Introduction
Picture yourself in the Cretaceous period. It’s a day like most any other, a sunny afternoon in the Hell Creek of ancient Montana about 66 million years ago. The ground is a bit mushy, a fetid muck saturated from recent rains that caused a nearby floodplain stream to overrun its banks. If you didn’t know any better, you might think you were wading on the edge of a Gulf Coast swamp on a midsummer day. Magnolias and dogwoods shoulder their way into stands of conifers, ferns, and other low-lying plants gently waving in the light breeze drifting over the open ground you now stand upon. But a familiar face soon reminds you that this is a different time.
A Triceratops horridus ambles along the edge of the forest, three-foot-long brow horns slightly swaying to and fro as the pudgy dinosaur shuffles its scaly, ten-ton bulk over the damp earth. The dinosaur is a massive quadruped, seemingly a big, tough-skinned platform meant to support a massive head decorated with a shield-like frill jutting from the back of the skull, a long horn over each eye, a short nose horn, and a parrot-like beak great for snipping vegetation that is ground to messy pulp by the plant-eater’s cheek teeth. The massive herbivore snorts, making some unseen mammal chitter and scramble in alarm somewhere in the shaded depths of the woods. At this time of the day, with the sun still high and temperatures above 80 degrees, there’s barely another dinosaur in sight—the only other “terrible lizards” plainly in view are a couple of birds perched on a gnarled branch peeking out from just inside the shadow of the forest. The avians seem to grin, their tiny insect-snatching teeth jutting from their beaks.
This is where we’ll watch the Age of Dinosaurs come crashing to a fiery close.
In a matter of hours, everything before us will be wiped away. Lush verdure will be replaced with fire. Sunny skies will grow dark with soot. Carpets of vegetation will be reduced to ash. Contorted carcasses, dappled with cracked skin, will soon dot the razed landscape. Tyrannosaurus rex—the tyrant king—will be toppled from their throne, along with every other species of non-avian dinosaur no matter their size, diet, or disposition. After more than 150 million years of shaping the world’s ecosystems and diversifying into an unparalleled saurian menagerie, the terrible lizards will come within a feather’s breadth of total annihilation.
We know the birds survive, and even thrive, in the aftermath of what’s to come. A small flock of avian species will carry on their family’s banner, perched to begin a new chapter of the dinosaurian story that will unfold through tens of millions of years to our modern era. But our favorite dinosaurs in all their toothy, spiked, horned, and clawed glory will vanish in the blink of an eye, leaving behind scraps of skin, feather, and bone that we’ll unearth eons later as the only clues to let us know that such fantastic reptiles ever existed. Through such unlikely and delicate preservation our favorite dinosaurs will become creatures that defy tense—their remains still with us, but stripped of their vitality, simultaneously existing in the present and the past.
The non-avian dinosaurs won’t be the only creatures to be so harshly cut back. The great, batwinged pterosaurs, some with the same stature as a giraffe, will die. Fliers like Quetzalcoatlus, with a wingspan wider than a Cessna and capable of circumnavigating the globe, will disappear just as quickly as the non-avian dinosaurs. In the seas, the quad-paddled, long-necked plesiosaurs and the Komodo dragon cousins called mosasaurs will go extinct, as well as invertebrates like the coil-shelled squid cousins, the ammonites, and flat, reef-building clams bigger than a toilet seat. The diminutive and unprepossessing won’t get a pass either. Even among the surviving families of the Cretaceous world, there will be dramatic losses. Marsupial mammals will almost be wiped out in North America, with lizards, snakes, and birds all suffering their own decimation, too. Creatures of the freshwater rivers and ponds will be among the few to get any sort of reprieve. Crocodiles, strange reptilian crocodile mimics called champsosaurs, fish, turtles, and amphibians will be far more resilient in the face of the impending disaster, their lives spared by literal inches.
We know the ecological murder weapon behind this Cretaceous case study. An asteroid or similar body of space rock some seven miles across slammed into Earth, leaving a geologic wound over fifty miles in diameter. Most species from the Cretaceous disappeared in the aftermath. It’s difficult to stress the point strongly enough. The loss of the dinosaurs was just the tip of the ecological iceberg. Virtually no environment was left untouched by the extinction, an event so severe that the oceans themselves almost reverted to a soup of single-celled organisms.
We are fearfully enraptured with the idea of such terrible devastation. When the impact at the end of the Cretaceous was scientifically confirmed, news of the disaster inspired not one but two blockbuster films about planet-killing asteroids in the summer of 1998. That such a huge rock could kill more than half of Earth’s known species suddenly seemed as obvious as the lethality of a gunshot. Simply knowing the terrible consequences of this disaster has been enough for us to look at the night sky with continued suspicion. If it happened before, it may happen again. NASA keeps an eye on the sky through their Sentry program, hoping to identify threatening asteroids and comets before they get too near.
But we often forget the unusual nature of the K-Pg crisis. Experts have often spoken of the calamity as part of the Big Five—a quintet of mass extinctions that have radically altered life’s history. The first extinction crisis, between 455 to 430 million years ago, reshaped the oceans, erasing entire families of archaic invertebrate weirdos and allowing fish to thrive. Rapid global cooling and plummeting sea levels killed about 85 percent of known marine species, reshuffling the evolutionary deck. The second event, spanning 376 to 360 million years ago, shook life up once more. Precisely what caused the disaster is unknown—a drop in ocean oxygen levels is suspected—but the sudden change killed about half of known creatures, reducing the diversity among organisms like trilobites and corals that formed the basis of ancient reefs.
Worse still was the third, peaking about 252 million years ago. This was the Great Dying, fueled by incomprehensibly violent and sustained volcanic activity that wiped out about 70 percent of known species on both land and sea through climate and atmospheric changes. Our protomammal ancestors, who had held sway in terrestrial ecosystems, were almost entirely extinguished. Their downfall is what allowed reptiles, including dinosaurs, to stage their evolutionary coup. Following that, about 201 million years ago, another disaster killed off a great number of the crocodile relatives that ruled the land and gave dinosaurs their shot at dominance. Once again, intense eruptions were to blame. Greenhouse gases belched into the atmosphere, spurring a burst of global warming followed by intense global cooling. Atmospheric oxygen levels dropped, the seas became more acidic, and the drastic shifts between too hot and too cold were too much for many species to cope with.
But none of these catastrophes were quite like the extinction event that ended the Mesozoic. These previous apocalypses took place over hundreds of thousands or even millions of years, with phenomena like intense volcanic activity and climate change creating grinding, protracted transformations that shifted the makeup of life on Earth over long time spans. The causes of death were also highly variable—ocean acidification prevented shell-building creatures from constructing their calcium carbonate homes, for example, while decreased atmospheric oxygen might have slowly choked terrestrial organisms. What happened at the close of the Cretaceous, however, had global reach. And it happened fast.
The happenstances that triggered the Late Cretaceous extinction culminated in one terrible instant, a rare sliver of time that we can pinpoint as the very moment that life would never be the same. Before the strike, thousands of species flourished on every continent. There were so many varieties of dinosaurs and assorted other creatures that paleontologists are still clocking overtime to find them all, with new toothy, sharp-clawed wonders being named every year. Experts even expect that there were scores of species we’ll never know as they lived in places where the circumstances of deposition and sedimentation did not allow them to be preserved, such as dinosaurs that lived in the mountains or other environments that were eroded rather than laid down as layers in stone. Mesozoic life was at its peak. Then, almost overnight, the dinosaurs were all but extinct and the planet’s ecosystems were in disarray. This was the worst single day in the history of life on Earth, followed by tens of thousands of years of struggle for the survivors.
Our view of the K-Pg extinction has been hard-won. In fact, the task has involved overcoming our greatest weakness—human hubris. When the famously cantankerous British anatomist Richard Owen coined the name “Dinosauria” in 1842, the great reptiles weren’t all that much of a mystery. At the time, only three were known to scientists and the scaly trio seemed to mark part of life’s expected progression. Geologists had identified an Age of Fishes, an Age of Reptiles, and an Age of Mammals, moving from low, squishy, squiggling forms of life through scaly monstrosities who were little more than a paleontological sideshow before mammals took up their starring roles. Whether understood as part of a creator’s plan or evolution’s great march, dinosaurs fit into a world of progress and refinement. No one needed to ask why they went extinct. How could shambling, malformed monsters that looked like a herpetologist’s nightmare ever be the pinnacle of life’s story? Great catastrophes turned over the makeup of life on Earth, but there was always a sense that the extinct species deserved their fate. That in some way or another, they were simply practice for what was to come.
Experts in the early twentieth century carried on this fatalistic assumption. Dinosaurs were big, bizarre, and anatomically extravagant. The question wasn’t why they died out. The real mystery was how they could have persisted for so long, especially when the clearly superior mammals were waiting in the wings to take charge.
Our mammalian conceitedness held on for decades. Even when the disappearance of the dinosaurs became a more legitimate question, the explanations were most always delivered in such a way that the dinosaurs themselves were to blame. The great, trundling reptiles laid eggs and cared little for their young, so mammals feasted on dinosaur omelets. (At this point researchers had paid no attention to the admirable parental oversight of alligators or snakes.) Or dinosaurs clearly invested so much energy and growth into becoming huge, spiky, and strange that they simply ran out of nebulous vital juices. How could a ten-ton rhino look-alike, studded with three horns and a bony collar around its neck, compete with the up-and-coming mammals? The mental capacities of dinosaurs were famously small, to boot. A cold-blooded reptile like a Stegosaurus or Ceratosaurus was perfectly suited to a lush world of sweltering jungles and dim-witted prey, but the lazy dinosaurs simply did not care to innovate, or even be open to the possibility. And if this is all sounding a little corporate to you, it should come as no surprise that these ideas proliferated during America’s great industrialization; “going the way of the dinosaur” is still a phrase used to tar competitors in financial circles.
In time, scientists began to accept the fact that animals do not have internal timers that regulate when species are “born” or “die” according to some cosmic clock, and the ideas about the expenditure of evolutionary energies was misplaced. There had to be some natural explanation. Refining the geological timescale made the question all the more puzzling. Dinosaurs did not represent a primitive lull as the world waited for the rise of mammals. Non-avian dinosaurs persisted for over 150 million years before abruptly disappearing at what seemed to be their apex. There had to be a reason.
Almost everyone had an opinion. Maybe the climate got too hot. Or maybe the climate got too cold. Perhaps some terrible disease ripped through their populations, or sea level rise ruined their favored habitats. Specialists from other fields chimed in, too. An opthalmologist proposed that dinosaurs had terrible cataracts, meaning that the impressive headgear of dinosaurs like the crested shovel-beak Parasaurolophus and the many-horned herbivore Styracosaurus had evolved as the world’s first sunshades. An entomologist spitballed that early caterpillars ate vegetation at such a voracious rate that there was no green food left, meaning that soon after there was no meat, either. Or maybe the time was simply right for mammals. Dinosaur diversity at the end of the Cretaceous seemed low compared with what it had been 10 million years prior. Maybe, after tens of millions of years, mammals started to flex their muscle a little bit and carve out more of the landscape for themselves.
The problem was that many experts focused on dinosaurs alone when the real devastation cut much deeper. Yes, an army of very hungry caterpillars could have denuded Cretaceous forests at a terrible rate, but that explanation did not explain why the flying pterosaurs of the air or the broad, flat rudist clams of the sea went extinct 66 million years ago, much less species of armored amoebas called forams that precisely track the extinction even though their witness testimony to the disaster will never be a cover story. Everyone was so tensely focused on the dinosaurs that the larger pattern was obscured even as experts continued to tabulate the Cretaceous body count.
It was only in the late twentieth century, when the signature of mass extinctions began to coalesce for paleontologists focused on the comings and goings of ancient mollusks and arthropods, that the fate of the dinosaurs started to take on a new gloss. The invertebrate record showed a sharp uptick in extinction at the end of the Cretaceous. The forams and armored balls of algae called coccoliths documented a sudden and horrible event. This is when dinosaurs disappeared, too. Something awful must have happened. Now the question was what.
Experts searched for a compelling cause to explain the devastation. At first, it seemed that some terrestrial trigger was to blame. At the end of the Cretaceous, right when the dinosaur record seems to evaporate in rock strata worldwide, the planet was changing. Sea levels dropped. The climate shifted. Volcanic rifts in the Earth’s crust emitted tons upon tons of greenhouse gases into the atmosphere.
It seemed as if dinosaurs simply couldn’t keep up with the Red Queen’s evolutionary race; they fell behind as mammals kept the adaptive beat. But this story didn’t quite fit either. Paleontologists working on the comings and goings of ocean mollusks and other invertebrates didn’t see a slow changing of the guard. Better fossil sampling and revised statistical techniques affirmed that life at the end of the Cretaceous was weathering the changes perfectly fine. Then suddenly life suffered a major shock. Something terrible had clearly befallen Earth’s biota. The answer didn’t come from fossils themselves but from the rock that entombed them.
Battered quartz crystals, vast amounts of prehistoric soot, and a rare metal called iridium, found just at the geological levels where the fossil record of non-avian dinosaurs disappears, suggested that some kind of extraterrestrial body had slammed into our planet. First proposed in 1980, at the fevered height of a new scientific interest in dinosaur biology, the idea set off an academic firestorm akin to the very impact it described. Paleontologists, geologists, and astrophysicists fought as fiercely as tyrannosaurs in conferences and journals over the proper interpretation of the results. But the discovery of an enormous impact crater in the Yucatán Peninsula in the 1990s settled the debate: a massive asteroid about seven miles across had struck Earth at just the moment the extinction becomes apparent in the strata. Nothing like this had ever been seen before. Physicists calculated that the initial impact that created the Chicxulub crater in Central America would have been powerful enough to blow many terrestrial dinosaurs in the vicinity off into space. But it wasn’t just the initial hit that sparked the extinction. The aftereffects of this dramatic event tipped the scales against the terrible lizards and many, many other forms of life.
Often, this is about as far as the discussion goes: an immense rock smacked into the planet and myriad species were summarily snuffed out. Simple as that. The asteroid becomes a cosmic bullet shot into the Earth’s gut. Yet there have been other impacts of similar or greater scale throughout our planet’s history—impacts that did not trigger biological disasters. About 35 million years ago, another large asteroid struck ancient Siberia and carved out the Popigai crater, sixty-two miles across. That’s more than ten miles wider in diameter than the impact crater in the Yucatán. But this more recent strike did not cause a mass extinction. There was local upheaval and damage, certainly, but life on the rest of the planet kept trotting along much as before. Not all impacts are equal.
The Cretaceous killer thus becomes a special case, standing out from other impacts through time. The size of the K-Pg asteroid, its speed, its angle, and the nature of the rock it struck all came together in the worst possible way for life on Earth—a set of complete happenstances that coalesced into nothing short of an apocalypse. It wasn’t just that Earth was hit by a massive asteroid. It’s that the aftermath of the impact played out in such a way that life was pushed to the breaking point, with many organisms unable to cope with the rapid changes. Earth swung between a world of fire and ash and one of withering, persistent cold and darkness. Dinosaurs didn’t just collapse when the asteroid struck. The real extinction played out over hours, days, months, and years in a constant state of flux as a new world emerged from the cosmic shake-up.
The K-Pg disaster was a global event, its story told through evidence gathered from many places across the planet. But the fossil record is uneven, yielding a collection of pinholes to look through to try to ascertain the whole. As the naturalist Charles Darwin famously observed, the world’s geological strata are like a book that lacks entire pages, paragraphs, sentences, and words from the story, leaving us to piece together the narrative from what might seem like isolated parts. By luck, good or bad, some chapters are richer than others. So far as the K-Pg transition goes, the best of these is in the western United States among the Hell Creek Formation beds of central Montana and the Dakotas. This relatively narrow expanse of our planet documents the last days of the dinosaurian reign up through the earliest days of the Paleogene period that followed. The impact boundary is clearly visible in the rock record itself. Sections of these strata displayed in museums look like the world’s most regrettable chunk of chocolate cake, dark brown and deadly. In this place we know the cast of characters who ambled across this ancient stage well and can track their fates across time and their changing environment. Their stories tell us how life suffered greatly, yet still survived.
But the reason we’ve gone back to this place and this one infamous moment is to understand not only why there are no Ankylosaurus descendants at the zoo but also how and why we came to exist. The Age of Mammals, a marker literally set down in stone, would never have dawned if this impact hadn’t allowed for evolutionary opportunities that were closed for the previous 100 million years. The history of life on Earth was irrevocably changed according to a simple phenomenon called contingency. If the asteroid’s arrival had been canceled or significantly delayed, or if it had landed on a different place on the planet, what transpired during the millions of years that followed the strike would have unfolded according to an altered script. Perhaps the non-avian dinosaurs would have continued to dominate the planet. Maybe marsupials would have held sway as the most common beasts. Perhaps some other disaster, like massive volcanic eruptions in ancient India that picked up around the same time, would have sparked a different sort of extinction. It’s likely that the Age of Reptiles would have marched on unimpeded, but without the origin of any species introspective enough to engage in such ruminations about time and its flow. This day was as critical for us as it was for the dinosaurs.
Now, after decades of fierce scientific debate, our picture of what transpired is starting to become clearer. Paleontologists, geologists, astronomers, physicists, ecologists, and others have assembled a more detailed tableau of what happened to the planet following the collision. It wasn’t the impact itself that caused such dramatic damage, but the long-lasting aftereffects that permanently reshaped the nature of life on Earth and allowed for the eventual and unintended emergence of humans. By imagining ourselves in the heyday of the dinosaurs at Hell Creek, on Extinction Eve and what follows, I’m going to walk you through what happened in the seconds, days, months, years, centuries, and millennia after the impact, tracking the sweeping disruptions that overtook this one spot and imagining what might have been happening elsewhere on the globe.
We’re about to watch the world change with unprecedented speed and violence. And we’ve carried plenty of cargo in with us to appreciate the dinosaurian drama. Not in terms of goods and gear, but in our ideas: over two centuries of scientific gleanings that describe everything from how the branching arms of the monkey puzzle tree grow to the taxonomic breakdown of the species that reside in this place. And for all that, the Hell Creek is perhaps the best known of any dinosaurian habitat. It’s often both our introduction to the dinosaurs’ world and the backdrop for their last great act—overture and finale all in one. This was the close of one era and the beginning of another. As much as we love dinosaurs—enshrining them like scientific relics in our museums, bringing them back to life in film—we know we exist only because they ceded the evolutionary stage to our ancestors. We owe them a debt.
Consider the dinosaur browsing along the forest’s edge, sun and shade sliding along its back as the quills growing from its hips and tail lilt with the creature’s amble. The name we’ve given this animal is Triceratops horridus, a label codified back in 1889 based upon an even older system of assigning every organism a genus and species name. In our own time, the petrified bones of the dinosaur are incredibly common—known from hundreds of skulls extracted from states huddled around the Rockies—which means we know a bit more about its variations, growth, and behavior than most other dinosaurs, informing our view of the reptilian grazer as its pillar-like legs pump back and forth.
Copyright © 2022 by Riley Black
