The Rise and Fall of the Dinosaurs

by Steve Brusatte

  Around the same time we were doing our study, other researchers were examining the dinosaur extinction from other angles. Teams led by Paul Upchurch and Paul Barrett in London undertook a census of dinosaur species diversity over the course of the Mesozoic—a simple count of how many dinosaurs were alive at every given point of their reign, corrected for biases caused by the uneven quality of the fossil record. They found that dinosaurs as a whole were still very diverse at the time the asteroid hit, as numerous species were frolicking throughout not only North America but the entire planet. Curiously enough, however, the horned and duck-billed dinosaurs underwent a decline in species numbers right at the end of the Cretaceous, coincident with their decline in disparity.

  What would all of this have meant in real-world terms? After all, it was a curious mix: most dinosaurs doing fine, but the big plant-guzzlers showing signs of stress. This question was addressed by a clever computer modeling study by one of the new breed of highly quantitative graduate students: Jonathan Mitchell from the University of Chicago. Jon and his team built food webs for several Cretaceous dinosaur ecosystems, based on careful review of all the fossils that had been found at particular field sites—not only the dinosaurs, but everything they lived with, from crocodiles and mammals down to insects. Then they used computers to simulate what would happen if a few species were knocked out. The result was startling: those food webs that existed when the asteroid struck, which had fewer large herbivores at their bases because of the diversity decline, collapsed easier than the more diverse food webs from only a few million years before the impact. In other words, the loss of some of the big herbivores, even without the decline of any of the other dinosaurs, made end-Cretaceous ecosystems highly vulnerable.

  Statistical analyses and computer simulations are all well and good, and there’s no doubt that they are the future of dinosaur research, but they can be a little abstract, and sometimes it’s useful to simplify things. In paleontology, that means going back to the fossils themselves: holding them in your hands and thinking deeply about them as living, breathing animals, considering them as the very animals that first had to cope with those Late Cretaceous volcanic eruptions and temperature and sea-level shifts, then later stare down an asteroid the size of a mountain.

  What we really want to study are the fossils of those last surviving dinosaurs, the ones that witnessed or came close to witnessing the asteroid do its dirty work. Unfortunately, there are only a few places in the world that preserve these types of fossils—but they are starting to tell a convincing story.

  The most famous place, without doubt, is Hell Creek. People have been collecting the bones of T. rex, Triceratops, and their contemporaries for well over a hundred years now throughout the upper Great Plains of the American West. The rocks of Hell Creek are very well dated, too. And that means you can track the diversity and abundance of dinosaurs through time, right up to the iridium layer that fingerprints the asteroid. A number of scientists have done just that—my friend David Fastovsky (author of the best dinosaur textbook on the market) and his colleague Peter Sheehan, a team led by Dean Pearson, and other crews led by Tyler Lyson, a gifted young scientist who grew up on a sprawling ranch in North Dakota in the heart of some of the best dinosaur-bone badlands. They’ve all found the same thing: dinosaurs were thriving all throughout the time the Hell Creek rocks were laid down, as the Indian volcanoes were erupting and temperatures and sea levels were changing, right up to that moment the asteroid hit. There are even Triceratops bones a few centimeters below the iridium. It seems that the asteroid caught the residents of Hell Creek blissfully unaware, right at the peak of their glory days.

  Things were similar in Spain, where important new discoveries are emerging from the Pyrenees, along the border with France. This area is being scoured by an energetic duo of thirty-something paleontologists—Bernat Vila and Albert Sellés, two of the most dedicated guys I know, who often find themselves working for months on end without a salary, victims of Spain’s torturously slow recovery from a series of financial crises that began in the late 2000s. Somehow that hasn’t stopped them. They keep finding dinosaur bones, teeth, footprints, and even eggs. These fossils show that a diverse community—including theropods, sauropods, and duckbills—persisted here into the very latest Cretaceous with no indication that anything was amiss. It’s interesting that, a few million years before the asteroid hit, there was a brief turnover event, when armored dinosaurs disappeared locally and more primitive plant-eaters were replaced by advanced duckbills. It’s possible that this is related to the decline of the big plant-eaters in North America, although this is hard to prove. It may be that changes in sea level were to blame; as seas rose and fell, they carved up the land that dinosaurs could live on, which led to some small changes in the composition of ecosystems.

  Finally, the story appears to be the same in Romania, where Mátyás Vremir and Zoltán Csiki-Sava have been collecting a great diversity of latest Cretaceous dinosaurs, and also in Brazil, where Roberto Candeiro and his students keep finding more teeth and bones of big theropods and enormous sauropods that probably made it to the end. The drawbacks of these places are that the rocks are still not dated very well, so we can’t be absolutely sure where the dinosaur fossils sit relative to the Cretaceous-Paleogene boundary, but no doubt the dinosaurs in both areas are latest Cretaceous in age, and there are no signs that they were in any type of trouble.

  There was so much new evidence from fossils, statistics, and computer modeling that Richard Butler and I figured the time had come to synthesize it. We came up with something of a dangerous idea: perhaps we could recruit a crack team of dinosaur experts to sit down, discuss everything we currently know about the dinosaur extinction, and try to come to a consensus on why we thought dinosaurs died out. Paleontologists had been arguing for decades on this topic, and in fact it was dinosaur workers who were some of the most ardent skeptics of the asteroid hypothesis in the 1980s. We thought our subversive little plot might end in deadlock or, worse, in a shouting match, but quite the opposite happened. Our team came to an agreement.

  Dinosaurs were doing well in the latest Cretaceous. Their overall diversity—both in terms of species numbers and anatomical disparity—was fairly stable. It had not been gradually declining for millions and millions of years, nor was it clearly increasing. The major groups of dinosaurs all persisted into the very latest Cretaceous—theropods big and small, sauropods, horned and duck-billed dinosaurs, dome-headed dinosaurs, armored dinosaurs, smaller plant-eaters, and omnivores. At least in North America, where the fossil record is best, we know that T. rex, Triceratops, and the other Hell Creek dinosaurs were there when the asteroid destroyed much of the Earth. All of these facts rule out the once popular hypothesis that dinosaurs wasted away gradually due to long-term changes in sea level and temperature or that the Indian volcanoes had started to pick away at the dinosaurs earlier in the Late Cretaceous, a few million years before the end.

  Instead, we found that there is no doubt about it: the dinosaur extinction was abrupt, in geological terms. This means that it happened over the course of a few thousand years at most. Dinosaurs were prospering, and then they simply disappear from the rocks, simultaneously all over the world, wherever latest Cretaceous rocks are known. We never find their fossils in the Paleogene rocks laid down after the asteroid impact—nothing, not a single bone or a single footprint anywhere. This means a sudden, dramatic, catastrophic event is likely to blame, and the asteroid is the obvious culprit.

  However, there is a nuance. The big herbivores did undergo a bit of a decline right before the end of the Cretaceous, and the European dinosaurs experienced a turnover as well. This decline apparently had consequences: it made ecosystems more susceptible to collapse, making it more likely that the extinction of just a few species would cascade through the food chain.

  All told, then, it appears the asteroid came at a horrible time for the dinosaurs. If it had hit a few million years earlier, before the dip
in herbivore diversity and perhaps the European turnover, ecosystems would have been more robust and would have been in a better position to deal with the impact. If it happened a few million years later, maybe herbivore diversity would have recovered—as it had countless other times over the preceding 150-plus million years of dinosaur evolution, when small diversity declines occurred and were corrected—and ecosystems again would have been more robust. There’s probably never a good time for a six-mile-wide asteroid to shoot down from the cosmos, but for dinosaurs, 66 million years ago may have been among the worst possible times—a narrow window when they were particularly exposed. If it had happened a few million years earlier or later, maybe it wouldn’t just be seagulls congregating outside my window but tyrannosaurs and sauropods too.

  Or perhaps not. It’s possible the massive asteroid would have done them in regardless. Maybe there was no escape from something that big, packing that kind of punch when it barreled its way into the Yucatán. Whatever the exact sequence of events, I’m confident the asteroid was the primary reason that the non-bird dinosaurs died out. If there is one, single straightforward proposition that I would stake my career on, it would be this: no asteroid, no dinosaur extinction.

  THERE IS ONE final puzzle that I haven’t addressed yet. Why did all the non-bird dinosaurs die at the end of the Cretaceous? After all, the asteroid didn’t kill everything. Plenty of animals made it through: frogs, salamanders, lizards and snakes, turtles and crocodiles, mammals, and yes, some dinosaurs—in the guise of birds. Not to mention so many shelled invertebrates and fishes in the oceans, although that could be the subject of another book entirely. So what was it about T. rex, Triceratops, the sauropods, and their kin that made them a target?

  This is a key question. We want to answer it particularly because it’s relevant to our modern world. When there is sudden global environmental and climate change, what lives and what dies? It’s case studies in the history of life—recorded by fossils, like the end-Cretaceous extinction—that provide critical insight.

  The first thing we have to realize is that, although some species did survive the immediate hellfire of the impact and the longer-term climate upheaval, most did not. It’s estimated that some 70 percent of species went extinct. That includes a whole lot of amphibians and reptiles and probably the majority of mammals and birds, so it’s not simply “dinosaurs died, mammals and birds survived,” the line often parroted in textbooks and television documentaries. If not for a few good genes or a few strokes of good luck, our mammalian ancestors might have gone the way of the dinosaurs, and I wouldn’t be here typing this book.

  There are some things, however, that do seem to distinguish the victims from the survivors. The mammals that lived on were generally smaller than the ones that perished, and they had more omnivorous diets. It seems that being able to scurry around, hide in burrows, and eat a whole variety of different foods was advantageous during the madness of the postimpact world. Turtles and crocodiles fared pretty well compared to other vertebrates, and that is probably because they were able to hide out underwater during those first few hours of bedlam, shielding themselves from the deluge of rock bullets and the earthquakes. Not only that, but their aquatic ecosystems were based on detritus. The critters at the base of their food chain ate decaying plants and other organic matter, not trees, shrubs, and flowers, so their food webs would not have collapsed when photosynthesis was shut down and plants started to die. In fact, plant decay would have just given them much more food.

  Dinosaurs had none of these advantages. Most of them were big, and they couldn’t easily scamper into burrows to wait out the firestorm. They couldn’t hide underwater, either. They were parts of food chains with big plant-eating species at the base, so when the sun was blocked and photosynthesis shut down and plants started to die, they felt the domino effects. Plus, most dinosaurs had fairly specialized diets—they ate meat or particular types of plants, without the flexibility that came with the more adventurous palates of the surviving mammals. And they had other handicaps as well. Many of them were probably warm-blooded or at least had a high metabolism, so they required a lot of food. They couldn’t hunker down for months without a meal, like some amphibians and reptiles. They laid eggs, which took between three and six months to hatch, about double the time for birds’ eggs. Then, after the eggs hatched, it took dinosaur youngsters many years to grow into adults, a long and tortured adolescence that would have made them particularly vulnerable to environmental changes.

  After the asteroid hit, there was probably no one thing that sealed the dinosaurs’ fate. They just had a lot of liabilities working against them. Being small, or having an omnivorous diet, or reproducing quickly—none of these things guaranteed survival, but each one increased the odds in what was probably a maelstrom of chance as the Earth devolved into a fickle casino. If life in that moment boiled down to a game of cards, dinosaurs were left holding a dead man’s hand.

  Some species, however, cashed in on a royal flush. Among them were our mouse-size ancestors, which made it to the other side and soon had the opportunity to build their own dynasty. Then there were the birds. Lots of birds and their close feathery dinosaur cousins died—all of the four-winged and batlike dinosaurs, all of the primitive birds with long tails and teeth. But modern-style birds endured. We don’t know why exactly. Maybe it was because their big wings and powerful chest muscles allowed them to literally fly away from the chaos and find safe shelter. Perhaps it was because their eggs hatched quickly, and once out of the nest, the fledglings grew rapidly into adults. It could be that they were specialized for eating seeds—little nuggets of nutrition that can survive in the soil for years, decades, even centuries. Most likely, it was a combination of these assets and others that we have yet to recognize. That and a whole lot of good luck.

  After all, so much about evolution—about life—comes down to fate. The dinosaurs got their very chance to rise up after those terrible volcanoes 250 million years ago wiped out nearly every species on Earth, and then they had the good fortune to sail through that second extinction at the end of the Triassic, which felled their crocodile competitors. Now the tables had turned. T. rex and Triceratops were gone. The sauropods would thunder across the land no more. But let’s not forget about those birds—they are dinosaurs, they survived, they are still with us.

  The dinosaur empire may be over, but the dinosaurs remain.

  Epilogue:

  After the Dinosaurs

  Chapter Title art by Todd Marshall

  EVERY MAY I HEAD OUT to the desert of northwestern New Mexico, not too far from the Four Corners. It’s a bit of a break, coming on the heels of exams and paper grading and the usual end-of-semester mania. I usually stay for a couple of weeks, and by the end of my trip, the calm of the empty desert and the spicy food we make every night in camp have succeeded in easing my stress.

  It’s not a vacation, however. As is usual when I travel these days, I’m here on business—to do what I’ve spent the last decade doing all over the world, in Polish quarries and on the frigid tidal platforms of Scotland, in the shadows of Transylvanian castles, the outback of Brazil, or the radiating sauna of Hell Creek.

  I’m here to find fossils.

  Many of these fossils, of course, are dinosaurs. In fact, they’re among the last surviving dinosaurs of all, ones that were living about a thousand miles south of Hell Creek during those final few million years of the Cretaceous. They were flourishing at a time when history seemed to be standing still, when it appeared that dinosaurs would keep on ruling the world forever, as they had done for over 150 million years. We find the bones of tyrannosaurs and huge sauropods, the skull domes that pachycephalosaurs used to head-butt each other, the jaws that horned and duck-billed dinosaurs sliced up plants with, and lots of teeth of raptors and other small theropods that skittered around underneath the big boys. So many species, living together in harmony, not a hint that things would soon go horribly wrong.

  Truth be told,
though, I’m not here for the dinosaurs. That may seem like sacrilege, as I’ve spent most of my young career on the trail of T. rex and Triceratops. No, I’m trying to understand what happened after the dinosaurs disappeared—how the Earth healed, a fresh start was made, and a new world was forged.

  Most of the candy-striped badlands in this part of New Mexico—in the vast and mostly uninhabited areas of Navajo country, around the towns of Cuba and Farmington—are carved from rocks laid down in rivers and lakes during the first few million years after the asteroid hit. Gone are the tyrannosaur teeth and big chunks of sauropod bone that are so common in the latest Cretaceous rocks of the area, deposited only a few feet below the rocks we’re looking at now, which date from the subsequent Paleogene Period (66 to 56 million years ago). There was a sudden change here; the asteroid blew away one world and ushered in another. Many dinosaurs, then all of a sudden, none at all. It’s a pattern eerily similar to what Walter Alvarez saw in the forams of the Gubbio gorge.

  I walk these dry New Mexican hills with one of my best friends in science: Tom Williamson, a curator at the natural history museum in Albuquerque. Tom has been collecting out here for twenty-five years, starting as a graduate student. He often brings along his twin sons, Ryan and Taylor, who through countless camping trips with their dad have developed a knack for finding fossils that rivals that of almost any paleontologist that I know—even Grzegorz Niedźwiedzki in Poland and Mátyás Vremir in Romania. Other times, Tom comes out here with his students, young Navajos from the surrounding reservations, whose families have lived on this sacred land for generations. And once a year in May, Tom meets me and my students from Edinburgh. Ryan and Taylor—who are now in college—usually tag along, and we have endless fun finding fossils by day and sitting around the campfire at night, telling the sort of inside jokes that develop after many years of being in the field together.