The Rise and Fall of the Dinosaurs

by Steve Brusatte

  The spiking temperatures lit forests on fire—maybe not all over the world, but certainly in much of North America and anywhere else within a few thousand miles of the Yucatán. We see the singed remnants of leaves and wood—the kind of stuff left after a campfire has been extinguished—in rocks that were laid down right after the asteroid hit. The soot from the fires, with other dust and grime kicked up by the impact but too light to fall back down to earth, would have floated up into the atmosphere, clogging the currents that circulate air across the globe, until the entire planet was dark. The ensuing period—thought to be equivalent to a global nuclear winter—probably killed off most of the dinosaurs in areas far from the smoldering crater.

  I could go on and on, exhausting my thesaurus, but if I go much further, you probably won’t believe me. Which would be a shame, because all that I write really happened. And we know that because of the work of one man, a geological genius who is one of my scientific heroes: Walter Alvarez.

  WE’VE ALREADY ESTABLISHED that I did some silly things in high school, when my obsession with dinosaurs overtook my better judgment. My fanboy stalking of Paul Sereno wasn’t nearly the worst of it. Nothing was more brazen than when I picked up the phone one day in the spring of 1999 and cold-called Walter Alvarez at his office in Berkeley, California. I was a fifteen-year-old kid with a rock collection; he was the eminent National Academy of Sciences member who nearly twenty years earlier had proposed the idea that a giant asteroid impact killed off the dinosaurs.

  He answered on the second ring. Even more astounding, he didn’t hang up as I rambled on about the purpose of my call. I had read his book T. rex and the Crater of Doom—still, to my mind, one of the best pop-science books on paleontology ever written—and was captivated by how he put together the clues that pointed to the asteroid. His book explained how the detective game started in a rocky gorge on the outskirts of the medieval commune of Gubbio, in the Apennine Mountains of Italy. It was here where Alvarez first noticed the unusual character of the thin band of clay that marked the end of the Cretaceous. As chance had it, my family was gearing up for a trip to Italy to celebrate my parents’ twentieth wedding anniversary. It would be my first time outside of North America, and I wanted to make it memorable. For me, that wasn’t basilicas and art museums, but a pilgrimage to Gubbio, to stand on the spot where Alvarez started to figure out one of the biggest riddles in science.

  But I needed directions, so I decided to go straight to the source.

  Professor Alvarez gave me detailed instruction that even a kid without any modicum of Italian could follow. We also talked for a while about my interest in science. Looking back, I am astonished that such a scientific giant could be as kind and generous with his time as he was. But alas, it turned out to be for naught, because my family never made it to Gubbio that summer. Floods closed the main rail line from Rome, and I was devastated. My whining nearly ruined my parents’ second honeymoon.

  Five years later, however, I was back in Italy for a college geology field course. We were staying in a small observatory in the Apennines run by Alessandro Montanari, one of the many scientists who made a name in the 1980s studying the end-Cretaceous extinction. On our first-day tour we passed through the library, where a solitary figure was scrutinizing a geological map under a flickering light.

  “I want you all to meet my friend and mentor, Walter Alvarez,” Sandro said in his singsong Italian accent. “Some of you may have heard of him.”

  I was paralyzed. Never, before or since, have I been as gobsmacked. The rest of the tour was a blur, but afterward I sneaked back to the library and gently opened the door. Alvarez was still there, hunched over the map in a trance of concentration. I felt bad about interrupting him—maybe he was homing in on some other unsolved mystery of Earth history. I introduced myself and was gobsmacked a second time when he remembered our conversations from a few years back.

  “Did you make it to Gubbio back then?” he asked me.

  I could only mutter an embarrassed no, not really wanting to admit that I had wasted his time with that phone call—and several e-mails that followed.

  “Well then, get ready, because I’m taking your class there in a few days,” he replied. I flashed a megawatt smile.

  Days later we were in Gubbio, gathered in the gorge, Mediterranean sun beaming down and fast cars whizzing by, a fourteenth-century aqueduct perched precariously on the cliffs above. Walter Alvarez stepped in front of us. His khakis were stuffed with rock samples; he wore a wide-brimmed hat and reflective aquamarine shirt to ward off the sun. He pulled his hammer out of its holster, and pointed downward and to his right, to a thin gouge in the rock that cut through the rosy pink limestone forming most of the gorge. This rock was softer, finer; it was a layer of clay, about one centimeter thick, a bookmark separating the limestones of the Cretaceous below from those of the postextinction Paleogene period above. It was here—this man, standing at this spot, looking at this strip of clay—where the asteroid theory was conceived a quarter century earlier.

  Afterward, we stopped for truffle pasta, white wine, and biscotti at a five-hundred-year-old restaurant just down the road. Before our lunch, we dutifully signed a leather guest book, inscribed with the names of many of the geologists and paleontologists who have come through Gubbio to study the gorge and its celebrity clay. It read like a Hall of Fame roster, and I’ve never taken more pride in signing my name. For the next two hours, I sat across from Walter as, between mouthfuls of linguine, he told my starstruck classmates and me the story of how he cracked the dinosaur mystery.

  Walter Alvarez pointing to the boundary between the Cretaceous rocks (below) and Paleocene rocks (above) in Gubbio, Italy. The boundary is the divot located between his rock hammer and right knee.

  Courtesy of Nicole Lunning.

  In the early 1970s, not long after Walter finished his PhD, the plate-tectonics revolution had consumed the science of geology, and people now realized that continents moved around over time. One way you could track their motions was by looking at the orientation of small crystals of magnetic minerals, which point themselves toward the North Pole when lavas or sediments harden into stone. Walter reckoned that this new science of paleomagnetism could help untangle how the Mediterranean region was assembled—how small plates of crust rotated and crushed into each other to form modern-day Italy and raise the Alps. That is what first brought him to Gubbio, to measure microscopic bits of minerals within the thick limestone sequence of the gorge. But when he was there, he became intrigued by an even bigger mystery. Some of the rocks he was measuring were crammed with fossil shells of all shapes and sizes, which belonged to a great diversity of creatures called forams—tiny predators that float around in the ocean plankton. Above these rocks, however, were nearly barren limestones, sprinkled with a few tiny, simple-looking forams.

  Walter was observing a line between life and death. It’s the geological equivalent of listening to those last few moments on a cockpit voice recorder before it gives way to silence.

  Walter wasn’t the first person to notice it. Geologists had been working in the gorge for decades, and painstaking work by an Italian student named Isabella Premoli Silva had determined that the diverse forams were Cretaceous in age, the simple ones from the Paleogene. The knife-edge separation between them corresponded to what had long been recognized as a mass extinction—one of those unusual times in Earth history when lots of species disappear simultaneously all over the world.

  But this wasn’t your average mass extinction. Specks of plankton weren’t its only casualties, and it wasn’t confined to the water. It decimated the oceans and the land, and killed off many other types of plants and animals.

  Including the dinosaurs.

  No way could that be coincidence, Walter thought. What happened to the forams must have been linked to what happened to the dinosaurs and all of the other things that perished, and he wanted to figure it out.

  The key, he realized, was hidden in that tiny stri
p of clay between the fossil-rich Cretaceous limestones and the sterile Paleogene ones. But when he first saw it, it didn’t seem all that special. It wasn’t heaving with mangled fossils, streaked with flamboyant colors, or rotten in scent. It was just clay, so fine that you couldn’t even see the individual grains with the naked eye.

  Walter called his dad for help. His father just so happened to be a Nobel Prize–winning physicist: Luis Alvarez, who had discovered a host of subatomic particles and had been one of the key players in the Manhattan Project. (He even flew behind the Enola Gay to monitor the effects of Little Boy when it was dropped on Hiroshima.) Alvarez the younger thought that Alvarez the elder might have some unconventional ideas for chemically analyzing the clay. Maybe there was something hidden in there that could tell them how long it took the thin layer to form. If it formed gradually, the product of millions of years of slow accumulation of dust in the deep ocean, then the death of the forams, and thus of the dinosaurs, was a drawn-out affair. But if it was deposited suddenly, that meant the Cretaceous must have ended in catastrophe.

  Measuring the length of time it took a rock layer to form is tricky—one of the headaches faced by all geologists. But in this case, the father-and-son team came up with what they figured was a clever solution. Heavy metals—some of those elements in the nether regions of the periodic table, like iridium—are rare on Earth’s surface, which is why most people have never heard of them. But tiny amounts of them fall at a more or less constant rate from the deep reaches of outer space as cosmic dust. The Alvarezes reasoned that if the clay layer had only a tiny peppering of iridium, then it had formed very quickly; if it had a larger amount, then it must have formed over a much longer time period. New instruments now allowed scientists to measure even very small concentrations of iridium, including one in a lab at Berkeley run by one of Luis Alvarez’s colleagues.

  They weren’t prepared for what they found.

  They found iridium all right—lots of it. Too much of it. There was so much iridium that it would have taken many tens of millions of years—maybe even hundreds of millions of years—of steady cosmic dusting to deliver it all. Which was impossible, because the limestones above and below the clay were dated well enough that the Alvarezes knew that the clay layer could have been deposited over only a few million years at most. Something was amiss.

  Maybe it was a mistake, some local quirk of the Gubbio gorge. So they went to Denmark, where rocks of the same age jut into the Baltic Sea. Here, too, they found an iridium anomaly right at the Cretaceous-Paleogene boundary. Before long, a tall young Dutchman named Jan Smit caught wind of what the Alvarezes were doing and reported that he had also been sniffing around for iridium—and had found a spike at the boundary in Spain. More reports of iridium soon followed, from rocks formed on land, in shallow water, and in the deep ocean, all at that fateful moment when dinosaurs disappeared.

  The iridium anomaly was real. The Alvarezes went through the possible scenarios: volcanoes, flooding, climate change, and others, but only one made sense. Iridium is super-rare on Earth but much more common in outer space. Could something from the deep expanses of the solar system have delivered an iridium bomb 66 million years ago? Perhaps it was a supernova explosion, but more likely a comet or an asteroid. After all, as the many craters pockmarking the surface of the Earth and moon attest to, these interstellar visitors do occasionally bombard us. It was a bold idea, but not a crazy one.

  Luis and Walter Alvarez, with their Berkeley colleagues Frank Asaro and Helen Michel, published their provocative theory in Science in 1980. It unleashed a decade of scientific frenzy. Dinosaurs and mass extinctions were constantly in the news, the impact hypothesis was debated in countless books and television documentaries, a dinosaur-killing asteroid made the cover of Time, and hundreds of scientific papers went back and forth on what really killed the dinosaurs, with scientists as diverse as paleontologists, geologists, chemists, ecologists, and astronomers weighing in on the hottest scientific issue of the day. There were feuds, egos clashed, but the crucible of fierce debate put everyone at the top of their game, as they gathered (or disputed) evidence for an impact.

  By the end of the 1980s, it was undeniable that the Alvarezes were correct: an asteroid or comet did hit the planet 66 million years ago. Not only was the same iridium layer found all over the world, but other geological oddities pointing to an impact were found alongside the iridium. There was a strange type of quartz in which the mineral planes had collapsed, leaving a telltale sign of parallel bands shooting through the crystal structure. This “shocked quartz” had previously been found in only two places: the rubble of nuclear bomb tests and the inside of meteor craters, formed from the fierce shock waves of these explosive events. There were spherules and tektites—spherical or spear-shaped bullets of glass forged from the melted products of a big collision that cooled as they fell back down through the atmosphere. Tsunami deposits were discovered around the Gulf of Mexico, dating right to the Cretaceous-Paleogene boundary, showing that a monumental event caused monstrous earthquakes right when the quartz was being shocked and the tektites were falling.

  Then, as the 1990s dawned, the crater was finally found. The smoking gun. It had taken a while to find it because it was buried under millions of years of sediment in the Yucatán. The only detailed studies of the area had been carried out by oil-company geologists who kept their maps and samples locked up for many years. But there could be no doubt: the 110-mile-wide (180 km) hole buried under Mexico, called the Chicxulub crater, was dated right to the end of the Cretaceous, 66 million years ago. It is one of the largest craters on Earth, a sign of just how big the asteroid was, how catastrophic the impact. It was probably one of the biggest, perhaps the biggest asteroid to hit Earth in the last half billion years. The dinosaurs probably didn’t stand a chance.

  BIG DEBATES IN science—particularly those that spill out of the specialist journals and into the public eye—always attract skeptics. So it was with the asteroid theory. These dissenters couldn’t argue that there was no asteroid—the discovery of the Chicxulub crater made such a claim foolish. Instead, they contended that the asteroid was wrongly accused, an innocent bystander that just so happened to smash into the Yucatán when dinosaurs and the many other things that died out at the end of the Cretaceous—the flying pterosaurs and sea-living reptiles, the coiled ammonites, the big and diverse foram communities in the ocean, and many others—were already on their way out. At worst, the asteroid was the coup de grace that finished a holocaust nature had already started.

  It might seem too coincidental to take seriously—a six-mile-wide asteroid arriving exactly when thousands of species were already on their deathbed. However, unlike the flat-earthers and global-warming deniers, these skeptics had a point. When the asteroid fell from the sky, it didn’t rudely interrupt some kind of static, idyllic, lost world of the dinosaurs. No, it hit a planet that was in quite a bit of chaos. The big volcanoes in India that the asteroid kicked into overdrive had actually started erupting a few million years before. Temperatures were gradually getting cooler, and sea levels were fluctuating dramatically. Maybe some of these things factored into the extinction? Perhaps they were the primary culprits; maybe these longer-term environmental changes were causing dinosaurs to slowly waste away.

  The only way to test these ideas against each other is to look very closely at the evidence that we have—dinosaur fossils. What we have to do is track dinosaur evolution over time, to see if there are any long-term trends and see what changes occurred at or near the Cretaceous-Paleogene boundary, when the asteroid hit. This is where I enter the picture. From the time I first spoke with Walter Alvarez on the phone, I was hooked on the riddle of the dinosaur extinction. My addiction spiraled as I stood next to Walter in the Gubbio gorge. Then, as a graduate student I finally had a chance to make my own contribution to the debate, using one of the specialties I had developed as a young researcher: using big databases and statistics to study evolutionary trends. />
  My venture into the extinction debate was a joint one, with my old friend Richard Butler. A few years earlier, we were bushwhacking through Polish quarries on the hunt for footprints of the very oldest dinosaurs; now in 2012 as I was starting to wrap up my PhD, we wanted to know why the descendants of these wispy ancestors disappeared over 150 million years later, after they became so phenomenally successful. The question we asked ourselves was this: how were dinosaurs changing during the 10 to 15 million years before the asteroid hit? The way we addressed it was using morphological disparity, the same metric that I used to study the very oldest dinosaurs, which quantifies the amount of anatomical diversity over time. Increasing or stable disparity during the latest Cretaceous would indicate that dinosaurs were doing rather well when the asteroid came, whereas declining disparity would suggest they were in trouble and maybe already on their way to extinction.

  We crunched the numbers and found some intriguing results. Most dinosaurs had relatively steady disparity during that last gasp before the impact, including the meat-eating theropods, long-necked sauropods, and small to midsize plant-eaters like the dome-skulled pachycephalosaurs. There was no sign that anything was wrong with them. But two subgroups were in the midst of a disparity decline: the horned ceratopsians like Triceratops and the duck-billed dinosaurs. These were the two main groups of large-bodied plant-eaters, which consumed enormous amounts of vegetation with their sophisticated chewing and leaf-shearing abilities. If you were standing around during the latest Cretaceous—anytime between about 80 and 66 million years ago—it was these dinosaurs that would have been most abundant, at least in North America where the fossil record of this time is best. They were the cows of the Cretaceous, the keystone herbivores at the base of the food chain.