down, or replacement by supposedly superior mammals. Edmontosaurus and its kind were cut down in their prime, the unsuspecting victims of cosmic chance. In our own era, the unfathomable devastation echoed Cold War fears of nuclear annihilation. Worries over “nuclear winter”—driven by the possibility of mutually assured atomic destruction—were fueled by the idea that dinosaurs had suffered such a fate millions of years ago.

  * * *

  Despite what I saw onscreen, most paleontologists in the mid-1980s bridled at the thought that the Cretaceous catastrophe was caused by a cosmic shock. This “impact hypothesis” looked like a deus ex machina that was far too simple to explain the death of so many different kinds of organisms all over the world. Climate change, the regression of Cretaceous seas, and ecological change following intense volcanic activity were the traditional extinction triggers. The fact that the Alvarez team came from disciplines outside paleontology didn’t help the matter. Dinosaur experts spent countless hours picking over outcrops, studying the bones of the animals themselves, and trying to measure the pulse of life, and here come these snooty strangers to say that all that hard work had gone in the wrong direction.

  Paleontologists resented the social and political fallout of the Alvarez debate. In 1985, a New York Times reporter relayed fears from paleontologists that papers critical of the impact hypothesis were being blocked by high-profile journals. Maintaining that changes in global climate, sea level, and habitat had killed the dinosaurs suddenly became a controversial point of view as the impact hypothesis grabbed the public’s imagination. The idea was seductive, sensational, and didn’t pay any heed to what paleontologists thought they were seeing in the fossil record. Paleontologist Robert Bakker was particularly incensed at the rapid acceptance of the hypothesis:

  The arrogance of those people is simply unbelievable. They know next to nothing about how real animals evolve, live and become extinct. But despite their ignorance, the geochemists feel that all you have to do is crank up some fancy machine and you’ve revolutionized science. The real reasons for the dinosaur extinctions have to do with temperature and sea-level changes, the spread of diseases by migration and other complex events. But the catastrophe people don’t seem to think such things matter. In effect, they’re saying this: “We high-tech people have all the answers, and you paleontologists are just primitive rock hounds.”

  Bakker’s feelings were echoed by many paleontologists—but, as in any debate, some pushed back and thought the Alvarez team had a point. One of the early converts to the impact hypothesis was the paleontologist David Raup, a specialist in large-scale patterns of evolution and decline over millions of years. His work was a central facet of a new paleobiology that promoted the study of fossils from what was perceived to be an intellectual backwater to one of the most essential components of evolutionary biology. While he admitted that he had an initial gut reaction against the impact hypothesis, Raup later changed his mind. In an essay about the debate (which continued for over a decade), Raup wondered why he himself had had such a negative reaction to the hypothesis. Why had paleontologists reacted with almost “universal derision” at the idea?

  Tradition certainly played a role. “When I was training to be a paleontologist, in the 1950s,” Raup wrote, “I was taught that most meteorite impacts on Earth were confined to what was known as the ‘early bombardment,’ the relatively short period of accumulation of debris left over from the formation of the Solar System.” Comets, meteors, and asteroids were not supposed to be a regular feature of Earth history, and paleontologists largely ignored the growing body of evidence that such bodies, sometimes of considerable size, had continued to fall to the planet. The end-Cretaceous impact wasn’t an unusual event that required special pleading for its possibility. The idea fit in with the long history of impacts on the Earth.

  And, not long after the Alvarez team made their contentious proposal, the actual impact crater was finally recognized. The 111-mile-wide depression in the Earth’s crust—situated in the vicinity of the Yucatan Peninsula—had been discovered years before by oil geologists, but no one understood the pit’s significance until a decade after the asteroid controversy started. In 1991, the geologist Alan Hildebrand and coauthors tied the crater to the iridium evidence the Alvarez team found, and the geologic wound was dubbed the Chicxulub Crater. Based on its location, the asteroid strike was close enough to the home range of Tyrannosaurus, Triceratops, and other famous end-Cretaceous dinosaurs to wipe them out instantaneously. (As my Paleontology 101 professor, William Gallagher, once told my class, the impact would have created a devastating wave of fire, steam, and debris that would have “flash fried” the dinosaurs of North America.) There could no longer be any dispute about whether the impact happened. Suddenly, the pattern of life on Earth seemed very different.

  Until the impact debate, most paleontologists had long upheld the Darwinian view of extinction as an outcome of the struggle for survival. Novel, better-adapted forms outcompeted and replaced their predecessors, and so the flow of life—while brutal—remained orderly. This traditional view was fading away thanks to a growing body of knowledge about the fossil record and the creatures themselves. There were no obvious signs that mass extinction victims were objectively inferior or that survivors were absolutely superior. Any number of dinosaurian liabilities could be—and had been—concocted to explain away their extinction, but, as Raup noted, just as many “biological failings” could have been found among mammals if the extinction had turned out differently. (And, according to a recent study by Mark Norell and colleagues, dinosaurs were still thriving at the very end of the Cretaceous. There was no global dinosaur decline that might indicate that they were already on their way out.)

  This was one of the most unsettling aspects of the impact hypothesis. Death in a mass extinction was partially attributable to luck; a species could not foresee or prepare for the test of survival. The natural history of some species would make them more vulnerable to extinction while others would be more resistant. Yet identifying a possible extinction trigger isn’t the same as showing how that event actually wiped out a species. We know that an enormous asteroid struck Earth around 66 million years ago, but what happened between that instant and the death of the last non-avian dinosaurs?

  Almost everything we know about the evolutionary changing of the guard comes from a relatively small area of Montana and adjacent states. We’re still piecing together what happened. The mass extinction was undoubtedly a global phenomenon—there is no solid ev
idence that dinosaurs survived into the Paleocene at any site in the world—but most of what we know about life just before the disaster comes from this narrow swath of western North America. The reason why doomsday scenarios in dinosaur documentaries invariably focus on Tyrannosaurus, Triceratops, Troodon, and Edmontosaurus as the stars is not just because these dinosaurs are among the most famous, but because this assemblage of dinosaurs is the only terminal Cretaceous community that has been extensively studied so far. There are other end-Cretaceous sites in Europe and Mongolia, and there may be more around the world, but so far we’ve only just begun to reconstruct what the entire world was like just before the asteroid hit.

  Without clear and comprehensive Before and After shots of the end-Cretaceous transition, it’s extremely difficult to detect the secret of the non-avian dinosaurs’ demise. We know when it happened, and we now know it happened fast. Yes, an asteroid struck the planet at a time already assailed by massive volcanic outpouring, climate change, and receding sea level, but how did any or all of these different factors translate into pressures that wiped out my favorite creatures? It’s not enough to simply correlate a mass extinction with a particular event. We need to gauge how a specific cause or causes triggered an ecological cascade ending in the survival or demise of a specific group of animals.

  The asteroid impact is such an unfamiliar and sensational event that it will undoubtedly dominate the discussion for years. Based on our current evidence, it’s the simplest explanation available for why dinosaurs exited what the paleontologist W. D. Matthew once called life’s “splendid drama.” All the same, the debate over what wiped out the dinosaurs isn’t over.

  Nothing summarized the current state of affairs better than a recent academic kerfuffle sparked by a 2010 Science paper that reaffirmed the impact hypothesis as the chief cause of the end-Cretaceous mass extinction. More than forty geologists and paleontologists collaborated on the position paper, which concluded that the vast weight of evidence pinned the responsibility for the extinction on an asteroid impact. Not all paleontologists agreed. Soon after, Science published dissenting opinions from several other groups of scientists. Dinosaur specialists and other vertebrate paleontologists, in particular, said that the asteroid impact probably played a major role in the extinction, but that other causes—such as sea level change and volcanic activity—could not be discounted.

  In light of this exchange, I called a few paleontologists to get a read on the current state of the debate. The University of Rhode Island paleontologist David Fastovsky, who has been a vocal advocate of the impact hypothesis for years, told me that the global pattern—on land and in the seas—points to a single, catastrophic cause. An asteroid impact best fits the terrible trend. An appeal to multiple causes, Fastovsky said, “philosophically bothers me” because “they only explain a piece of the data” rather than the big picture. Thus far, the idea that an asteroid impact was the chief culprit has survived the myriad tests and criticisms thrown its way.

  Mark Goodwin took a different view when I asked him about the extinction. The impact hypothesis is “a sort of hook to hang one’s hat on for the demise of the dinosaurs,” and it “certainly caused some stress in the environment,” Goodwin said, but we don’t know enough yet to determine whether it entirely explains the massive destruction at the end of the Cretaceous. Goodwin likened the state of affairs to the current scientific investigation of climate change. “We all agree that the world is warming up” and that humans are behind it, Goodwin said, “but we disagree on the tempo and mode.” The points of contention are not major theoretical differences, but “fine points” that require additional data and refined techniques that we don’t yet have access to.

  I called the Mesozoic mammal expert Anne Weil, too. After all, some mammal lineages were cut back or died out at the end of the Cretaceous. Perhaps, by studying the record of our tiny relatives, we can better understand what happened to the dinosaurs and other animals present at the end of the Cretaceous. “This was the biggest event in mammalian evolution,” Weil said. “It was amazing.” About half the mammals present at the end of the Cretaceous were lost, and the ones we find on the other side of the extinction—at the dawn of the next geological era—formed the basis for the beasts we see around us today. Determining what happened in that small slice between the end of the Cretaceous and the earliest part of the Paleocene is the trick. We simply don’t have the high-definition data to tell whether the extinction took place over the course of weeks or thousands of years, Weil said. Since we presently lack the refinement to track how ecosystems changed in the days, months, and years after the asteroid strike, we can’t confidently say that the impact was the one and only extinction trigger. “I don’t have any trouble with there being an impact,” Weil said, “but show me how it works.”

  More than thirty years after the impact hypothesis reinvigorated the debate over what happened to the non-avian dinosaurs and so many other life forms that shared their world, we’re really only just starting to understand what happened at the end of the Cretaceous. Speaking for myself, I believe that geologists and paleontologists have pinpointed the major players in the extinction, but we have a long way to go before we can tease apart how these factors contributed to one of the most devastating episodes in the history of life. We still lack the delicate resolution needed to comprehend why the Cretaceous went out with a bang. With so much left to discover, I’m in no rush to declare the case closed.

  All we can say for certain about that great extinction is that the swift desolation forever changed the history of life on Earth. As Weil pointed out, the event gave mammals their big evolutionary chance, and the loss of almost all the dinosaur lineages irrevocably changed our world. Dinosaurs didn’t just happen to live on this planet. They were interacting parts of connected ecosystems, and their long tenure on Earth helped set the stage for life as we know it today.

  The paleontologist Jack Horner once said that he wasn’t interested in how dinosaurs died. He was interested in how they lived. He said this at the height of the extinction controversy, in the 1980s. I can’t blame him for wanting to just stay out of the whole mess. But dinosaurs’ lives and their ultimate demise are inextricably tied together. If we can better understand the biology of dinosaurs, we can start to figure out why avian dinosaurs survived while the most monstrous forms were lost forever. Mulling over ideas about dinosaur extinction will lead to discoveries, yielding information on aspects of dinosaur biology we hadn’t expected. The more we learn about dinosaurs, the more complex they become. Even as we find answers, we’re met with new questions. If dinosaurs survived millions of years of climate change, continental drift, and even other mass extinctions, then why did most dinosaurs perish 66 million years ago? Dinosaurs certainly had the flexibility to survive. Some of them did, in feathery avian garb. The riddle is why the non-avian sort—the ones that stalk our dreams and nightmares—did not.

  Epilogue: My Beloved “Brontosaurus”

  When I was little, all I wanted was a pet dinosaur. I kept hoping that someone would find one in a distant jungle, or, taking a cue from Michael Crichton’s fictional genetic engineers, that scientists would hurry up and find a way to bring dinosaurs back to life. I had no idea that Sweetie—my pet budgie—was a dinosaur, and, even if I had known, she wasn’t the same as a sauropod I could ride to school every day. It was unfair that dinosaurs were lost to history. I wanted the dinosaurs to come back. So now, over twenty years after I first met my “Brontosaurus,” it feels a little strange to say that I’m glad that the most wonderful and awesome of dinosaurs are extinct.

  Sometimes I still imagine parting the curtains one morning to find an Apatosaurus plucking leaves from the tree on my front lawn. I’ll admit that, and I wouldn’t look a gift dinosaur in the mouth. But my attitude has changed since I was a kid. I know that dinosaurs do more for us dead than alive. Just like Obi Wan Kenobi in Star Wars, when he warned Darth Vader that he would be more powerful in death than in life, th
e lessons and wisdom we gain from dinosaurs could have come only from their tragic demise.

  If Apatosaurus and other dinosaurs had been given a stay of execution, they wouldn’t seem so special to us. Even though we know birds are dinosaurs, we don’t cherish them the way we do their Mesozoic relatives. They’re too familiar. The same is true of the bizarre and varied fossil mammals that thrived in the wake of the end-Cretaceous extinction. They’re just as spectacular as dinosaurs, but they’re too close to animals we see around us today. A large part of why we keep going back to dinosaurs, I think, is because they’re so alien and exotic. There hasn’t been anything quite like them in 66 million years. The deep gulf created by extinction, separating us from them, makes dinosaurs all the more fantastic. Just think of all of our favorite dinosaurs—Apatosaurus, Tyrannosaurus, Triceratops, Stegosaurus. There is nothing in our era that comes close to their magnificence.

  Time matters. Let’s say a descendant of Apatosaurus had held on, bucked the trend, and persisted to the present day. We could observe, study, and carefully dissect that dinosaur to solve any number of mysteries about its prehistoric counterparts. But we don’t have that straggling sauropod thread to draw answers from. At least the truth about dinosaurs is kept safe in their remains. If we’re going to understand how dinosaurs like Apatosaurus lived, we have no choice but to carefully and gradually piece together what we can from their petrified vestiges. The most amazing of dinosaurs come to us as stone outlines of what they once were, and the struggle of science is to interpret their bones.