by Peter Ward
A previously unrecognized aspect of the changing oxygen levels would be their effect on species migration and gene flow. Mountain ranges in our world are often barriers to gene exchange, producing different biota on either side of the range. At the end of the Permian, just living at sea level would have been equivalent to breathing at five thousand meters, a height that is greater than that found atop Mount Rainier in Washington State. Thus even low altitudes during the Permian would have exacerbated this, so that even a modest set of hills would have isolated all but the most altitude- or low-oxygen-tolerant animals. The result would be a world composed of numerous endemic centers hugging the sea level coastlines.
Ranges of vertebrate fossils from the Karoo of South Africa, from approximately 260 to 250 million years ago. Each vertical line represents a genus of vertebrate animal (based on fossils recovered from the strata). While most extinctions took place over a fairly narrow interval, this pattern is nothing like that seen at the end of the Cretaceous. The “smeared” appearance of extinctions shown here is characteristic of “greenhouse mass extinctions”—not a single level of extinction, but successive extinctions.
The high plateaus of many continents may have been without animal life save for the most altitude tolerant. This goes against expectation based on continental position: because the continents at this 250-million-year-old time were all merged into one gigantic supercontinent (named Pangaea), we would expect a world where there were very few terrestrial biotic provinces, since animals would be able to walk from one side of the continent to the other without an Atlantic Ocean in the way. But altitude became the new barrier to migration, and new studies of various vertebrate faunas appear to show a world of many separate biotic provinces, at least on land.
The late twentieth- and early twenty-first-century work of Roger Smith, Jennifer Botha, and coauthor Ward in the Karoo desert, Mike Benton in Russia, and Christian Sidor in Niger12 showed that each of these separate localities in Africa had distinct and largely nonoverlapping faunas. Thus, during times of low oxygen, altitude would have created significant barriers to migration and gene flow.13 Low-oxygen times therefore should have had many separate biotic provinces, at least on land. The opposite occurred during high-oxygen times: there would be relatively few biotic provinces and a worldwide fauna.
The drop in oxygen did more than make mountain ranges barriers to migration. It made most areas higher than a thousand meters uninhabitable during the late Permian through Triassic time interval. This effect, called altitudinal compression, could have had a major impact on Triassic land life in the time of lowest oxygen. The removal of habitat because of altitudinal compression would have caused species from the highlands to migrate toward sea level or die out. Doing so would have increased competition for space and resources, and perhaps would have introduced new predators, parasites, or diseases in the previously populated lowlands, causing some number of species to go extinct. We calculated that by the end of the Permian more than 50 percent of the Earth’s land surface would no longer have been habitable because of altitudinal compression. There may even have been extinction caused by the effects modeled long ago by Robert MacArthur and E. O. Wilson in The Theory of Island Biogeography. Those two scientists noted that diversity is related to habitat area, and that when islands or reserves of some sort became smaller, species died out. Altitudinal compression would accomplish the same by making the continental landmasses functionally lower in usable area.
PERMIAN EXTINCTION REDUX
A final aspect of the Permian extinction comes from research not yet published, but because it comes from coauthor Ward it will be reported here, as it is most pertinent to the topic of the Permian mass extinction. One of Ward’s graduate students, Frederick Dooley, combined with Lee Kump to produce an unexpected discovery. Dooley studies the effect of hydrogen sulfide on plants and some animals; Kump has been modeling ocean conditions at the end of the Permian, including estimated amounts of hydrogen sulfide in the global oceans’ surfaces. Kump arrived at a value that Dooley then used in actual experiments on single-celled oceanic phytoplankton, as well as the most important oceanic zooplankton, the tiny shrimp-like creatures called copepods. The levels were not sufficient to kill the algae, and surprisingly actually made them grow faster. The copepods, on the other hand, died almost instantly. Without copepods to feed on phytoplankton and keep it in check, these tiny plants sink to the sea bottom and rot, removing any last vestige of oxygen. This would produce a great oscillation in the carbon isotope pattern, as well as kill off every marine animal species that has an early life history in the upper water column as temporary plankton. The result would be a planet choked in rotting plants but nearly without animals. This is exactly what happened at the end of the Permian—in the oceans, anyway. On land it would have been very much like some combination of World War I and II combined. Roger Smith of South Africa now has very credible evidence of an extraordinary period of drying and sudden heat in the South Africa of 252 million years ago, while our own work on the vertebrates in the Karoo, published in 2005, remains the best record of land animal extinction across that boundary.14 Roger Smith thinks that the drought and heat alone can account for the extinction of most vertebrates. We maintain that the world war analogy is apt: great armies dying in the desert, and in World War I, killed by poisonous chlorine gas. Long ago it was death in the desert from poisonous hydrogen sulfide in the air and sea.
CHAPTER XIII
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The Triassic Explosion: 252–200 MA
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One of the great joys of academics is the sense of community among the faculty, be it a community college or the most high-powered research institution in the land. Much of this comes from the very nature of the American university system, which requires a six- or seven-year trial period, followed by tenure. Permanence. Perhaps more than in any other profession, university faculties have a high stability, and compared to most other professions, a relatively low rate of turnover. The result is that relationships can literally last for appreciable parts of one’s lifetime. In this the university faculty systems are indeed much like the system they were spawned from, the cloistered seminaries where monks would start as young men and then pass through life with others of their kind. And as was true in the old abbeys, with age and wisdom one learns to respect those with even greater experience—and listen to them.
In the year 2000 or thereabouts, the authors of this book were at lunch with several of the eldest of the science faculty of the California Institute of Technology. One of these elder statesmen was the great Sam Epstein, one of the most distinguished professors of geochemistry, perhaps of all time. Sam was present in the halcyon days at the University of Chicago, when Nobel laureate chemist Harold Urey discovered a way to measure the temperature at which ancient carbonate rocks were formed by comparing the isotopes of oxygen found in the precipitated carbonate rock. The ratio of the isotope O16 varied with the much more rare isotope O18 in proportion to the temperature of formation.
Sam eventually moved to Caltech and spent his career making high-precision measurements of many kinds of samples, using many different methodologies. But his first love seemed to be ancient temperatures. After a wonderful lunch, he took Kirschvink and Ward to his downstairs lab, which was in the process of being dismantled. The geochemical equipment of the 1950s and 1960s, Sam’s heyday, was mainly composed of handmade and hand-blown glassware, walls of thin tubes spiraling, crisscrossing, making spider webs of glass interrupted by strange flask-like shapes, rubber tubing coming and going, greased glass stopcocks of exquisite manufacture—everything custom made by the artisans who kept science in those days going, the skilled technicians now banished by budget cuts and the new generations of solid-state technology.
We walked through the lab, and conversation moved on to a topic then of our keen interest: the Permian mass extinction and its possible causes. At that moment the impact hypothesis was still viewed as the probable cause. Sam, however, woul
d have none of it. He turned to us with a smile and told us the following short story. In his younger days he had taken samples of marine limestone that dated to the earliest Triassic, samples that had probably been formed in a very shallow seaway somewhere near the Permian equator in what is now Iran. On a whim, or because that is what he loved most, Sam began analyzing these samples for their ancient temperatures. He was stunned, he said, to find that all had been formed in temperatures above 40°C, with some of the temperatures exceeding 50°C—from 104°F to over 120°F! The samples had come from ancient corals, creatures that need water of normal salinity.
Such temperatures can be found in the stagnant pools and lagoons. But brachiopods do not live in such places. The temperatures found by Sam Epstein could not have been formed anywhere on our Earth. They spoke of a postextinction world of unreal water temperature in the main ocean.
Sam, then in his eighties and with only another year ultimately to live, smiled a sad smile. He told us that he never had the guts to publish these data. Any paleotemperature analysis requires really pristine samples to be accurate, and quite often samples looking as if they had not been reheated or exposed to groundwater or chemically changed in any obvious way had, in fact, had their oxygen isotope temperatures “reset,” and such resets were normally to produce what looked like abnormally high temperatures. This process becomes ever more common the older the sample. But Sam was quite convinced that he had proof of ocean water temperatures above 100°F in the first million years after the Permian extinction—in the first million years of the Triassic.
Several years later, in analyzing paleotemperatures from a different, lower Triassic site, we too found what looked like one-hundred-degree-plus water. This time the depth was even greater than the estimated ancient water depths where Sam Epstein’s Triassic brachiopods had grown so long ago. Like Sam Epstein, we did not publish these results.
The prize never goes to the faint of heart. In 2012 a joint Chinese-American research team, trying to understand why it took so long for life to recover in the seas after the Permian extinction, published an amazing paper.1 Their findings: water temperature of 104°F in the sea, and a blistering 140°F on land! Unlike the work of Epstein, this study involved the analysis of over fifteen thousand samples, making it the most detailed and painstaking look yet at the environmental conditions in the aftermath of the Permian extinction.
The scientists completing this study allowed themselves to speculate about what that ancient hot world would have been like. Most marine organisms die above the plus-100°F level found by the investigators; in fact photosynthesis essentially stops at temperatures much above this. In that world, the entire zone of the topics would have been devoid of animals, and complex life would have hung on only at high latitudes. Land animals would have been rare even in the mid-latitudes. In such heat there would have been enormous volumes of moisture in the air, and the topics would have been wet year-round. But it might have been a wet desert, with no plant life at all.
Ever better geochronology now shows that this time of high temperature extended at least for the first 3 million years of the Triassic, and indeed may have been climbing ever higher during that time, with a maximum temperature occurring during a time interval known as the Smithian stage (a million-year time interval of around 247 million years ago) having the highest of all known temperatures since the time when animals first occurred. Sam Epstein was right. Our data from Opal Creek2 were right. We were wrong in not publishing those data.
The Permian extinction was clearly one of the most fundamentally catastrophic of all events—if, that is, one was a multicellular plant or animal. From a microbe’s point of view—especially one of the sulfur-loving, oxygen-hating microbes that made up the majority of all life on Earth from its very inception right up to the first evolution of animals—that event was like a return to paradise. Seen from our vantage point so long after, the Permian extinction was a repeat of what happened at the end of the Devonian, itself the first of what we now call greenhouse extinctions. Many more were destined to come at the end of the Triassic, multiple times in the Jurassic and Cretaceous, and ending with the last-known greenhouse extinction at the end of the Paleocene epoch, some 60 million years ago. But none were ever to be so great as the Permian event, or to unleash a more diverse assemblage of animals in the aftermath of extinction.
The Permian extinction gave the world many new creatures, but for us, two entirely new lineages, both thriving and evolving by the end Triassic period. In no small way the Permian extinction brought to life mammals, and brought about the means that would create our long-term nemesis, the dinosaurs. Yet while being among the most important of all land animals (few animal groups are awarded an “age of …” before their names), the Triassic dinosaurs and mammals were late arrivals in the Triassic explosion, and remained both relatively small of stature (especially the mammals, which rarely exceed rat size) and small in both absolute abundance and species diversity. The age of dinosaurs was not to start until the successive Jurassic period, while the still-running age of mammals had to await the Cenozoic era.
Long before the late (Triassic) arrival on the evolutionary stage of dinosaurs and mammals, the other animals and plants of the Triassic period make up a most interesting assemblage of organismal characters, cast with new versions of already long-running taxa mixed with entirely new entrants, new designs arising yet radically different from the actual survivors of the Paleozoic era. It is this mix that makes the Triassic appear to be a veritable crossroads in time. In some ways it was not unlike the Cambrian explosion—a slew of newly invented body plans filling up an empty world, just as the first animals rapidly evolved into the cornucopia of body plans that filled the seas after the extinction of the first animals, the Ediacarans. And like the great Cambrian explosion, many of the body plans of novelty turned out to be but short-term experiments, to be pushed into extinction by the competition and/or predation of better-designed organisms. There is no time period other than the Cambrian and Triassic in which such a diversity of new forms appeared. Two reasons seem paramount: The Permian extinction emptied the world to such a degree that virtually any new design would work, for a while at least. But there is a second, new view of the Triassic that may be just as (or more) important than this.
Just coming out of the most devastating of all mass extinctions, this early Triassic world was very, very empty of life. At the same time, all modeling suggests that a long interval of the Triassic was a time of oxygen levels lower than those today. Earlier we suggested that times of low oxygen, especially following mass extinction, foster disparity: the diversity of new body plans. These two factors combined to create the largest number of new body plans seen since the Cambrian, and here we propose that it is to that seminal Cambrian time that we most accurately compare the Triassic. We call this time and its biotic consequences the Triassic explosion.
The Triassic was a time of amazing disparity on land and in the sea. In the latter, new stocks of bivalve mollusks took the place of the many extinct brachiopods, while a great diversification of ammonoids and nautiloids refilled the oceans with active predators. Fully a quarter of all the ammonites that ever lived have been found in Triassic rocks, a time interval that is only 10 percent of their total time existence on Earth. The oceans filled with their kind, in shapes and patterns completely new compared to their Paleozoic ancestors, and why not, for, as shown above, this kind of animal was the preeminent, low-oxygen adaptation among all invertebrates. A new kind of coral, the scleractinians, began to build reefs,3 and many land reptiles returned to the sea. But it is on land that the most sweeping changes in terms of body plan replacements and body plan experimentation took place. Never before and never since has the world seen such a diverse group of different anatomies on land. Some were familiar Permian types: the therapsids that survived the Permian extinction diversified and competed with archosaurs for dominance of the land early in the Triassic, but this ascendance was short lived. The many kind
s of reptiles were locked in a competitive struggle with them, and with each other, for land dominance. From mammal-like reptiles to lizards, earliest mammals to true, the Triassic was a huge experiment in animal design.
On the face of it, the mammals should have come out competitively “ahead” of the pure reptiles. After all, most of the mammal-like reptiles by this time were warm-blooded, probably capable (as now) of far more parental care than the presumably egg-laying dinosaurs; the mammalian teeth, one of the main reasons that mammals eventually did dominate the world, in their endlessly malleable tooth morphologies allowed all kinds of food acquisition, from small seeds to grass to meat of many kinds. Yet they did not win. Their extinction closed out the first age of mammals and gave rise to the second—composed of a very different group of mammals.
One of the major changes that has and continues to allow entirely new kinds of study of all groups of extinct animals is the great revolution in communication, morphological characterization and image analyses, and profound literature search skills that the computer revolution has allowed. Now large databases can be produced and then searched and analyzed in lightning blazes of microprocessor skill.
No longer does each fossil have to be laboriously measured by hand with micrometers, and no longer is it a single investigator traveling from museum to museum to do the work. Almost every new study that brings change to our history of life comes from large teams of investigators, ultimately inputting huge numbers of numbers to be crunched. Now the machines do much of this for us. And the results can produce new insights.