“IT IS VERY hard to imagine what happened,” the paleontologist Jan Smit said. He was describing the minutes and days following the impact of a massive meteorite, possibly 10 kilometers wide, that slammed into the Earth roughly 65 million years ago. Smit is one of the scientists who first discovered evidence for this violent event back in the 1970s. Today many of his colleagues agree that it’s what caused the Cretaceous-Tertiary (K-T) mass extinction—or, as it’s better known, the extinction that ended the dinosaurs.
Though nearly everyone is familiar with the story, Smit finds himself constantly correcting people’s misconceptions about it. “It wasn’t like [the movie] Armageddon at all,” he chuckled. The Earth wasn’t wrapped in fire. There were no enormous dust storms choking the life out of the soon-to-be-extinct dinosaurs. Instead, Smit said, most of the molten splash-back from the hit would have been hurled right back into space. And that’s why it was so deadly.
The energy released by the meteorite slamming itself thirty meters deep into Mexico’s Yucatán Peninsula was enough to punch a hole in the atmosphere. As Smit put it, “For this kind of impact, blowing away the atmosphere is a piece of cake.” Tiny droplets of liquified rock and metals shot into space, quickly wreathing our stratosphere in a thick layer of extremely high clouds. The biggest problem was that the meteorite hit Earth in a particularly tender spot, geologically speaking. Beneath the Yucatán, Smit explained, “are three kilometers of limestone, dolomite, magnesium, and gypsum, and salt. Gypsum is about half sulfur. There aren’t that many areas in the world that contain that much sulfur.” Essentially, the meteorite vaporized a hidden cache of explosives and poisons, scattering them everywhere. Still, the mass death that swept the world afterward was not caused by acid rain or other poisons, according to Smit. Instead, it had to do with a peculiar property of vaporized sulfur: When reduced to tiny droplets in the upper atmosphere, the resulting cloud becomes highly reflective. “From space, the planet would have looked brilliantly white,” Smit speculated. For at least a month, Earth became a giant reflector, and little to no sunlight could have penetrated the sulfur-laced clouds. It would have been a very extreme version of what happened after the Permian megavolcano shot sulfur into the atmosphere and cooled the planet.
Below the cloud, it would have been dark for weeks or months. Death would have come quickly to anything that drew sustenance from sunlight, including most plants. Next to die would be plant-eaters whose food sources were gone, followed by starvation among the dinosaur predators at the top of the food chain. Imagine a near-instantaneous food-web collapse, a fast-motion version of the collapses that Roopnarine described as choking off early Triassic life over millions of years. What this means is that one of the planet’s most notorious disasters, complete with cinematic explosions, caused global mass extinction simply by shutting down photosynthesis.
Perhaps more than any of the other mass extinctions we’ve talked about so far, the K-T extinction dramatizes how mass death on Earth is tied to environmental changes. The area around the Yucatán would have been devastated after the meteorite hit. Toxic gas, fire, and extreme tidal waves would have sterilized the region around what is now called the Chicxulub crater. But even the death by darkness that followed would have been just the opening act. It would have taken centuries, and perhaps millennia, before the K-T event achieved full mass-extinction status. The dinosaurs did not die out during one long, sulfur-enhanced night. In fact, Smit underscored that the truly devastating effect of the sulfur cloud was most likely a temperature drop of 10 degrees Celsius that lasted for at least half a century, and probably a lot longer. The lush, green tropics of the Cretaceous cooled, ocean temperatures dropped, and animals who couldn’t migrate found themselves trapped in hostile ecosystems. The mass extinction took out as many as 76 percent of species, including all the non-avian dinosaurs. Meanwhile, a group of mouse-sized furry creatures we know today as mammals began to thrive and grow.
The K-T mass extinction is the most recent one in Earth history, and the evidence it left behind is richer than what we’ve got for any comparable event. As a result, scientists who study the K-T have had to confront the full complexity of life when it collapses. Not surprisingly, this has led to some of the bitterest debates in paleontology.
Smit and a UC Berkeley colleague, the geologist Walter Alvarez, endured years of doubt and ridicule when they first began speculating that the dinosaurs’ demise began with a meteorite. Previously, paleontologists accounted for the mass extinction by suggesting everything from cosmic-ray bombardment to starvation. It took a while for the scientific community to accept the idea of a flaming ball from space. But Smit and Alvarez had pretty compelling evidence. Working on opposite sides of the globe—Smit in Spain, and Alvarez in the Americas—the two researchers uncovered physical remains of the impact and overturned the previous theories about why dinosaurs suddenly went extinct after ruling the planet for over 100 millennia. Alvarez worked with his Nobel Prize–winning physicist father, Luis Alvarez, and the two published a history-making paper in 1980 showing that rock layers at the K-T boundary contained a high concentration of iridium, a metal found almost exclusively in space. Meanwhile, Smit had discovered “spherules,” tiny balls of rock that had been heated up and then cooled down quickly, in the same layer all over the world. Smit published a paper about the spherules the same year Alvarez published his about what’s come to be known as the “iridium anomaly.” The one-two punch of these papers—chronicling metals from space and the remains of superheated rock scattered across the planet—suggested an event whose magnitude could easily have accounted for global mass death.
But the flaming-ball controversy is still far from over. In the late 1980s, Princeton geologist Gerta Keller began publishing papers questioning whether the meteorite impact actually had a global effect after all. She claimed she had a better explanation: megavolcanoes in India. And she spent the next two decades gathering evidence to prove her hypothesis, despite widespread scorn from the scientific community. UC Berkeley paleontologist Charles Marshall said that “nobody in the scientific community takes [Keller] seriously,” and Smit told the BBC that her ideas “are barely scientific.” Like Smit and Alvarez before her, Keller cheerfully met doubt with documentation.
When I spoke to Keller, she had recently returned from India, where she’d made a series of incredible discoveries. She and a group of local scientists managed to get samples three kilometers deep underground in a region called the Deccan Plateau, long known to be the site of an ancient megavolcano. The area has been off limits to scientists ever since India’s Oil and Natural Gas Corporation started drilling there. An outspoken person who clearly loves a good scientific fight, Keller put her considerable powers of persuasion into a campaign to gain access to what she suspected might reveal the truth about how the dinosaurs died. She finally got her wish and, joined by a group of Indian scientists, she found more than she’d ever hoped.
Using special tools that produce “cores,” cylindrical rock samples pulled up from deep underground using cannulated drills, Keller and her colleagues discovered that the Deccan Plateau was the result of at least four major eruptions following closely on each other. One eruption was so enormous that the team found a single uninterrupted lava flow stretching 1,500 kilometers from the volcanic vent all the way to the sea. But the most valuable discovery was the layers of sediment in between each lava flow—in those sediments, Keller and her colleagues found fossils that helped date the volcanic eruptions. Based on the evidence so far, it appears that the Deccan supervolcano began spewing lava and toxic gas about 67.4 million years ago—about 1.6 million years before the K-T boundary. The timing was right. And so were the deadly patterns of extinction she observed in those layers of sediment. After each lava flow, fewer and fewer animals were recovering from the devastation. “By the time the fourth flow came, nothing was left,” she said.
Keller believes that the flows may have come so rapidly that life nearby had no chance to recover—and that the toxins and carbon released by the explosions wound up killing off creatures across the globe with environmental changes similar to those at the end of the Permian. A runaway greenhouse effect, combined with acid rain and ocean dead zones, would have made the planet unlivable for the majority of its inhabitants. “That’s the likely killing mechanism,” Keller concluded matter-of-factly.
Who is right? It’s entirely possible that both Smit and Alvarez on one side and Keller on the other have identified causes of the K-T mass extinction. There are other theories, too. A fungal spike in the fossil record during the mass extinction has led at least one scientist to suggest that the dinosaurs died of fungal infections like the ones that are causing extinctions among amphibians and bats today. When the evidence in the geological record is relatively fresh, it becomes obvious that most mass extinctions on Earth have multiple causes. And as Keller’s work suggests, evidence gathered outside Europe and the Americas can offer a new perspective on old theories. We know that the bodies start piling up when environments change, but many events all over the world may have set those changes in motion.
One of the difficulties in sorting out what happened to the dinosaurs has nothing to do with geological evidence and everything to do with human culture. Dinosaurs have been so widely misrepresented in pop culture about the prehistoric world that it’s hard for us to step back and appreciate this diverse array of creatures for what they actually were, and how they really died out.
To get the real story, we’ll return to the chaotic Triassic period that followed the Great Dying, when many species evolved and died out rapidly. Late in the Triassic, about 220 million years ago, dinosaurs began to evolve. At this time, they were, as Brown University geologist Jessica Whiteside put it, “about the size of German shepherds and not very diverse.” Their main competitors were the crurotarsans, those fierce carnivores that eventually evolved into crocodiles and alligators. How did a relatively small group of mini-dinos prevail against these toothy, occasionally armored beasts? “If you were in the Triassic, you would bet on crurotarsans,” Whiteside said. But surprisingly, most crurotarsans did not survive the mass extinction that ended the Triassic, leaving the dinosaurs to take over lands once dominated by their mega-gator counterparts.
Whiteside attributes this bizarre turn of events to one of the most stupendous underwater volcanoes in Earth history. Known as the Central Atlantic magmatic province (CAMP), the eruption started about 200 million years ago in a narrow body of water separating the eastern Americas from West Africa. (At that time, the two continents were still joined into the Pangaea supercontinent.) The lava flow from CAMP was tremendous. It forced the continental plates so far apart that an entire ocean grew between once-connected regions known today as Canada and Morocco.
If this eruption could create one of Earth’s biggest oceans, just imagine the high volumes of carbon, methane, and sulfur it was pumping into the water and the atmosphere. A superhot greenhouse gripped the planet as the Triassic wound to a murky close. Whiteside ticked off the deaths that followed: As the temperatures climbed higher, the world-spanning tropical forests of the Triassic dried out and succumbed to enormous wildfires. The burned remains of forests slipped into the oceans along with carbon-rich soil. The oceans became acidic, which led to anoxia and die-offs there. Coral reefs were the first to go, and their deaths set off a cascading effect where anything that fed higher in the food chain died too. It was the perfect storm for destroying food webs, starting in the oceans and creeping up onto a warming land whose trees were being eaten by fire. Once again, climate change was killing the world.
There are many well-preserved plant fossils from this era, so it’s possible to visualize how the extreme greenhouse conditions changed the environment between the Triassic and the Jurassic. Jennifer McElwain, a paleobotanist at University College Dublin, has spent years studying this transition in Greenland, excavating everything from leaves and flowers to microscopic bits of pollen, to reconstruct the world where dinosaurs ultimately triumphed. Today, coastal Greenland is hard tundra that’s too cold for trees, but in the late Triassic and early Jurassic, it was full of lush vegetation. McElwain called it “a cross between New Zealand conifers and the Florida Everglades.” It was a world of “broad, meandering rivers” and “big, wide floodplains” bordering forests full of towering trees and stubby, thick-trunked plants called cycadeoids with palmlike fronds bursting from their tops. And then came CAMP, with its carbon emissions and rising global temperatures.
Tens of thousands of years of greenhouse conditions led to fire after fire. Ultimately, McElwain believes, the environment of diverse trees, shady forests, and thick vegetation was reduced to swamps full of ferns. “There would have been ferns as far as the eye can see, with hardly any trees, and lots of fire,” McElwain said. There was no complex, multitiered canopy in the forests, so the landscape would have been much brighter. But within another 100,000 years, the region went back to being conifer-dominated. What emerges from this fast-motion vision of ancient forests rising, burning, and rising again is something approaching the truth of where the dinosaurs began. They were among the only survivors of radical environmental changes that drove their competitors to extinction. Most crurotarsans were extinguished in the burned threads of food webs, but those small, early dinosaurs were able to spread out and adapt to the new environments and continents.
When forests at last returned to the land, dinosaurs evolved to be much larger. They diversified into armored herbivores like triceratops and plate-backed stegosaurus, sneaky scavengers, and predators like the 40-foot-long T. rex that we’ve seen in movies from the 1933 version of King Kong to Jurassic Park. Dinosaurs were as diverse as mammals are today, and their behavior probably varied a great deal from species to species. Many of them walked on two legs, with body postures similar to birds—their heads would have been thrust far forward, their spines nearly horizontal, and their tails held out stiffly behind them rather than dragging on the ground. Indeed, most paleontologists today accept that birds evolved from therapods, a group of bipedal, feathered dinosaurs that included the infamous T. rex. If you ever want to imagine what it would be like to face down a dinosaur, imagine a hulking, 40-foot-long crow whose beak has become a toothy mouth.
Recent evidence suggests that many dinosaurs weren’t feathered in quite the way birds are today. Most had dark gray or reddish proto-feathers (often called dinofuzz) that looked something like spiny down. Indeed, dinosaurs may have had proto-feathers for millions of years before birds evolved the ability to fly. Also like their bird relatives, many dinosaurs made nests and laid eggs. Though it’s hard to piece together how these different Cretaceous animals might have behaved, some paleontologists theorize that they may have been social, like birds, forming flocks and possibly mating for life.
What we do know is that when the catastrophes of the Cretaceous period hit, dinosaurs were in a position similar to their old rivals, the mega-gator crurotarsans. A lot of dinosaurs had evolved into specialists, and were therefore deeply connected to food webs that were all too easy to unravel with a few shifts in global temperature. This time, a group of mouse-like, furry animals called mammals—the descendants of the Permian survivor Lystrosaurus—were the survivors.
The Earth these mammals began to colonize with their strange paws and non-feathered faces had come into being through extremely complex events, whose true impact can only be measured in tens of millions of years. Environments had died and been reborn from the effects of liquid rock deep in the Earth and flaming balls from space; the mixture of gases in the atmosphere had been altered by microbes, mountains, and plants; temperatures had fluctuated between extreme icehouses and greenhouses; and the very shape of the planet’s landmasses and oceans had transformed quite radically dozens of times. If there had been a paleogeologist among the last of the dinosaurs, she could hardly have pinned the blame for her peers’ demise on any single factor. The entire ambiguous history of the planet would have had to stand trial for murdering brachiosaurus and letting a bunch of little monkeys take over.
Adding to our paleogeologist dinosaur’s conundrum would be another issue, which is that the dinosaurs didn’t entirely die out. An evolutionary offshoot of therapods—birds—survived into the present to become one of the most successful animal classes on the planet. They are highly diverse, exhibit a wide variety of social behaviors, and undertake some of the most incredible migratory journeys of any creatures we know. Of course their dinosaur forebears are extinct, much the way humans’ forebears are. But the dinosaur evolutionary line appears to have continued unbroken.
Perhaps the single most common misconception about dinosaurs among humans is that these creatures and their world have disappeared. Like mammals, dinosaurs are survivors. But their children, who flash through the skies and leave us in awe, are so different from their ancestors that we find it hard to draw a connection between them. What we should ponder, as we move from geological history into the world where human evolution takes place, is whether our understanding of survival is as clouded as our understanding of dinosaurs.
The dinosaurs survived two mass extinctions, but the crows who like to hang out in the tree next to my house are nothing like those dog-sized dinosaurs who beat out the crurotarsans. In fact, it’s not entirely accurate to say the crurotarsans have been pushed off the environmental stage either. Are we not witnessing a strange tableau of survival whenever a bird alights on the head of a crocodile, bringing together the evolutionary offspring of Triassic and Jurassic? Instead of saying the dinosaurs died out, it might be more accurate to say that dinosaurs changed.
Can humans possibly expect to remain unchanged as we face the next mass extinction? History suggests that it’s unlikely. But if survival means that our species will evolve into creatures like ourselves, but with new abilities—like, say, flight—that’s not so bad. Some would even call it an improvement. Survival may be far weirder, and better, than we ever imagined.