The Permian‑Triassic Extinction: Brushing with Annihilation

Deaths can be hard to decipher, whether they are the deaths of individuals or of an entire species. In the Karroo Desert in South Africa there is a graveyard on an abandoned farm. Locals know that a family named the Fouches owned the farm in the late 1800s: a mother, father, and two sons. They all died in the 1890s, but no one knows how. It has been barely more than a century, and the fates of the Fouches have been lost.

Since 1991 the University of Washington paleontologist Peter Ward has been coming to the mountains near the Fouche farm. The rocks that make up the mountains contain clues to the biggest mass extinction ever recorded. John Phillips recognized this extinction, which marked the end of the Paleozoic era and the beginning of the Mesozoic era 250 million years ago. (Today, paleontologists refer to it as the boundary between two smaller units of time, the Permian period and the Triassic period.) Over 90 percent of all species on Earth disappeared, but their deaths lie in an obscurity far deeper than the Fouches’, with hardly any clues as to how they vanished. Ward comes back to the Karroo every year to look for those clues. “We need to understand what happened,” he explains. “It is important because the rules of evolution that held for that extinction could apply to our current event. We are undergoing a mass extinction event today, and understanding what may have happened in the past can help us predict how life on Earth will be affected.”

Today the Karroo is a stark, mountainous region, but 250 million years ago, just before the mass extinctions, it would have been a very different place. “It would have been a big, wide river valley,” says Ward. “The rivers would have been gigantic, Mississippi‑sized. The forests lining the side were made of plants unlike those today. There were no flowers. There would have been no birds flying overhead. It would not have looked anything like our Earth as we know it today. A totally foreign, alien world.”

The dominant animals of the ancient Karroo were reptilian creatures called synapsids, from which all mammals would later descend. By 250 million years ago, some synapsids had evolved into stocky hippo‑like herbivores and bizarre carnivores, some of which looked like lizards with the head of a saber‑toothed turtle. By 250 million years ago, these synapsids had already evolved a few of the key traits of mammals. They had evolved jaws and teeth that could chew food rather than slashing and gulping it whole, helping to create a more efficient digestive system that increased their stamina; legs that no longer sprawled to their sides but fit instead underneath their bodies, giving them a more powerful run; and a metabolism that was more warm‑blooded than cold.

Synapsids were not alone in the ancient Karroo–amphibians, turtles, crocodiles, and even the forerunners of dinosaurs lived alongside them–but synapsids ruled. The fossils of the Karroo hint that synapsids swarmed over its conifer forests and fern savannas in numbers that rivaled the wildebeest and antelope that roam East Africa today. Life seemed like it could only get better for synapsids. But all of that would change in a geological instant.

“The Karroo is an amazing place,” says Ward. “In paleontology, it’s really one of the sacred places. There is no other place on Earth where the fossils of mammal‑like reptiles are either as abundant, as easily recovered, or, to this point, as well studied. This is really the center of the Earth for understanding what happens on land during the Permian‑Triassic extinction.”

In a gully in the Lootsberg Pass, Ward and his colleagues can see the final years of the Permian period. The green and olive strata turn to red and purple, indications that the Karroo was turning hot and dry. The fossils of tetrapods, so easy to find in older rocks, become rarer and rarer. Eventually only three species of synapsid fossils survive–one synapsid that endured from earlier times, along with two newcomers: a predator called Moschorinus and an ugly, hippo‑like herbivore called Lystrosaurus. And the rocks from the very end of the Permian, a green layer of rock, lack any sign of life whatsoever.

“The mass extinction happened in these beds,” says Ward. “We have no fossils whatsoever. All the Permian creatures that we saw right down there have disappeared entirely. A few of them, we know, survived, because one or two species will be found a little higher up. But in these beds we find nothing. Not only are there no fossils, there aren’t any of the burrows or the tunnels or the traces of animal activity. We see, instead, layers of rock that could only have formed in the absence of animal life. So catastrophic was that mass extinction that even the small creatures have died out. Not just the mighty, but the meek as well. This place is dead.”

These rocks paint a picture of pure desolation. Ward and his colleagues can trace impressions on them that show how the disappearance of trees released the rivers of the Karroo from their narrow channels. They wandered across the basin as braided streams, clogged with eroding soil. Only in the very highest rocks of the gully do fossils of Lystrosaurus appear again, a rugged survivor, along with synapsids more closely related to mammals, as well as the forerunners of dinosaurs. After millions of years the trees anchor the landscape again.

Life on land suffered not just in the Karroo but around the world. Almost all species of trees on Earth became extinct, and many of the smaller plants disappeared as well. Even insects–which have not succumbed to any other mass extinctions in their 500‑million‑year history–vanished in great numbers. In the oceans, the destruction was even more devastating. Entire reefs died away. Trilobites, ribbed arthropods that had been among the most common marine animals for 300 million years, were claimed by the Permian‑Triassic extinction. Giant sea scorpions known as eurypterids–some of which grew up to 10 feet long–emerged 500 million years ago and thrived for 250 million years. At the end of the Permian they too became extinct. All told, an estimated 90 percent of all species on Earth disappeared.

In the oceans, at least, they disappeared in a hurry. Near the village of Meishan, in southern China, there are abandoned limestone quarries whose walls record the extinction of marine animals at the end of the Permian. The carbon atoms that make up the limestone speak of a global disaster as well. Limestone is made out of the skeletons of microscopic creatures. They build skeletons by combining calcium and carbon dioxide in the seawater to form calcium carbonate. The carbon that they use may come from some living sources–a rotting leaf, a dead bacterium–or it may have an inorganic origin in a volcano. Because photosynthesis filters out a lot of carbon 13, organic carbon has a different ratio than inorganic carbon. By measuring the ratio of carbon isotopes in limestone, scientists can figure out how much organic carbon was being produced at the time the skeleton‑building creatures were alive.

During the Permian‑Triassic extinction, the isotopes in the limestone at Meishan went through a wild swing. Their gyrations hint that the oceans ecosystems completely collapsed, and dead organic matter flooded the seas. Geologists have found the same isotopic lurch in rocks from Nepal, from Armenia, from Austria, from Greenland. What makes the Meishan quarries special is that their limestone is interspersed with layers of volcanic ash from eruptions that took place just before and after the extinctions. And in those ash layers are zircons, the time‑telling crystals. Meishan can put a limit on just how long the collapse lasted.

In 1998 Samuel Bowring of the Massachusetts Institute of Technology and his colleagues measured the uranium and lead in zircons just above and below the extinctions and the swing of carbon isotopes. They concluded that the interval lasted less than 165,000 years, and perhaps much less. Volcanic ash layers from elsewhere in China gave the same result. In the scope of geological time, the Permian‑Triassic extinction happened in a flash.

Any explanation for the Permian‑Triassic extinction must be able to slip into these narrow constraints. One popular hypothesis was based on the slow but immense drop in sea levels around the extinction at the end of the Permian. About 40 percent of the continents and their surrounding shelves was covered in water 280 million years ago. By 250 million years ago that figure had dropped to 10 percent.

But if this retreat of the oceans was the cause of the mass extinctions, they should have taken millions of years to unfold, not the brief pulse that Bowring and others have documented. The world’s ecosystems collapsed like a house of cards, not like a slowly eroding hillside. Scientists are looking for other culprits instead.

Volcanic eruptions might have been able to do their damage in such a short period of time. No earlier than a few hundred thousand years before the extinctions, lava began flowing out of giant vents in what is now Siberia. Over the course of about a million years, these vents belched up 11 massive eruptions, a total of 3 million cubic kilometers of lava–enough lava to cover the entire planet’s surface to a depth of 20 meters. The Siberian volcanoes might have been able to cause the extinctions by wrecking the climate and chemistry that made life possible. Along with the lava, the volcanoes may have released huge clouds of sulfate (S0₄). In the atmosphere, these molecules would have acted like seeds for fine droplets, creating a haze that could have reflected sunlight away and chilled the planet. When these droplets fell out of the sky as sulfuric acid rain, they would have poisoned the ground.

In both these ways, the volcanoes could have killed off most of the trees on the planet. The insects that depended on them would have gone extinct as well, along with many vertebrates. The acid rain and cold clouds might have lasted for a few years. As they faded, the volcanoes could have wreaked havoc in another way. The Siberian volcanoes may have released trillions of tons of carbon dioxide, which gradually would have absorbed heat and created global warming. The global climate appears to have heated up quickly, perhaps in only a few decades. The heat wave would have put enormous stress on a biosphere that was already crippled.

According to Andrew Knoll of Harvard University and his colleagues, volcanic eruptions may have destroyed life in the ocean by upsetting its delicate chemical balance. There is evidence that 250 million years ago, the deep ocean had accumulated poisonously high levels of carbon dioxide. The organic carbon that fell to the seafloor produced C0₂ gas, and thanks to a sluggish circulation of water in the ocean, the gas remained trapped in their depths. The volcanic eruptions liberated the C0₂ by altering the climate, thereby stirring up the oceans. When the C0₂ reached the shallower waters, it acidified the blood of animals there, driving most of them to extinction.

Scientists have not narrowed down their search to a single culprit, and it may well be that many factors created the Permian‑Triassic extinction. “If we look at the rock record across this boundary, between the Permian and the Triassic, we have so many clues that things were really getting bad, bad in many ways,” says Ward. “We have this great drought, we have an increase in temperature. The very nature of how rivers worked, the ways in which sediment was accumulated on sea bottoms, the level of the ocean was changing–globally things were rapidly changing in many different directions. And this may have been a mass extinction brought about by many things going to hell in a handbasket very quickly.”

Rebirth

It strains the human imagination to picture the world just after mass extinctions. We have nothing in our experience to compare it to. But the volcanoes that have erupted during human history can offer a glimpse of what life must have been like after the dying stopped 250 million years ago.

In the Sunda Strait, running between Java and Sumatra, there once was an island called Krakatau. Before 1883, those who sailed past it could look up at the forested flanks of a quiet volcano. The Dutch set up a naval station on Krakatau in the 1600s, mined it for sulfur, and logged its trees. Indonesians lived in a few villages on the island, growing rice and pepper, until the 1800s. By 1883 Krakatau was uninhabited.

In May of that year, the volcano began to rumble. A group of Dutch volcano watchers sailed to the island and climbed the rim of one of its craters, measuring 980 meters across. They saw steam, ash, and pumice fragments the size of baseballs shoot up into the air. Then for three months Krakatau became quiet again, preparing for a climax. On August 26 the island erupted, with explosions that could be heard hundreds of miles away. A column of ash rose 20 miles. Mud rained down from the dark sky. Clouds of vaporized rock glided over the strait at 300 miles an hour. When they hit land, they raced uphill, incinerating thousands of people. Tsunamis rolled out from Krakatau, washing away dozens of villages and then heading out across the globe. They even made the English Channel bob. For months afterward, the ash from the eruption floated in the sky, turning sunsets around the world bloodred. In November 1883 fire engines in New York and Connecticut were called out because the red glow in the west looked like entire towns ablaze.

The day after the eruptions stopped, a ship named the Gouverneur‑Général Loudon, passing by Krakatau, reported that two‑thirds of the island was gone. Where the volcano had been, there was a submerged pit reaching down hundreds of feet underwater, surrounded by a frail archipelago of bare, burned earth. Nothing that had lived on Krakatau had survived, not even a fly. Nine months later a naturalist visiting the islands wrote, “In spite of all my searching I could find no sign of plant or animal life on the land, except a solitary very small spider; this strange pioneer of the renovation was in the process of spinning its web.”

Within a few years a thin coat of life was covering the islands again. Cyanobacteria formed a gelatinous film over the ash, and later ferns, mosses, and a few flowering beach plants sprouted. By the 1890s, a savanna, with fig and coconut trees scattered across it, had grown on the islands. Along with the spiders lived beetles, butterflies, and even a monitor lizard.

To cover the 27 miles from the mainland to the islands, plants and animals had to travel by sea or air. Seeds of some plants could float on the currents of the Sunda Strait. The monitor lizard could swim, and other animals could ride on top of driftwood and rafts of plants. The spiders arrived on Krakatau by spinning silk balloons that carried them over the water. Birds and bats (including the Malay flying fox, with a 5‑foot wingspan) could fly to Krakatau, and bring in their stomachs the seeds of fruits they had eaten on the mainland.

Yet life on Krakatau did not come back randomly. The first species to come were weedy, pioneering organisms well adapted to catastrophes. In time, other species arrived and created a succession of ecosystems, each opening the way for the next. A grassland ecosystem assembled itself first, and any animal that arrived on the islands had to be ready to survive on the food it had to offer. Emerald doves and savanna nightjars settled successfully. So did pythons and geckoes and foot‑long centipedes. Many other species did not. Others had to wait as the grasslands gradually gave way to forests.

For some trees, the timing had to be exquisite. Fig trees, which were among the most successful, depend on a single species of wasp to pollinate them; if a fig arrived at Krakatau, its only hope of colonizing the island was for its pollinating wasp to come to the island soon afterward. Apparently this improbable event did happen, because figs began to spread. Animals feasted on the figs, making the diversity of the forest swell. New shade‑loving species such as orchids have now established themselves. In years to come, the forest will continue to mature. Bamboo may arrive and take root, and it will allow bamboo snakes and other animals adapted to it to settle on the island.

As the forests overtook the grasslands, many of the pioneer species that had arrived on Krakatau early on disappeared. The zebra dove vanished from the island by the 1950s. Others eked out an existence by colonizing patches of the forests where a tree crashed down and opened up the canopy. Now, after almost 120 years, the flow of immigrants has slowed down considerably. Krakatau seems headed for an equilibrium.

The theory that islands have an equilibrium of diversity was pioneered by two ecologists in the 1960s. Robert MacArthur and E. O. Wilson argued that you can predict how many species an island will have from its size. The first species that arrive on an island have lots of room to spread. As more species arrive, though, they have to compete for food or sunlight, and their numbers go down. As more predators arrive, they can drive down the numbers of their prey as well. If the population of a species on that island drops too low, a hurricane or a disease can wipe out the last few individuals. The arrival of new species, in other words, raises everyone’s risk of extinction.

So there are two forces pushing and pulling on the total number of species on an island–the addition of new species (those that arrive and those that form on the island) and the extinction that their competition brings. Eventually the diversity on the island reaches a point where both forces cancel each other out.

That balancing point depends on the size of the island. On a small island, there are few habitats and little space, which means that competition will be fiercer, extinctions more intense, and species fewer. Bigger islands can accommodate more species. Before Krakatau blew, it presumably had more species on it than any of the smaller islands that were left afterward.

Before Krakatau erupted, hardly anyone bothered to make note of its wildlife. What little information there is, however, suggests that ecologically it’s not the same place it was before. An early explorer noted 5 species of land mussels on its beach; today, there are 19 species, none of which match the original ones. The forests that have taken hold on the new islands are not the same; they are dominated by different kinds of trees.

When an ecosystem rebuilds itself, it may follow MacArthur and Wilson’s rules of diversity, but it doesn’t duplicate itself. Different species scramble to take over the niches waiting to be filled. Krakatau’s fate depended in large part on what plants and animals got there first and how much time they had to establish themselves before they had to face competition.

At the beginning of the Triassic period, the world was a patchwork of Krakataus. Species that could survive in awful conditions had the world to themselves, and they spread like weeds for thousands of miles. Carpets of bacteria rolled through the shallow coastal waters, unmolested by grazing animals. A few tough species of animals and plants also flourished. A single species of bivalve named Claraia raced across the shallow oceans of the western United States; today you can walk for miles over pavements made only of its fossil shells. On land, lush jungles were replaced by patches of quillworts and a few other weedy species, as botanically boring as an Iowa cornfield. Quillworts belong to a primitive branch of plant evolution that, by 250 million years ago, had been pretty much outcompeted by gymnosperms (a group that includes conifer trees). But quillworts can survive in harsh conditions that kill most gymnosperms, so the extinctions of other plants revived them.

For 7 million years, life on Earth remained covered in weeds. Researchers don’t know why these grim conditions lasted as long as they did; the climate and the chemistry of the oceans may have been too hostile for anything but disaster‑loving species to survive. Even after the physical conditions improved, it would still have taken a long time for the world’s ecosystems to recover. Forests could only grow once soil had been created by the plants that came before them.

Slowly the world’s ecosystems recovered, but they were never the same again. The oceans’ reefs, once composed of algae and sponges, were now made up of colony‑forming animals called scleratinian corals, which still make up the majority of reefs found on Earth today. Before the extinctions, the fauna that lived on a typical reef would have been dominated by slow‑moving animals or ones that were rooted to the reef itself–creatures such as sea lilies, bryozoans, and lampshells. Today only a remnant of each group survives. Since the Permian‑Triassic extinction, fish, crustaceans, and sea urchins have dominated the reefs instead.

On land, quillworts and other weedy plants rebuilt the soil, and conifers and other plants emerged from their refuges. They beat back the quillworts in only half a million years, rebuilding forests and shrublands. But once life on land had recovered from the mass extinctions, it was changed for good as well. Before the extinctions, the dominant insects were dragonflies and other species that keep their wings unfolded. But after the extinctions and ever since, insects with folded wings have been most common.

Almost all the synapsids, which had been diverse and dominant vertebrates before the Permian‑Triassic extinction, disappeared, and during the recovery they did not take back their dominance. Reptiles became more common, evolving into new forms, like crocodiles and turtles. And about 230 million years ago, one slim bipedal reptile gave rise to the dinosaurs. Dinosaurs soon became the dominant land vertebrates, a position they would hold for 150 million years.

The Permian‑Triassic extinction shows that there is something to Cuvier’s revolutions after all. Millions of species can be wiped away in a geological flash, and the sort of life that takes over afterward is often profoundly different from what came before.

Mass extinctions put the normal rules of evolution on hold. At the end of the Permian period, conditions suddenly became too harsh for almost any species to survive. As species disappeared, the ecological web they helped form collapsed, and other species went extinct. Some of the survivors might have had some intrinsic qualities that kept them from vanishing. Their ranges may have spanned an entire continent or ocean, raising the chances that a few individuals might have survived in some isolated refuge. They might have been able to tolerate low levels of oxygen in the ocean or a sudden rise of temperature on land. But most of these adaptations mattered only for the short stint during which Earth became its own hell.

Once mass extinctions end, evolution returns to its normal rules. Competition between individuals and between species begins again, and natural selection invents new kinds of specialization. But a lineage that might do well playing by these normal rules can’t win if it has been wiped out by a catastrophe.

Extinctions also bring bursts of change in their wake. They can clear away dominant forms of life that under normal conditions would shut out any aspiring species that have the potential to compete with them. Without this overbearing competition, the survivors are free to explore new forms. Dinosaurs may have emerged only because the dominant synapsids were overthrown.

Yet the liberation that extinctions bring to survivors isn’t infinite. Even after the Permian‑Triassic extinction, when competition dropped to nearly nothing, evolution did not invent any new phylum of animals. No vertebrate lineage evolved nine legs. After the Cambrian explosion, animals may have become too complex to be radically reworked by evolution. The new evolution that took place in the wake of extinctions were only variations on these basic plans.