There are always complications, Roopnarine admitted. Many species share the same potential prey or predators, and it’s hard to know which species might have been generalists with many food sources, or specialists with just a few. But the beauty of using computers to simulate food webs is that you can go through as many iterations as you like, creating different worlds each time paleontologists discover more about a fossil predator’s range or appetites. Plus, there are a few rules of thumb, including the fact that there are usually far more specialists than generalists. Once the ancient food web has been set up in his program, Roopnarine said, he can simulate food-web disturbances like the one in the early Triassic.
Based on what he’s figured out so far, Roopnarine’s theory is that a basic imbalance in early Triassic food webs led to millions of years of maimed ecosystems rising and collapsing in rapid succession. Initially, the problem was that so few creatures had survived the Permian mass extinction. Of the survivors, he said, “you have small carnivores and some seriously big, bad amphibians who are the precursors of crocodiles.” Among herbivores, he said, lystrosaurs were the only game in town. The problem was that nobody around seemed to be eating Lystrosaurus, perhaps because they were the wrong size or in the wrong environments for most predators. In fact, the food web began to unravel because there were so many carnivores and very few prey.
Those “big, bad amphibians,” known as crurotarsans, were in fierce competition with each other. With their huge, toothy mouths and muscular tails, they would have been deadly predators—and Roopnarine says that the competition between carnivores during the early Triassic was more intense than in any other food web he’s looked at. The carnivores competed with each other so intensely for the tiny amount of available prey that they wound up driving each other to extinction over and over. New creatures would evolve, then get crushed out of existence. Only the herbivore Lystrosaurus, and eventually other herbivores, really recouped their losses. It took tens of millions of years before there were a small enough number of carnivores for food webs to stabilize.
Community Selection
This raises the question of what makes for a stable food web over the long term. And there’s an easy answer. “Diversity,” Roopnarine said firmly. A food web needs to be “robust,” full of many kinds of carnivores, herbivores, and plants, in order to withstand an environment that can often hammer creatures with everything from volcanoes to drought and sea-level shifts. As long as there are many nodes in a food web, a healthy balance of predator and prey, you have a community of life-forms that can remain steady even when the environment wobbles.
“So does that suggest some communities are better than others when it comes to survival?” I asked.
Roopnarine offered a conspiratorial nod. “This can be controversial, but yes, you could say this is natural selection at the community level.” Food webs don’t compete the same way two species might because they don’t exist next to each other, trying to eat the same things and live in the same caves. Instead, they compete with each other temporally, replacing each other in the same geographical places over time. To “win” the natural-selection game, a food web must outlast other food webs, remaining stable for as long as possible in the same place. Looked at from this perspective, you might consider all of Earth’s geologic history a competition between food webs struggling to last through as many environmental disasters as possible, simply by retaining their robustness in the face of calamity.
Survival is never just a matter of one species being exceptionally adept. We only survive in the context of our food webs. And when a food web starts to unravel, the extinction of one creature will mean the “secondary extinctions” of others.
Roopnarine and his colleagues have run enough simulations of food-web collapse that they’ve discovered a pattern. You can take away up to 40 percent of the life-forms in a system, and the number of secondary extinctions doesn’t increase significantly. “But there’s a critical interval after that where things happen rapidly—a threshold effect,” Roopnarine said. “The secondary extinction numbers rise dramatically.”
Imagine a world like the one we live in today, with a variety of creatures in many different environments. Let’s say we begin to chip away at one of those environments, like the American prairies. People clear grasslands, kill both predator and prey animals, and destroy insect pests. Still, the food web seems stable. Creatures and plants go extinct in the region, but there seems to be no ripple effect. And then, after centuries, we hit a tipping point. Forty percent of the nodes in the prairie food web have been knocked out. Suddenly, there are predators with very few prey. Catastrophic deaths among predators result: They are competing for scarce or no resources. And then a drought hits, killing the few remaining prairie grasses. Now our tiny herbivore population goes mostly extinct. We are left with few predators and virtually no prey. The already unstable food web falls apart, one death leading to another—and making the community more vulnerable to climate fluctuations.
“Don’t expect the unraveling to be linear,” Roopnarine warned. The deaths will be exponential. Once we hit the threshold, our food web has lost in the war of community selection. A new food web will rise up to take its place, turning the American prairie into a whole new world full of strange predators and grasses unlike any we’ve ever seen.
So the Permian extinction event yields a double lesson in survival. First, it offers compelling evidence that climate change caused by greenhouse gases can kill nearly every creature on the planet. Regardless of how that greenhouse scenario starts—whether it’s a massive volcano or an industrial revolution—climate change can kill more effectively than a meteorite impact. Of course, atmospheric changes were only the first phase in a problem that lasted 30 million years. One could argue that food-web collapse is really what makes the Permian mass extinction a “Great Dying.” The scourge started by Permian megavolcanoes echoed for millions of years, rending food web after food web until at last equilibrium was achieved in the Triassic period.
Still, there were survivors. Humans and many other mammals on Earth owe our existence to a bunch of piglike creatures with beaks who loved the sunny southern climate. That Lystrosaurus survived for millions of years (much longer than Homo sapiens has been around) proves that complex life can make it through even the most terrible disasters. These lumbering proto-mammals also left behind a few tips for what to do when we hit that wall of toxic air. By following in the lystrosaurs’ footsteps, mammals dodged the next major mass extinction—even though many dinosaurs didn’t.
4. WHAT REALLY HAPPENED TO THE DINOSAURS
“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.
The Cretaceous Period (145.5 Million–65.5 Million Years Ago): Meteorite Impact
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 th
e 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 flaming-ball-of-death controversy
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.
The Late Triassic Period (220 Million–200 Million Years Ago): The Beginning of Our World
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.
Scatter, Adapt, and Remember: How Humans Will Survive a Mass Extinction Page 5