Next Species 1 A MASS EXTINCTION: THE CRIME SCENE
IF YOU’RE CURIOUS as to what a mass extinction looks like, you might want to visit the remains of the Capitan Reef at Guadalupe Mountains National Park, the highest mountains in Texas. Life abounded in the seas back then, but the dinosaurs had yet to appear. The creatures that walked the dry land were not as enormous, nor as diversified, as they would become later. The continents were bound together in a single landmass, but as it broke up and drifted apart, the movement provided the isolation necessary for new species to evolve. Still, life had to sidestep the Permian extinction before it could truly flourish. The story of life’s decimation at this point, followed by its resurrection, has multiple lessons for our own predicament.
The Capitan Reef, though long dead, once thrived between 272 and 260 million years ago in the middle of the Permian period, just before the greatest mass extinction the world has ever known. The International Union of Geological Sciences has selected three points within the park as “golden spikes,” the standard against which all other rocks of the Middle Permian period are compared. (The actual markers that indicate these points are brass plaques.)
At the bottom of the trail up to the reef one day, I met Guadalupe Mountains National Park geologist Jonena Hearst. After she patiently
answered questions from a park visitor and showed me some maps and geological charts, a process that took twenty minutes, she loaded up her day pack and lifted it onto her back. With a big broad smile she signaled it was time to get going. I followed after her. It was fall, a transitional time in West Texas weather, when mornings bore the chill of impending winter, yet the afternoons carried a remembrance of summer heat. McKittrick Canyon before us cut a slash through the Guadalupe Range, exposing the backbone of the Capitan Reef—one of the most extensive fossil reef formations on earth.
The surrounding terrain was dry, open desert, with cactus and creosote brush sheltering an assortment of rabbits, snakes, and lizards—a marked contrast from the tropical rain forests of Vilcabamba. Whereas the rain forest is full of moisture and life, the desert is bashful about any display of exuberance. Farther up McKittrick Canyon, cottonwoods surrounded a portion of the stream that surfaced intermittently. The trees were full of inviting autumn colors, but our path quickly pulled away from the stream and veered up a steep embankment toward the Capitan Reef above us.
What we saw of the reef exposed here displayed the calcified remains of an enormous formation of shelled creatures and sponges that once lay beneath an ancient sea, not unlike the coral reefs of today. A huge fault lifted a section of the reef high into the air, brandishing the dark rock for all to see. The trail was steep—a gauntlet of narrow switchback turns, full of slippery boulders that tested one’s stamina and balance. Yet the site was still quite popular, particularly with geologists and paleontologists, for it led into the fossil remains of an ancient world.
Park geologist Hearst was the keeper of this treasure, and was astute and knowledgeable about its intricate secrets. But she was as exuberant as she was scholarly. She told me she had last hiked up this reef just two weeks earlier, yet she wore a big smile despite some heavy breathing. “It’s a geological Disneyland,” she proclaimed. “Every time I go up there, I learn something new. How many times have I been on that ride, you ask?” She shook her finger toward the reef. “Don’t know, but I want to do it again!”
The hike began at the bottom of an
enormous depression known as the Delaware Basin, which spread out into Texas. The reef had formed over a distance stretching many miles around the lip of the basin, the horseshoe mouth of which once pointed out to an ancient sea. A quarter of a billion years ago, this reef was still glowing with a halo of life formed by millions of juvenile fish and other marine creatures that once used the nooks and crevices of the reef to avoid large predators.
Back then, two enormous continents—Laurentia (made up of North America, Europe, and Asia) and Gondwana (made up of South America, Africa, Antarctica, and Australia)—formed the terrestrial landscape at the surface of the planet. These two landmasses were on a collision course, soon to form Pangaea, the single continent that would take the world through the Permian extinction, an event that came the closest to ending life on earth than at any other time in the last 600 million years.
Our trail told the grand story of life before that event. We scrambled up the loose rock beneath the slopes of the giant reef head. We gained altitude quickly as the trail rose above the desert landscape. This was a deepwater reef, different from the shallower coral reefs most recreational divers are familiar with today. In Permian times, we would have been walking 5,000 feet (1,500 meters) below the surface of the ocean. “A very long snorkel to get to the top,” said Hearst.
As we rose upward, larger boulders and layered outcrops gradually displaced the loose, rocky slopes. Hearst stopped before a large boulder that reached our height and stared at the markings on it. At first I didn’t see anything special; it was just a big boulder. But then she pointed out the many fossils contained in the rock. It turned out that we weren’t staring at a plain rock—we were gazing on the calcified remains of ancient reef animals that had once been bound together in a mass of life.
During the Permian period this gallery of life included flowerlike crinoids, which sat atop stalks attached to the seafloor, their numerous tentacles coated with mucus extended out to capture prey, and
you could see the fossil remains of these creatures in this rock. There were also bryozoans, small animals that superficially resembled corals, which grew in tightly packed colonies resembling intricate fans, lacy fronds, or fruitlike displays that accumulated into massive stony buttresses. Also here were clamlike creatures called brachiopods, which were filled with a tangle of filaments that helped the animal sift food from the water but which would have made a poor clam chowder. There were numerous species of sponges as well as nautilus-like creatures housed in large spiral shells. The boulder was filled with such animals, surrounded by algae, which acted as cement to hold everything together. As she pointed to other rocks nearby, my astonishment grew. All the boulders housed similar amazing displays.
From the base of the reef we pressed on up the trail. As we approached the part of the reef formerly within the reach of sunlight and the energy of the waves, the reef fauna began to change from marine communities dominated by sponges and bryozoans to those dominated by algae and large clamlike gastropods.
Toward what was once sea level, the sponges disappeared. We entered the intertidal zone where outgoing tides would have periodically exposed the reef to sunlight and air, and this produced still more shifts in the animal communities. Ahead we could see the remains of limestone barrier islands. Behind the barrier islands were sand and gravel bars cut through by tidal channels, and beyond that the dry remains of a large lagoon facing a shoreline of salt flats.
The Permian period stretched from 298 million to 251 million years ago, the reef thriving across the West Texas terrain along the margins of what was once a warm tropical sea. In its prime, it would have been about four hundred miles in length.
Reefs are among the most biologically diverse of any ecosystems. They are the rain forests of the sea. Yet they leave more evidence than a rain forest for the paleontologist to study because they are made up of hard-bodied organisms that make fine fossils. It’s why paleontologists have made the pilgrimage to McKittrick Canyon for decades to witness what nature has exhumed almost intact.
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It hasn’t been that long since man would have looked at this towering monument to the history of life and not understood what he was seeing. The recognition and study of fossils in rocks grew out of an incident in the late fifteenth century when two fishermen caught a giant shark off the coast of Livorno, Italy. The local duke sent the shark to Niels Stensen (aka Nicolas Steno), a Danish anatomist working in Florence. Steno dissected the animal and noted how much the shark’s teeth looked like “tongue stones,” triangular pieces that rock collectors had been gathering for ages. Few at the time would have conjectured that tongue stones or any other fossils might be remnants of ancient sea life, but Steno started making a case for it and was widely credited with giving birth to the science of paleontology.
The awareness of fossils grew, and in 1815, William Smith, a geologist from the county of Oxford, England, published a complete geological map of England and Wales. He was the first to use fossils as a tool for dating and mapping rocks by their stratigraphy, the lines and layered elements of earth that are visible when sedentary rocks are cut into—though it wasn’t until after Darwin that scientists realized the importance of these fossils to understanding the timing of evolution.
Geologists discovered that layers of rock in North America could correspond in time to layers of rock in Asia or even Africa and that similarities in the fossils within them could be used to determine their synchronicity. But what geologists began to realize was that the layered record of earth’s history at times told the story of evolution a bit differently from Darwin. The master believed that evolution advanced in tiny increments over multiple generations and that the process was geologically slow. Natura non facit saltum (“Nature makes no leap”) was his credo. But other scientists began to note a number of upheavals captured in the rock record of earth’s history, which showed radical, sudden changes in animal fossils.
These upheavals presented an amended look at Darwin’s grand scheme, and were known as mass extinctions. Evolution continued
after them, but mass extinctions reordered nature, abruptly ushering out older forms of life and allowing for the creation of newer ones.
Simple animals without shells or skeletons appeared about 635 million years ago during the Ediacaran period, when oxygen in the atmosphere began to build toward present levels. Since then, there have been five mass extinctions. Evidence of the Permian period, which preceded the Permian extinction 252 million years ago, surrounded National Park Service geologist Hearst and me.
Perhaps the most famous of the five extinction events was the one that wiped out the dinosaurs at the end of the Cretaceous period about 65 million years ago. Scientists long argued over what had killed off the dinosaurs until, in the late 1970s, a team of scientists at the University of California, Berkeley, came up with a theory. Luis Walter Alvarez, a bespectacled Noble Prize–winning nuclear physicist and leader of the team, found unusually high levels of iridium—a heavy substance rarely found on the surface of the planet, but quite common in meteorites—in layered deposits of earth that represented the Cretaceous extinction in both Italy and Demark.
Alvarez, his son the geologist Walter Alvarez, and colleagues shook the scientific community with their announcement that the mystery of the Cretaceous extinction had been solved: an asteroid got the dinosaurs.
Scientists were at first skeptical. Older hypotheses cited volcanism or glaciation as the primary cause of this mass extinction. But eventually high levels of iridium were found at more than one hundred sites, all marking the Cretaceous extinction, and the evidence couldn’t be ignored. But where was the crater?
The Alvarez team went looking for a depression somewhere on the planet big enough to have fit the job. The team calculated that the asteroid must have been about seven miles in diameter. In June 1990, a decade after the original Alvarez proclamation, geologists discovered a huge crater underlying the northern tip of the Yucatán Peninsula near the town of Chicxulub (“Chick-sha-loob”), Mexico, from which the crater eventually took its name.
The crater revealed that the asteroid must have been about
7.5 miles (12 kilometers) wide and was traveling about 44,640 miles per hour (20 kilometers per second) on impact, roughly twenty times the speed of a bullet. The collision would have released
a million times more energy than the largest nuclear bomb ever tested.
The impact blasted thousands of tons of rock as well as the mass of the asteroid back into the atmosphere, with some elements going into orbit, while others returned to the ground in a barrage of flaming meteors. These fireballs ignited the verdant late Cretaceous landscape, burning half the earth’s vegetation in the weeks following the impact. Dust along with the smoke from the fires obscured the light of the sun, dealing a deadly blow to plant life.
In the ocean,
huge tidal waves spread out to the continental shores, leaving a line of beached and bloated dinosaurs skewered on shoreline trees. Scavengers had a field day on the plentiful carcasses. After the initial fires burned out, the earth descended into a period of perpetual night caused by a blanket of smoke and dust in the air. Trees and shrubs began to die, as did the animals that ate them and the carnivores that ate the plant eaters. The Cretaceous extinction killed off the dinosaurs and many but not all of the mammals.
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At the top of the Capitan Reef, we looked out over the fossils, rocks, precipices, and the valley below us, and imagined life over 250 million years ago at the pinnacle of the Permian period. Dry land, which was then about fifteen miles northwest of the reef, was growing drier. The lush swamp forests that had existed before the Permian had been replaced by conifers, seed ferns, and other types of vegetation that were drought-tolerant. Giant cattail-like trees grew up to eighty feet. Ten-foot relatives of the centipede splashed through inshore water.
The first vertebrates had crawled out onto the land only about 100 million years earlier. Giant amphibians, which roamed the marshlands, were up to six feet in length and two hundred pounds in weight. They sucked down dinner with enormous mouths filled with sharp teeth, tossing their captives little by little back into their deep throats,
like a crocodile or alligator would. There were flying lizards and large armored herbivores the size of oxen. There were a number of sharks in the Permian oceans, the most bizarre being Helicoprion, which had a spiral jaw fitted with backward-leaning teeth that looked like a buzz saw. Primitive pelycosaurs about ten feet (three meters) long with smooth bodies spread over much of the land with giant swordfish-like fins on their back for capturing the sun.
The Permian world was a lively one, as proven by the numerous fossils that adorn the earthen walls of McKittrick Canyon. But something caused the annihilation of most of these animals.
THE SECOND CREATION OF LIFE
The Capitan Reef that decorates the top of the Guadalupe Mountains above McKittrick Canyon is similar to the structure of Mount Rushmore, only carved not with US presidents but with the force of life that thrived before the mass extinction. Yet the rocks in McKittrick Canyon do not display evidence of the end of the Permian.
To see that, Sam Bowring, a bearded and amiable professor of geology whom I visited earlier at MIT, had to travel to China. Bowring showed me a photo of himself and Zhu Zhuli, a Chinese researcher, in Meishan, standing on the face of a rock quarry. Zhuli had his feet on a dark line in the rock that represented the end of the Permian. The change in color was caused by a dramatic change in the geology and chemistry of the rock. It was the geological boundary line between the Permian and the Triassic periods, the point where one era of life encased in sediments of earth ceased to exist and another was laid down on top of it. In the photo, Bowring stood above the line in early Triassic ash beds. It is one of the
best-studied Permian-Triassic boundary sequences in the world. Fully 333 species have been identified in the fossils below where these two scientists were perched. But above that line almost all of them disappear, an extinction rate of 94 percent.
John Phillips, a mid-nineteenth-century English geologist who
published the first global geological time scale, found that the fossils were so different on either side of the Permian-Triassic boundary that he referred to the line in the stratigraphic layers that Bowring stood above and the difference in fossils on either side as the Second Creation of Life. He never saw the line in Meishan, China, but had studied this event at similar stratigraphic sites elsewhere in the world.
The catastrophe that created this boundary has similarities to the destruction humans are inflicting through greenhouse gas buildup, ocean acidification, and global warming. No, it wasn’t a giant spectacular meteor falling out of the sky. The primary villain of the Permian extinction was the Siberian Traps.
This eruption occurred about 252 million years ago, according to new findings from Bowring. At that time a viscous magma flowed out of the ground and spread over the land, filling in the valleys and basins around it like honey finds the crevices on a piece of toast. The total amount of lava flow was mind-boggling. In one area it grew 6,500 meters thick, almost four miles. “In the end it covered much of Siberia, an area close to the size of the continental United States,” Bowring told me.
Still, there was not just a single cause to this extinction. It was more the perfect storm, the coming together of multiple perpetrators, as it has been with other extinction events. The lava that created the traps burned up through an enormous coal reserve at its center, and the heat of the molten lava converted much of the black rock to CO2. But as temperatures rose, some of that coal would have converted to methane, which is twenty times more potent a greenhouse gas than CO2, and this would have accelerated warming.
The end result of the buildup of CO2 and methane, among other causes, was one of the few mass extinctions of insects in earth’s history. Their numbers descended from sixty families during the height of the Permian period to almost zero at the end of it. The air was silent, since birds had yet to evolve. The coal that had thrived in the marshy environments and plentiful vegetation disappeared as the earth grew drier. Whole forests and entire ecosystems of plants died but fungi flourished, since they fed off the dead plant and animal matter.
Though the asteroid that got the dinosaurs at the Cretaceous extinction may have produced a better fireworks display and spectacular tsunamis, in terms of pure raw killing power, the Permian extinction can’t be beat. Its witch’s brew of toxins poisoned the land for several hundred thousand years. Doug Erwin says that the eruption of the Siberian Traps caused global cooling from the erupted dust, global warming from the CO2, and acid rains from billowing clouds of sulfur. Couple this with ocean acidification and the death of oxygen in the deep seas due to the melting of polar ice and the loss of ocean currents, and you have a lethal force that far exceeded the destruction caused by the falling asteroid during the Cretaceous.
The resulting excess CO2 entered the ocean, making the water acidic enough to prevent animals from forming exterior skeletons and destroying most of the reef-making organisms of the Permian seas and most of the reefs. The acidic nature of the seawater, coupled with the lack of oxygen in the deep oceans, wreaked havoc on marine plants and animals. The sulfates that emerged from the volcanoes reached the upper atmosphere, to be carried afar as sulfuric acid and lethal acid rains. These rains may have been strong enough, suggests Erwin, to kill off many of the terrestrial plants. This totally denuded landscapes over much of the earth’s surface. Scientists have found evidence that much of the rain that followed the Permian extinctions rolled off the land in flash floods, since there was no vegetation to contain the flow of water.
Floods skipped across the earth like oil does on a hot skillet, moving in every direction, leaving braided gullies in the rock record. I’ve witnessed fast-moving desert flash floods that carved out chunks of road like butter, but desert rains are meager. Imagine flash floods raging in plant-free tropical or coastal environments where annual rainfalls are twenty, fifty, one hundred inches, or more, racing in full and furious force across landscapes stripped of vegetation, and you’ll get an idea of what the floods that followed the Permian extinction must have been like.
But despite the evidence of multiple causes for the Permian
extinction, some scientists still champion their favorite antagonists. Andrew Knoll, a paleontologist at Harvard, thinks that many of the catastrophes—their causes and their results—can be boiled down to one chemical compound, CO2, the biggest villain of the day, and perhaps our greatest threat as well.
In a 2007 paper in Earth and Planetary Science Letters, Knoll and colleagues tried to work backwards from the extinction event, doing a computerized autopsy of the victims to see if the massacre matched the typical scenario caused by oxygen depletion, a breakdown of the food web, and acid rains, but none of them quite matched the autopsy except for CO2. He highlighted a gas that so many today ignore. “Only
30 percent of the species of plants and animals were tolerant of massive doses of CO2. But after the Permian extinction, that 30 percent suddenly becomes 90 percent of all living animals.”
Estimates vary on how long this extinction lasted. MIT’s Sam Bowring sets the duration at about sixty thousand years. The tiny chewing apparatuses of small eel-like animals are some of the first fossils to appear in layers of earth laid down after the Permian extinction. Fossils of Lystrosaurus, a mammal-like reptile that looked like a bulldog with tusks, but which survived the extinction and proliferated, mark the beginning of the Triassic recovery.
The irony of the Permian extinction is that though it devastated large portions of the planet, it created opportunities in newly emptied terrain. From the resurrection of life after the mass annihilation of the Permian came more-adaptive species, changes in ecosystems, and a world more diverse than the one before it. Perhaps these improvements could be in our future—if we survive the extinction.
The processes were similar to what Darwin witnessed in the Galápagos Islands. Of the twelve species of finches he collected, all were adapted from a few individuals from the mainland or other islands, which had arrived in the Galapágos and proliferated after finding no competition for the banquet of seeds available.
What emerged from the Permian extinction was a similar explosion of new animals and plants. Life not only survived, it eventually
thrived. By 225 million years ago, the first dinosaurs appeared; but by 65 million years ago the group, other than birds, was gone. Their reign on earth lasted close to 160 million years, a length of time that the family of man has barely approached.
Though the end was glorious, the millions of years it took to recover from the Permian extinction were excruciating.
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After a brief lunch at the end of the trail, looking out over western Texas and southeastern New Mexico, enjoying the cooler breezes at the top of the range, Hearst and I gathered our gear and headed back down the same path we’d come up on, still taking note of the various changes in the fossil communities, enjoying a second look, knowing them better.
Hearst explained that life as a whole eventually resurrected itself from the Permian extinction, but few of the individual species of plants and animals displayed here in the fossils of the mid-Permian made it across the boundary. “Life goes on. Life is incredibly resilient. But my work here has taught me that ecosystems and individual species are so very, very fragile,” she said. If history is our teacher, then life in the aftermath of our own era will prove equally resilient, though right now ecosystems and individual species are rapidly disappearing.
From the height of the trail we looked out over the vast desert below and reflected on our own situation. We stood in the middle of evidence of a past evolutionary catastrophe and gazed out over another in progress: our own. Some scientists believe our current situation started at the onset of the Industrial Revolution in Great Britain during the 1700s. This is when CO2 in the atmosphere began its upward climb, a change mirrored here in the aftermath of the Permian. But others date the commencement of our dilemma to 1800, when the human population reached one billion.
Still, others say we entered the present biodiversity crisis during the final moments of the last ice age from about fifteen thousand to twelve thousand years ago, when a substantial portion of the large
animals that once existed in North and South America disappeared. Similar scenarios took place in Australia, New Zealand, Europe, and Asia with
the arrival of man.
Hearst poured some water on a group of fossils, washing off the dust and making them more defined and lifelike for the moment. Of course, the evolutionary processes that produced their first spark of life were much more complicated.