My Beloved Brontosaurus

by Brian Switek

  Had the non-avian dinosaurs survived, though, I wouldn’t exist to mourn their loss. A delicate fragment of fossil jaw made this clear. After my brief break, I slowly made my way over to where most of the other field crew members were searching. I got down on my hands and knees and gradually shuffled my way around the hill, checking the little divots and washes for any fossils that might have settled in them after the last storm.

  I didn’t even see the tiny skull fragment until I was right on top of it. The fragile fossil was beautiful—a chocolate-colored piece of an upper jaw, with two coal-black teeth still embedded inside. This was no dinosaur. The arrangement and anatomy of the teeth gave the bone away as part of a small mammal—a little beast with a differentiated tool kit of complex teeth covered in bumps, troughs, and cones.

  The fossilized speck was the best thing I found that day. Dinosaur teeth are easy to find at microsites, but mammal fossils are very rare. As I turned the fossil over in my hand, I thought about my deep connection to the tiny critter. The jaw belonged to one of my Cretaceous family members, undoubtedly a fuzzy little thing with a twitchy nose surrounded by whiskers. This species lived in the dinosaur’s world, yet some members of its mammalian kin didn’t share the same fate as the great archosaurs. From my privileged perspective, I could look back and puzzle over why these meek little beasts eventually inherited the Earth.

  Homo sapiens wasn’t an inevitable evolutionary outcome. Our history has been molded by contingency and chance just as much as that of the dinosaurs. Even though our species is very young, at about 200,000 years old, our mammalian roots go back to diminutive insectivores that lived alongside some of the earliest dinosaurs. The delicate jaw I stumbled across among eastern Montana’s hills was one of those meek critters. Our ancestors and cousins were present for the entire dinosaurian reign. There was never any mammalian contest to dinosaurian dominance; no furry rebellion to overthrow the scaly and feathery rulers. The adaptability and ultimate success of the dinosaurs severely limited the evolutionary routes available to mammals. And although we might want to cheer for a few of our prehistoric relatives who ate dinosaurs—such as the badger-size carnivorous mammal Repenomamus, found with baby dinosaurs in its gut contents—mammals of the dinosaur era stayed small and inhabited the shadowy corners of the Mesozoic world.

  The dinosaur decimation opened up ecological and evolutionary possibilities—opportunities exploited by survivors of a global disaster at the end of the Cretaceous, mammals among them. This dramatic, rapid turnover in life’s history wasn’t simply a matter of dinosaurs handing over the world to mammals. This was one of the worst mass extinctions of all time, an event that took with it the flying pterosaurs, reptilian sea monsters such as mosasaurs and plesiosaurs, the exquisitely coil-shelled cephalopods called ammonites, bizarre rudistid clams, and even some mammal lineages. What could have triggered such a tragedy?

  * * *

  The acceptance of mass extinctions as a reality of life on Earth, and one worth studying, took a circuitous path, thanks to preconceived notions about patterns of change on our planet. The early nineteenth century is a good place to start tracing this thread. During this time, the French anatomist Georges Cuvier divvied up the history of life into successive eras punctuated by dramatic catastrophes. Even though geologists had not yet determined a way to assign absolute ages to rock layers, naturalists of Cuvier’s era perceived several major periods in the history of life on Earth. The first was dominated by fish and other sea life, the second was ruled by huge reptiles (such as the creatures that would eventually be called dinosaurs), and the third was ruled by mammals. There seemed to be clean breaks between successive eras, and Cuvier thought that sweeping calamities explained the sweeping changes to the sorts of life found in each era. Whatever species survived would then take over the vacated spaces and proliferate, and the next cataclysm would continue the cycle of devastation and renewal. (Unfortunately, Cuvier was never clear on what mechanism created the new forms of life that distinguished each age.)

  Partially due to misunderstandings created by the religious and scientific views of his translators, though, Cuvier’s work was associated with a kind of biblical literalism that was being washed away as geologists and paleontologists learned more about prehistory. A contrasting view, ushered in by geologist Charles Lyell and like-minded naturalists from the 1830s onward, considered the Earth to exist in a state of dynamic equilibrium in which gradual, stately change was the rule. There was no evidence for disasters of biblical proportion, and clinging to “catastrophist” views was chided as giving religion precedence over the evidence in the rock record. Paleontologists thought that life followed a similar tempo.

  Charles Darwin’s evolutionary theory—while not exactly welcomed by his peers when first announced—added to Lyell’s “uniformitarian” viewpoint. Life gradually evolved at a steady pace, and as new species were born, they usually drove their parent species into extinction. Earth, and the living things upon it, were transformed in a logical, orderly manner. And since both Lyell’s and Darwin’s ideas were founded on the idea that changes in operation today entirely explained changes in the past—both geological and biological—there was no room for a global catastrophe on the scale of a mass extinction. No one had ever observed such a dramatic change, and to argue that life had been marked by such extreme pressures was special pleading.

  Within this view of dignified, gradual change, the fact that dinosaurs disappeared wasn’t all that surprising to paleontologists of the nineteenth and early twentieth centuries. As Michael Benton has emphasized, earlier generations of scientists thought that “the extinction of the dinosaurs was regarded as a minor hiccup in the progression of life.” According to early-twentieth-century researchers such as Yale’s Richard Swann Lull, dinosaurs “died a natural death” that was long overdue. “[T]he marvel is, not that [the dinosaurs] died,” Lull wrote, “but that they survived so long.” Nature had given dinosaurs a longer tenure than they deserved, and, according to paleontologists of Lull’s generation, dinosaurs eventually succumbed to a built-in evolutionary self-destruct system that operated according to strange rules.

  I was repeatedly told, from elementary school all the way through college, that scientists immediately accepted Darwin’s evolutionary theory. But contrary to this textbook tale, naturalists didn’t immediately welcome the notion of evolution by natural selection, especially paleontologists. They turned a critical eye on the idea: natural selection seemed too weak a force to modify organisms, and to those scientists who wanted to see the benevolent hand of God in nature, the mechanism seemed excessively violent and uncaring for the Creation. So, until the 1940s, when biologists from various backgrounds reaffirmed that natural selection was the main driver of evolutionary change, many paleontologists preferred alternative ideas: that there were internal forces that propelled organisms along golden roads of increasing perfection, or that there were vague biological drives that controlled the birth and death of species.

  Lull subscribed to some of these ideas, and picked dinosaurs as a perfect example of evolution molded by mysterious internal forces. According to Lull, large size, a profusion of ornaments such as spikes and horns, and a general look of “degeneracy” were the telltale indicators of slipping evolutionary vigor, and dinosaurs showed all of these. Impressive as Stegosaurus and other armored dinosaurs appeared, Lull and other paleontologists presumed that the grotesque creatures were investing so much biological energy into growing horns and armor that they could hardly keep their other physiological and biological systems running. The same could be said of the biggest dinosaurs, like the high-shouldered, long-necked Brachiosaurus. These miserable dinosaurs were driven into extinction by evolutionary excesses, Lull and like-minded paleontologists believed, and the idea that dinosaurs grew too big and bizarre for their own good quickly spread through the public imagination. World War I protesters with the Anti-Preparedness Committee adopted Jingo the Stegosaurus as their mascot—a creature that w
as “All Armor Plate—No Brains,” showing the dangers of stockpiling weaponry. And dinosaurs easily served anti-industrialists, too. The massive, undoubtedly dim-witted sauropods might just as well have been the titans of big business, while nimble small businesses—the economic equivalent of mammals—were poised to overrun the reptilian behemoths. Nobody wanted to “go the way of the dinosaur.” This was the most embarrassing sort of extinction—an ultimate death from obsolescence and refusal to change.

  George Wieland, a colleague of Lull’s at Yale’s Peabody Museum of Natural History, was one paleontologist who didn’t follow this trend. Contrary to his early-twentieth-century colleagues, he didn’t think dinosaurs were inherently doomed to fail; he argued, instead, that dinosaurs ate each other into extinction. Inspired by earlier hypotheses that small mammals frequently cracked open dinosaur eggs, Wieland proposed that giant lizards, snakes, and even tyrannosaurs and other theropods consumed so many sauropod eggs that the demand eventually outstripped the supply. Dinosaurs watched over their nests to drive off such thieves, Wieland speculated, but even the most loving of mothers would lose out to the egg-hungry hordes.

  Wieland’s papers were not the last word on dinosaur extinction. Not even close. Without a note informing them that “The butler did it,” paleontologists were left to pick up a fossil cold case that had been lying dormant for 66 million years. The death of the non-avian dinosaurs was a scientific problem so nebulous, and so apparently irresolvable, that anyone who had a wild hair about the topic felt compelled to speak up. This long-running “dilettante” phase of idle speculation, as Michael Benton has called it, produced a slew of weird ideas that implicated just about every ailment or adverse environmental condition paleontologists and wannabe dinosaur experts could dream up. Global warming, global cooling, toxic plants, insufferable stupidity, cataracts, slipped discs, cosmic radiation, eggshells thinned by chemicals, and even reduced sex drive are just a few of the suspects out of more than a hundred ideas proposed during the twentieth century. As Benton has explained, “[T]he whole approach was apparently so easy and such fun that everyone felt that they had the opportunity, if not the duty, to solve the question of why the dinosaurs died out … It was as if, at the mere mention of ‘dinosaur extinction,’ scientists breathed a sigh of relief and felt freed from the straitjacket of normal scientific hypothesis-testing.” There had to be some reason the dinosaurs disappeared, and the floor was open to just about anyone who had even the slightest inkling on the subject.

  As far as I know, no one has yet implicated the CIA, the KGB, or Fidel Castro in the ultimate demise of the dinosaurs. That said, there has never been a shortage of truly outrageous hypotheses, including the sci-fi idea that aliens hunted dinosaurs out of existence. The idea is bunk, regardless of what the buffoons on Ancient Aliens might tell you, but is popular enough that the Utah State University Eastern Prehistoric Museum goes out of its way to list alien extermination along with disease and an ice age as unsupported hypotheses. (“There is no evidence of aliens or their garbage in the fossil record,” the sign advises in deadpan lettering.)

  But my favorite odd proposal was put forward by the entomologist Stanley Flanders in 1962. It is the David vs. Goliath (or, rather, Mothra vs. Godzilla) scenario of caterpillars versus dinosaurs. “The inherent weakness of the reptile was an extraordinary need for an abundance of plant material,” Flanders wrote, and pointed out that Cretaceous caterpillars would have competed with the horned dinosaurs, hadrosaurs, sauropods, and other dinosaurian herbivores for food. Dinosaurs were big, sure, but caterpillars had strength in numbers. In his scenario, the insect larvae gobbled up the world’s forests so quickly that a stunning profusion of butterflies eventually flittered over the carcasses of the starved dinosaurs. “Thus,” Flanders concluded, “the giant reptiles which had survived during eons characterized by great changes in climate, continental uplifts, and different diets may have been exterminated by the lowly caterpillar.” I guess we all have our favorite underdogs.

  Of course, all of this idle speculation stems from the fact that the extinction of the non-avian dinosaurs and their unfortunate contemporaries was an event so massive in scale that it’s difficult to comprehend what could have triggered such a tragedy. The conundrum has gotten only more frustrating with time. Dinosaurs were amazing creatures that, in one form or another, lived on this planet for 230 million years. They were not somehow due for extinction, or in decline—non-avian dinosaurs went out on top. In fact, as paleontologists have reconstructed dinosaurs with ever-greater clarity, and have realized that they were behaviorally complex and dynamic animals, the disappearance of Triceratops and company becomes ever more perplexing. What could have possibly wiped them out with such deadly discrimination? How could anything have destroyed them so mercilessly and completely? The list of possible killers has been narrowed down significantly, but it’s one thing to find a possible murder weapon. It’s another to figure out how and why it was used.

  At the moment, the most prominent suspect is an asteroid, meteorite, or similar chunk of extraterrestrial rock that smashed into Earth’s crust. There are a few competitors, mainly climate change, receding seaways, and intense volcanic activity. But none of these have the same public popularity. We know that all of these factors must have played some role. Intense scrutiny of the rock record has revealed that the world was drastically changing at the end of the Cretaceous. As the global climate cooled, the warm, shallow seas that straddled North America and other continents began to recede as ice built up at the poles. Simultaneously, a massive, sustained outpouring of lava at the Deccan Traps—which left wide flows of once-molten rock in what is now India—dumped greenhouse gases into the atmosphere, further altering climate and weather patterns.

  All these things happened, and yet they’ve been overshadowed by what paleontologists call the “impact hypothesis.” Gradual climate change never got its own big-budget natural-disaster flick, but the idea of extinction by extraterrestrial bolide got two mega-movies in the summer of 1998, not to mention the attendant Aerosmith single for Armageddon. There’s something beautifully, destructively simple about an asteroid impact—the vision of dinosaurs being swept off the evolutionary stage by an unforeseeable stroke of misfortune. To borrow Neil Young’s famous dichotomy, dinosaurs burned out rather than fading away.

  How scientists detected the asteroid’s impact has been told many times over. Still, it’s worth recapitulating the discovery and the reaction to it, considering how radically the idea changed our perception of what happened to the dinosaurs. During the late 1970s, geologist Walter Alvarez was studying the latest Cretaceous rocks in Gubbio, Italy, when his attention was drawn to a half-inch layer of clay that he believed separated the reign of the dinosaurs and the beginning of the age of mammals. In other words, he speculated that the clay layer—known to paleontologists today as the K/Pg (Cretaceous-Paleogene) boundary—was deposited during the time of the end-Cretaceous mass extinction. Alvarez reasoned that he could measure the rapidity of the catastrophe if he could determine how long it took for that clay band to form.

  Alvarez turned to his father, Nobel Prize–winning physicist Luis, to discuss the problem, and the elder Alvarez suggested a time-measurement technique that would ultimately spark a major controversy over the fate of the dinosaurs. Meteorites and other big-enough bits of cosmic flotsam and jetsam penetrate our atmosphere at a near-constant rate, he knew, and these alien rocks contain a relative abundance of elements rarely found in Earth’s crust. One such element is a platinum-group metal called iridium. If this rare-earth element accumulated at a constant rate, the scientists reasoned, then they could use the amount of iridium in the clay layer to constrain the timeline for the end-Cretaceous extinction. As it turned out, the Gubbio clay layer was unusually rich in iridium. It wasn’t an anomaly. When the Alvarez team investigated additional K/Pg samples from Denmark and New Zealand, they found that the spike was a consistent, real feature.

  The abundance of iridium
in the clay layers was not the result of an extended, slow period of deposition. Instead, in a 1980 Science paper, the Alvarez team concluded that the excess iridium came from an immense asteroid that, they hypothesized, struck the Earth right at the time of the Cretaceous mass extinction—the polar opposite of the slow, stately changing of the guard that paleontologists had suspected for decades was the real answer.

  The Alvarez team wasn’t the first to propose death from the skies, but they were the scientists who had the physical evidence to back it up. A few years before, the paleontologist Dale Russell and the physicist Wallace Tucker had proposed that a supernova triggered the end-Cretaceous extinction, but the Alvarez team had the answer locked in stone. The iridium band was the first undeniable clue that something strange had happened.

  * * *

  By the time my dinophilic self started pestering my parents to tape anything remotely dinosaur-related on TV in the mid-1980s, the problem of what caused the end-Cretaceous extinction seemed all but resolved. I knew this not because of Alvarez, but because Superman told me so. In 1985, Christopher Reeve (to me, the one and only Superman) hosted the Mesozoic extravaganza Dinosaur! The show was an inspiring mix of dinosauriana, from movie clips to notes on new discoveries and, best of all, superb stop-motion dinosaurs created by Phil Tippett. (While not entirely accurate, Tippett’s creations still look far better than many of the stiff, cheaply rendered CGI dinosaurs that stampede across basic cable networks.) The show wasted little time setting the stage for the doom of the dinosaurs. Just under six minutes in, as an asteroid ominously rotates through the vacuum of space, Reeve asks, “What terror came out of the skies to end the dinosaurs?” Cut to a pair of courting Edmontosaurus—gentle and sensitive herbivores who guide their sole offspring through a hazardous Cretaceous world where a hungry tyrannosaur might be lurking just behind the next tree. These were not mindless abominations, but unique creatures with family values who were snuffed out by an awful happenstance.