Watery habitats were everywhere—even on the continents. Enormous tropical inland seas dominated the landmass that later became North America. Most of the Midwest and the central United States were fully submerged, which is why paleontologists today find some of the best fossilized fish in the middle of the Midwest’s rolling prairies, which are about as far from the coast as you can get. Yet by the end of the Devonian, almost none of the gigantic armored fish and swarms of tentacled ammonites were left. What happened?
One paleontologist, Ohio University’s Alycia Stigall, has a theory that could explain why life during this period went from diverse to homogenous. She believes that invasive species took over the world’s oceans and inland seas, the same way cockroaches, kudzu, rats, and humans have spread across the globe today.
Stigall lives in Ohio, at the bottom of what was once a shallow Devonian sea. In fact, the vista from her windows is the former seafloor of an inland ocean hit particularly hard by the mass extinction. “We don’t have a good modern analogue for these types of oceans,” she said. They are a completely vanished ecosystem, though she imagines they might have been something like Hudson Bay. At the end of the Devonian, it’s likely that sea levels were high, pooling several inland oceans together. Earthquakes thrust new mountains from the land, which also brought previously separated ecosystems into contact with each other.
Many highly adaptable or generalist species began invading new watery territories. That meant they were competing with the local specialist species, like trilobites, for food. A specialist species requires very specific temperatures or food sources. They couldn’t cruise all over the Devonian oceans eating anything that came into view the way sharks could. So when invasive species came into their territories and stole their food, the specialists had nowhere to go. Their populations dwindled and they went extinct. By the end of the Devonian mass extinction, the planet was covered with giant, homogenous inland oceans where you’d see the same generalist species no matter where you looked. And out of those circumstances, Stigall believes, you had the makings of a mass extinction.
Though Stigall’s account is only one of many theories about mass extinction in the Devonian, her claims are backed up by evidence that many of the creatures who survived the period were generalists like sharks—creatures who could live anywhere and eat almost anything. Another survivor was the humble crinoid, a starfish-like creature with several feeding arms surrounding its mouth that make it look a little like the “face-hugger” stage of the creature in Alien. The crinoids went through a floating larval stage that allowed them to drift into many new environments before attaching to the ground and feeding on the abundant plankton in the water.
Still, homogenous ecosystems like the ones in the Devonian would have left all life-forms primed for disaster. Generalist species may be hardy, but they also share the same vulnerabilities. Say, for example, a drought hits an ecosystem in the Midwest. If there is only one type of wheat species, and it can’t deal with higher temperatures or less moisture, a short-term climatic change could kill off every blade of wheat in a given region. Without a diverse range of grain species, which might have different moisture tolerances, drought slays all the wheat. This in turn kills the animals who feed on that wheat, whose deaths leave predators hungry, too. Soon, you have multiple extinctions because the whole food chain has been ravaged. “The more we cull diversity, the more we are vulnerable to extinction,” Stigall concluded. It’s very possible that this is how the Devonian came to a close, with only a few invasive species scraping by until speciation, or new species evolution, made the ecosystem diverse again.
For Stigall, there’s a lesson in the fossils that surround her home, remnants of an inland sea unlike anything that exists now. “We’ve got the same problem with invasive species today,” she said. “It’s not caused by sea levels, but by humans, because we like to move things around.” She described how invasive species like pigeons and rats, as well as some trees and grasses, have gone from a few local regions to expand across the Earth. If this trend continues, she predicted that “long term, what you expect is a huge decimation of total biodiversity.” We could be on our way back to the late Devonian, in the early stages of a mass extinction that begins with a depression in speciation and ends with deadly homogenization.
Sometimes, the urge to live by expanding into as many territories as possible can backfire. As the invasive species of the Devonian reveal, life does not always beget more life. Some ways of living can actually kill just as handily as climate change and radiation.
As devastating as the Ordovician and Devonian mass extinctions were, they were nothing compared to what ravaged the planet about 75 million years later. The “Great Dying,” as it is known among geologists, was the worst period of mass death the Earth has ever known. It’s very likely that this event had no single cause; it was set off by a combination of disasters from the physical world and the biological.
3. THE GREAT DYING
THE BERKELEY GEOCHRONOLOGY CENTER, a lab devoted to studying the ancient ages of Earth, is located on a pleasant tree-lined ridge overlooking UC Berkeley. Somewhat puzzlingly, it shares a building with the Church Divinity School of the Pacific. While seminary students strolled by in the hall outside, I met Paul Renne, the center’s head geologist, a big, jovial man in a T-shirt. As he walked me through a warren of labs—full of the lasers and mass spectrometers that I’d come to expect in such places—I realized there was a peculiar kind of symmetry to the Geochronology Center’s tenancy arrangement with the Divinity School. After all, I had come to ask Renne about the closest thing to a total apocalypse the planet has witnessed.
The Permian Period (299 Million–251 Million Years Ago): Life in the Time of Megavolcanoes
Two hundred fifty million years ago, at the end of the Permian period, Earth spent thousands of years dying. At the end of those millennia of carnage, almost 95 percent of the species on the planet were dead. It was the worst mass extinction in our planet’s history, earning it the moniker “the Great Dying.”
The first phase of the mass extinction was caused by a disaster that has left an indelible and easily deciphered mark upon the Earth. If you visit the vast area known as the Siberian Traps today, you’ll find a beautiful, hilly terrain covered in short grasses. But 250 million years ago, the region was drowning under liquid rock spewing from the ragged mouth of a megavolcano. As its name implies, a megavolcano is far more powerful than your typical lava-filled mountain. Renne and other geologists estimate that as much as 2.7 million square kilometers of basaltic lava swept across the land in a fiery deluge. Almost a million square kilometers of the hardened basalt rock still remains here, smoothed by erosion into plateaus and valleys. It’s unclear whether this almost unimaginable ocean of lava was unleashed by one or two enormous eruptions, or a single, ongoing eruption that lasted for centuries.
But the Great Dying wasn’t caused by flaming tides of death. Volcanic eruptions on a large scale release a lot of gases, including greenhouse gases like carbon dioxide and methane. Jonathan Payne, a geologist at Stanford, estimates that the eruptions unleashed 13,000 to 43,000 gigatons (a gigaton is 1 billion tons) of carbon into the atmosphere. As if that wasn’t enough, they also released highly reflective sulfur particles that remained suspended in the atmosphere, scattering light away and cooling the climate very rapidly. The culprit responsible for the Great Dying was climate change.
Ironically, the roiling fires from this Siberian megavolcano may have caused a brief ice age. As glaciation locked coastal waters into ice sheets, the sea level dropped, and another source of greenhouse gas was unleashed. It’s possible that the water dipped low enough to expose methane clathrates, huge deposits of frozen methane that cling to the edges of continental shelves deep beneath the ocean. The clathrates melted and released ancient methane, a powerful greenhouse gas. As quickly as it began, the Permian ice age would have ended with a more intense greenhouse than before. These radical transformations in the atmosph
ere and climate made it impossible for most creatures to survive. Food sources dwindled. Species upon species died out.
It was an ugly ending for the Permian, which had been a time of rapid animal evolution on land. When the megavolcano began erupting, the earliest ancestors of today’s mammals were walking the Earth. Gingkos and conifers covered the coasts in forests, while seeded ferns evolved, uncurling their leafy fronds beneath tall pines. Mammal-reptile hybrids called synapsids roamed the land, some looking like giant lizards, and some like small rhinos. One of them, the enormous, dragon-like predator dimetrodon, had a tall sail attached to its back like a bony fin, and was such a badass hunter that paleontologists believe it may have fed upon sharks. These creatures all thudded around on the same continent because plate tectonics had finally pushed the planet’s landmasses together into one enormous continent called Pangaea, which stretched from pole to pole. A globe-wrapping ocean called Panthalassa teemed with sea creatures, from tiny single-celled organisms to corals and large fish.
A cutaway view of the megavolcano in Siberia that led to the Permian mass extinction. (illustration credit ill.3)
(Click here to see a larger image.)
These new forms of life, the forerunners of so many animals and plants we take for granted today, almost didn’t make it. What was especially unusual about the Permian mass extinction was that it took out nearly every form of life. Unlike in other mass extinctions, which sometimes hit sea creatures but not land creatures, or animals but not plants, this extinction was absolute. As many species were lost at sea as on land. When the megavolcano pumped carbon into the atmosphere, a lot of that got dissolved into the oceans. The water grew warmer, which destroyed the habitats of shellfish, who are sensitive to temperature changes. It also grew more acidic. The shells of shellfish are made of calcium carbonate, which dissolves in acid. Many sea creatures didn’t survive simply because their offspring couldn’t form shells in a highly acidic ocean environment.
Meanwhile, on land, so many trees and plants died that the continent’s surface was “denuded,” as Payne put it. The result was shockingly rapid weathering. As acid-tinged rain poured from the sky, followed by hot winds, more soil poured into the oceans, further raising the levels of carbon and acid. Vast areas of the coastal seas became anoxic dead zones—regions completely purged of oxygen. With oxygen supplies low in the water, large fish could not survive, especially ones that lived close to the deeply damaged ocean’s surface.
Even insects, which generally survive everything, suffered extinctions. An estimated 9 out of 10 marine species and 7 out of 10 land species went extinct. Across the planet, carbon levels suddenly skyrocket in rocks from this period examined by Renne and his colleagues, which suggests that the dead bodies of plants and animals were quite literally piling up on land and at sea. As the plants rotted, they released even more carbon into the environment. The devastation was so complete that we see a “coal gap” in the layers of rock left behind from this era. Plant life, which decays into coal, was so sparse in the 10 million years following the end of the Permian that none of the fossil fuel could form.
The planet had already endured ice ages, greenhouses, cosmic rays, and speciation depression. But only in the Permian mass extinction were almost 95 percent of all species cut down. And it happened in just 100 thousand years—the blink of an eye in geological time.
Slime World Survivors
Still, there were survivors. The Stanford geologist Payne showed me a rock that’s a slice of geological time from this period, where a layer of ocean-floor sediment filled with tiny shells is topped by a black layer of what looks like pure sludge. It’s easy to see that a diverse community of creatures was abruptly replaced by nothing but, well, slime. Payne and his colleagues have nicknamed this era Slime World, because the oceans were dominated by dark, oozing bacterial colonies, feasting on the dead bodies of their multicellular cousins.
On land, one of the great survivors was Lystrosaurus, an animal that managed to thrive. A heavy, clubfooted creature with a beaked snout and two tusk-like teeth, Lystrosaurus was a four-legged synapsid, or mammal-reptile hybrid. About the size of pigs, lystrosaurs were burrowing animals whose muscular hindquarters ended in short, wiggly tails. And they somehow managed to endure when even the precursors of the hardy cockroach were dying. They were herbivores, and their beaks probably allowed them to chomp on rough vegetation and dig for roots to eat.
For several million years after the end of the Permian, lystrosaurs were alone on a dead world. But they didn’t cower or retreat. Instead, they spread out as far as they could across the landmass that would one day fracture into the continents we know today. Their fossils have been found in Africa, Asia, and even Antarctica, which was a tropical region at the time. With no predators and no competition for their favorite foods, lystrosaurs could waddle anywhere they liked. They are, as far as we know, the only creatures ever to dominate our world so thoroughly: For millions of years, most four-legged land creatures were one type of lystrosaur or another.
Lystrosaurus was one of the few land animals to survive the Permian mass extinction, and its progeny spread across the Southern Hemisphere during the early Triassic. (illustration credit ill.4)
Why did these creatures—our distant ancestors—survive when so many of their fellow creatures didn’t? Theories abound. The Permian expert Mike Benton said it’s possible that they were “just lucky.” More likely, he added, they were well adapted for a world with depleted oxygen. They lived in underground tunnels, so they had a natural way to escape the heat and fire of the initial volcanic eruptions. Plus, the air they were used to breathing in their burrows was likely to be low in oxygen and full of dust—sort of like the air after carbon has been saturating it for a few centuries. Their barrel chests held lungs of a tremendous capacity, which meant more oxygen uptake. Lystrosaurus had the right respiratory system at the right time.
Over time, Lystrosaurus’s progeny repopulated the southern part of Pangaea, diverging into many subspecies. Their favored half of the supercontinent eventually broke off from the northern half and became its own continent, Gondwana (named after the southern Ordovician continent), packed with dinosaurs and proto-mammals. It took 30 million years for our planet to grow a robust ecosystem again, packed with predators and herbivores and a wide range of flora and fauna.
The Early Triassic Period (250 Million–220 Million Years Ago): Unraveling Food Webs
Those 30 million years of ecosystem struggles are their own story. Though every mass extinction unfolds differently, they all end when a new community of creatures has established itself—generally, a community that statistician Charles Marshall described as “completely different life-forms.” After the Permian, during the early millennia of the Triassic period, new communities of completely different life-forms rose and fell with alarming regularity. A new ecosystem would come together only to collapse in a few million years. Then another ecosystem would arise. This mass extinction just wouldn’t end.
Why did it take the planet so long to recover from the Great Dying? For answers, I visited Peter Roopnarine, a zoologist at the California Academy of Sciences who has a rather singular occupation among scientists. He’s developed a computer program that simulates food webs, the complex interplay between predators and prey within an ecosystem. Using this program, Roopnarine studies why the worst part of mass extinctions isn’t necessarily the fire, or the eruptions. It’s what comes afterwards, in the centuries of what scientists call “indirect extinctions” caused by food webs that are too unstable to support life.
In this food web illustration created by Peter Roopnarine, the arrows between life-forms indicate who eats whom. This is a Cretaceous-era food web. (illustration credit ill.5)
The old computer game Wator offers a perfect example of a simple food web simulation. In it, red pixels stand in for sharks (predators) and green pixels for fish (prey) as they battle it out for supremacy of the sea. You can set a few simple parameters, such as how
many sharks and fish there are to start, how often they breed, and how long it takes before they starve. Then you press “start” and watch generations unfold in seconds. When there are too many sharks, or the fish breed too slowly, the population of sharks eventually dwindles to zero and the waters of “Wator” become a sheet of uniform green. And that means you fail. What Wator reveals is that predators are as much at the mercy of prey as the reverse. Food webs can be knocked out of balance by life-forms at any point in the food chain.
Roopnarine’s simulations are infinitely more complex than Wator, incorporating the smallest planktons to the largest predators, and everything in between. In them, he describes relationships between predators and prey that lived millions of years ago. And from these models, he’s generated a theory about why the Triassic burned through so many food webs.
He began by coming up with a way to generate a realistic food web for species that no longer exist. He included every known form of life from the fossil record, and then he extrapolated predator–prey relationships based on what we know about how animals behave today. “You can’t ever know exactly what a fossil animal was eating—you can’t even know that with animals today,” Roopnarine explained. “But we can use the body size, tooth shapes, and other things to decide who their prey might have been.” Predators’ body sizes are helpful because, obviously, a small predator will prefer small prey, while a larger predator might be a generalist who can eat creatures of many sizes.
Scatter, Adapt, and Remember: How Humans Will Survive a Mass Extinction Page 4
