The greenhouse extinction – evidence of greenhouse effect in dinosaur fossils
Peter D. Ward
Life disappeared almost completely 250 million years ago. Now the fossils of the victims tell a tale of hideous global warming.
After a long day collecting cores of rock, it was finally time to relax. Along with my colleague Joseph Kirschvink of Caltech, I sat in the cool protection of the afternoon’s lengthening shadows with field gear and sample bags strewn about my feet. Slabs of eroding brown anti red shale were piled around us; randomly scattered through this shale were bits–and occasionally whole skeletons–of white fossilized bone. We were about halfway up a wide canyon, and as I stretched out my tired muscles I looked back, and down, in the direction we had traveled since morning.
At first light we had made our way to the banks of the broad Caledon River, which torpidly snakes across the region of parched South African countryside known as the Karroo. The Caledon River had created the canyon, near the town of Bethulie, by carving its way through rock dating back 245 to 255 million years ago. These ancient strata have escaped the tossing and turning of plate tectonics, and so they still lie in the same horizontal plane in which they were laid down. Thus as we climbed the canyon wall, we ascended into ever more recent times. From the river’s edge to where we now rested, the strata were green and olive sandstone and shale formed on an ancient floodplain. Above us, however, the somber green suddenly gave way to red and brown. For as much as a thousand feet above us, I could make out rocks painted every imaginable hue between carmine and ocher gleaming in the sun.
Our resting place was balanced on the fulcrum of this great color shift–green rocks below, red above, and around us hovered the unnumbered flies of Africa. They seemed fitting company, for this place is actually a vast ancient graveyard. The division between green and red marks the greatest mass extinction in Earth’s history: the end of the Permian Period and the beginning of the Triassic 250 million years ago, when 90 percent of species in the ocean and 70 percent of species on land disappeared.
Despite its scale, the Permian extinction remains a deep mystery. Kirschvink and I had traveled to this isolated piece of Africa to look for new clues about what had actually happened during this global catastrophe. Did the old species slowly die away over millions of years, one after another, to be gradually replaced by dinosaurs and other creatures? Or was the old fauna suddenly destroyed by a catastrophic event of much shorter duration? And most important, what caused this greatest of mass extinctions? Could it happen again?
Sitting in the Bethulie canyon, I could see the fossilized bodies of the victims scattered around me, eroding into green and red dust, and I wondered about their death. Perhaps some shift in the way our planet worked–in its climate, its sea level, or the chemistry of its oceans and atmosphere–killed them off slowly. Or perhaps their passing was a sudden result of an extraterrestrial event, such as the impact of a comet or the explosion of a nearby star. I felt a bit like a gumshoe looking for a murder weapon in a cemetery, hoping that the murderer wasn’t still lurking nearby.
In the 530 million years’ worth of fossils that animals and plants have left behind in any great numbers, we have found evidence of many mass extinctions but only five that have killed more than half the world’s species. The best known is the Cretaceous-Tertiary (also called the K/T) event 65 million years ago, caused by the impact of a comet or an asteroid. Yet despite its fame for claiming the dinosaurs, the K/T event destroyed only about 50 percent of the species on Earth, making it far gender than the Permian-Triassic event.
It’s been difficult to determine the cause of the Permian-Triassic extinction–largely because very few places on Earth preserve the sedimentary rocks formed at the time, most of which were deposited at sea rather than on land. The marine rocks demonstrate that a monstrous extinction swept away most of the animals then living in the sea; the few terrestrial deposits suggest that some extinctions happened at about the same time on land. But most of the land fossils have turned up in scattered layers below and above the Permian-Triassic boundary, making it hard to tell what the speed of extinction on land really was, or to determine whether it was happening at the same time as the extinction at sea.
South Africa may hold the solution to these mysteries. The great Karroo desert has long been revered by paleontologists as an exquisite boneyard, with the world’s best fossil collection of land-dwelling Permian vertebrates. Most of the Karroo fossils belong to one of three groups of animals: amphibians, big plant-eating reptiles called pareiasaur’s, and therapsids–the reptilian-looking ancestors of mammals. We know that therapsids were mammalian ancestors because despite superficial appearances, they share a number of distinctive features with us. Their teeth, for example, are differentiated for biting, chewing, and grinding, as ours are.
All three groups were abundant in the Karroo, with the therapsids the most abundant of all. Some of the therapsids, such as the animals known as dicynodonts, were strange-looking herbivores that sported a pair of large, downward-protruding tusks and a parrotlike beak, making them look a bit like saber-toothed tortoises. Dicynodonts may have used their tusks for digging up roots, or perhaps for defending themselves–these dog-size plant eaters were not alone in their world. Terrible predators lived there as well, veritable hellhounds from our worst nightmares, called the gorgonopsids. These large meat eaters of the ancient Karroo world grew to the size of lions, and while they weren’t very swift, they were probably able to prey on the herds of lumbering herbivorous dicynodonts.
This bizarre assemblage thrived for tens of millions of years in the lush splendor of the ancient Karroo, and then, 250 million years ago, they died away. Until quite recently, those few scientists who studied this mass extinction universally believed that it took place over a protracted period, perhaps as long as 10 million years. Once, the K/T extinction was viewed in much the same way, but with the accumulating evidence that the age of the dinosaurs was brought to an end suddenly by an impact, many scientists began to wonder if other extinctions had been misinterpreted as well. Naturally, many were drawn to the greatest mass extinction of all time.
I was one of them. Back in 1991, I began studying the Permian-Triassic boundary rocks of South Africa, and I was fortunate to get as my guide for this adventure Roger Smith of the South African Museum, the world’s authority on the Karroo’s strata and the fossils they hold.
Most of the work on the therapsids of the Karroo hasn’t been very concerned with what happened at the extinction. Paleontologists have focused their attention instead on how mammals evolved from therapsids, with the result that there has been only a small effort to track how their diversity changed through time. The general sense was that therapsids in the Karroo died out slowly–but that wasn’t the impression I got when Smith and I traveled to the desert for my first visit. Instead I was struck by how similar the pattern of fossils was to the sudden extinction of the dinosaurs at the K/T boundary. I eagerly looked for the telltale signs of a collision, such as the traces of iridium carried by asteroids, and the shocked quartz created in a tremendous impact. On hands and knees, I crawled over the gritty sediments in this South African wasteland, but I could find none of the evidence that I had seen for the K/T event.
I finished my visit to South Africa puzzled. I could only hope that Smith–who promised to keep looking at the P/T boundary and collect any fossils he found there–might find clues to help me understand this strange extinction. By 1906, he and his team from the South African Museum had decided that the Caledon River valley would make the best site for collecting from the boundary. This section of rock was packed with fossils, even by Karroo standards, and many could be found in ancient burrows, the skeletons neatly curled up and undisturbed since their death a quarter billion years ago.
Smith began to chart the fossils far more carefully than had ever been done before in the Karroo. He first measured the thickness of the rock layers and then began collecting the fossils they held, noting the location of each one extracted. It soon became clear to him that the site was truly exceptional. In the lowest strata, he found a wide diversity of Permian-age mammal-like reptiles. Then, around the sudden transition between the green rocks of the Permian and the red rocks of the early Triassic, there was a 20-yard stretch of rock in which the older therapsids mingled with a new animal, a pig-size herbivore known as Lystrosaurus. In these boundary rocks the fossils were sparse, but farther up the canyon wall they again became abundant–although now the vast majority were Lystrosaurus. Smith and his team had discovered a continuous, dense record of life on land before, during, and after the Permian-Triassic extinctions.
This discovery immediately showed that the extinction was not as sudden as the K/T event. At the end of the Cretaceous, dinosaurs just vanished, wiped out in a matter of months or years. Here we could see a longer dwindling of Permian forms. But to get a more precise fix on the timing of the extinctions, we needed to look at the chemistry of this exceptional span of rock.
The carbon in our atmosphere’s carbon dioxide comes in different forms, known as isotopes, depending on how many neutrons are in the nucleus. Carbon 12 is most abundant, but there is a small percentage of carbon 13 as well. As plants absorb carbon dioxide by photosynthesis, they shun carbon 13, so that carbon 12 tends to concentrate in their tissues. Most of this organic carbon gets returned sooner or later to the atmosphere, where its concentrated ratio of carbon isotopes leaves its mark. If for some reason the productivity of life on Earth slows down or collapses, returning organic carbon to the atmosphere, the ratios will shift accordingly. And any organisms that are alive at the time and incorporating atmospheric carbon into their tissues will have that isotopic ratio reflected in their bodies. We paleontologists can dig up those bodies and measure the ratio of isotopes in the fossils.
This sort of work had already been performed on marine fossils from the Permian-Triassic boundary, and researchers had found a distinctive change in the ratios of carbon 13 and carbon 12, which pointed to a worldwide ecological catastrophe. Plankton died out almost completely, and the animals that depended on this resource vanished. To do the same sort: of study for conditions on land, I enlisted the help of a specialist in this sort of analysis, Ken MacLeod, then at: the University of Washington, to study the carbon isotopes in the teeth of the fossils from the Karroo.
It took months of work, but MacLeod’s efforts paid off in the summer of 1997. He discovered a huge shift in the isotopic values across the Permian-Triassic boundary, just like the one found in the oceans. The change is consistent with the idea that the productivity of plants plummeted at the same time that Permian therapsids were dying out and Triassic lystrosaurs appeared. Judging from the rate at which the sediments built up in the layers where these teeth were found, we estimate that the change in isotopes lasted somewhere between 10,000 and 100,000 years. That range turns out to match the latest estimate for the isotope spike in marine rocks, reported in May by Samuel Bowring of MIT and Douglas Erwin of the Smithsonian Institution. It’s not as sudden as the K/T event, but it’s far faster than previously thought.
In the summer of 1997 I returned to South Africa to get a different kind of insight into the extinction, bringing with me Joseph Kirschvink, a specialist on Earth’s ancient magnetic field. When certain rocks form, magnetic particles inside them line up like little compasses. Hundreds of millions of years later it’s possible to read the directions they point in. One of the things you can learn from these particles is where on the planet the rock was when it formed. Getting these data is tough work, particularly in a place like the Karroo. You have to find the particular rocks that still preserve their magnetic orientation, and then you have to drill them out. The coring machine is a modified chain saw fitted with diamond-tipped drills; it has to be cooled with running water pumped through the drill core. The task takes two people: one person drills the rocks, the other pumps water through the machine, and all during the operation, clouds of muddy water are thrown over both. The noise is earsplitting, and more often than not the precious cores shatter. But the biggest trouble with this work is that it requires a lot of water. We were working in a desert, and much of our labor involved hiking back to the river or to small mosquito-infested water holes to refill our canisters. As we trudged across the profoundly lonely canyon, accompanied only by swarms of flies and the vultures soaring overhead, I laughed at the notion that fieldwork has any romance to it.
This trip was particularly frustrating for me as a paleontologist. My job there was to drill rocks, not collect fossils, and yet fossils were everywhere. I could see yard-long mud balls with lystrosaur bones extending from them, concretions that Smith has identified as burrows in which these animals died. Much as I wanted to experience the joy of exhuming these spectacular fossils, the best I could do was mark each site with a rocky cairn to alert Smith’s team on their next visit.
We are still analyzing the paleomagnetic samples, but some important clues have already emerged. Our results agree with previous research showing that at the end of the Permian, southern Africa was located well below the Antarctic circle. This is a strange notion to contemplate, given that the fossils suggested a lush landscape populated by abundant animals. Every other piece of evidence suggested warmth, not cold.
I believe that the only way to make sense of all this–the polar jungles, the death of the therapsids, the vast shifts of carbon on land and at sea–is to turn to the ideas of Harvard paleontologist Andrew Knoll and several of his colleagues. In 1995 they offered an explanation for the extinctions that happened in the oceans at the end of the Permian. They suggested that the ocean at that time was very stagnant compared with the present. Today very cold water from the Arctic and Antarctic ice caps sinks and flows along the bottom until it eventually encounters warmer tropical water and rises. During the Permian Period, however, there were no ice caps and thus no mixing. Knoll and his colleagues theorized that because of this sluggish circulation, organic matter that fell to the bottom of the ocean became locked in sediments. Then, as the continents drifted into a new configuration, the ocean circulation switched back on. The ocean began to mix again, and much of the extra carbon on the sea floor was swept up into the surface waters, where it turned into carbon dioxide. At the same time this carbon was rising from the deep, vast amounts of lava were flowing out of fissures in Siberia. Along with the lava came carbon dioxide, which eventually penetrated the oceans.
Carbon dioxide is extremely toxic to marine life. It interferes with the biochemistry that many plankton use to build their calcium carbonate shells, and it acidifies the blood. Knoll anti his colleagues proposed that this influx of carbon dioxide was so poisonous that almost everything in the oceans died.
This model, as elegant as it was, didn’t say much about what happened to life on land. But our new evidence suggests to me that land animals were also killed by carbon dioxide, although indirectly, Terrestrial plants and animals are far less sensitive to increases in carbon dioxide than marine life, so the change in atmospheric chemistry wouldn’t have been able to do any direct harm. But when the gas emerged from the volcanoes and the oceans, though it didn’t poison terrestrial life, it did heat up the atmosphere.
The surge in temperature is reflected in the red beds that appear suddenly along the canyon walls of the Caledon River at the P/T boundary. Red beds can be formed when sediments are exposed to the air at high temperature; the iron compounds they contain essentially rust, giving the sediments a ruddy hue. Such red beds are almost unheard of at high latitudes, and their abrupt appearance at the end of the Permian suggests a climate change of massive proportions.
The rapidly changing climate altered weather patterns around the world, and regions that were wet and rainy may have become dry, and vice versa–much as happened this winter during the latest El Nino. Most land animals couldn’t survive the change. The few that did, such as the lystrosaurs, seem to have adopted a burrowing habit, not to stave off cold as so many animals do today but perhaps to avoid the heat.
The Permian extinction is now shaping up as an entirely new type of mass extinction. It had nothing to do with extraterrestrial causes, yet it happened far faster than typical extinctions triggered by internal changes to Earth’s climate and chemistry. And if our hypothesis is correct, it raises some very disturbing implications about our current situation. We humans are producing carbon dioxide at a prodigious rate, and many climatologists believe that we are already raising temperatures and altering weather patterns. Are we walking down the same path that killed off so much life 250 million years ago–not from carbon dioxide liberated from the oceans but from carbon dioxide liberated by our cars and industry.? There is still far more work to be done and more trips to Africa to be made. But the image of an ancient killer now lies exposed in the red strata of the ancient Karroo desert, a killer I certainly hope is not currently coming back to life after its quarter-billion-year-long sleep.
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