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By now, the scientific story of the Cretaceous-Tertiary (K-T) catastrophe is well known. Sixty-five million years ago, the Cretaceous period ended when a rogue space rock the size of a mountain smashed into the edge of the Yucatan peninsula, resulting in giant earthquakes, an Atlantic Ocean sloshing in its basin, and a planet-wide rain of superheated rocks that ignited forest fires wherever they landed. A thick pall of smoke and dust blocked sunlight, triggering years of global winter and acid rain.
On land, vast numbers of plants and animals died out. Decaying flesh and plant matter produced a spike in atmospheric methane and carbon dioxide, both greenhouse gases. Meanwhile in lakes and oceans, shellfish populations plummeted as their shells dissolved in a suddenly acidic environment, releasing even more CO2.
When the skies cleared, the greenhouse effect took over, suddenly turning the wintry planet tropical. By the time the climate settled down in the early Tertiary period, more than half the genera, including all the dinosaurs except the ancestors of present-day birds, had gone extinct.
Yet the mass extinction caused by that catastrophe pales in comparison to the events that ended the Permian period, writes Douglas H. Erwin of the Smithsonian Museum of Natural History in his new book Extinction: How Life on Earth Nearly Ended 250 Million Years Ago. The fossil record shows that approximately 95% of the Permian species did not survive into the Triassic.
The cause of that near-total extinction remains one of science's great mysteries, and Erwin has spent most of his professional life searching for clues. The well-written Extinction is his invitation to amateur paleontologists to join him in his sleuthing.
Erwin structures the book as a mystery. He begins with what might be considered the scene of the crime, though he does not use that language. First stop: the American West, including the famed Permian Basin of West Texas where ancient coral reefs now tower as mountains above barren terrain. Near the top of those reefs, fossil marine species that are abundant and varied in the Permian rocks below the boundary give way to a much sparser assortment in both variety and number above it.
Since the extinction was a world-wide event, Erwin also takes his readers to South China, Australia, Siberia, and South Africa, and discusses geological findings in Antarctica. At the end of the Permian, these sites were in very different positions relative to each other than they are today. Two ancient super-continents were in the process of merging into Pangaea, and the Atlantic Ocean had yet to form.
Erwin turns next to the suspects in a chapter called "A Cacophony of Causes." The possibilities include the impact of an even larger space rock than the K-T killer. Alas, there is no evidence like the Chicxulub impact crater, whose discovery in the early 1990s was viewed as the "smoking gun" in the solution of the K-T case. There are wisps of evidence that support the possibility of an impact, but nothing as clear as the surprising excess of iridium in the layer of clay marking the K-T transition in rocks around the globe.
Other likely suspects include climate change as a result of massive volcanic activity that formed the flood basalts of Siberia, the largest such features in the world. The formation of Pangaea itself is also a suspect, because species from one colliding continent could invade the other. Such alien invasions generally lead to a decrease in biodiversity.
Glaciation, with its consequent cooling and decrease in sea level, is another suspect. In today's discussion of global warming, some worried scientists are speaking of a possible 6-meter (20-foot) increase in sea level this century. As devastating as that would be to the civilized world, it is barely significant in comparison to the estimated 200-280 meter drop in sea level that took place at the end of the Permian.
Another possibility is the disappearance of oxygen from either deep or shallow ocean waters, which could result from several different geologic events. Finally, there is the "Murder on the Orient Express" hypothesis, in which some or all of the suspects act in concert to produce the mass extinction.
Throughout the book's detective work, several questions about the extinction remain at the top of the agenda: How long did it take? Did it happen at the same time at different locations? Were there separate extinctions on land and in the oceans? Was it, in fact, a single event or were there multiple bursts of extinction?
Pursuing those questions requires immersion in considerable scientific detail. For the casual reader, the taxonomic names of fossils and the names of geologic sub-periods drawn from various European and Asian locations can become overwhelming. Likewise it is easy for readers to get lost in the details of scientific techniques used to date rock formations or to find subtle differences in samples, such as changes in the ratio of the two non-radioactive isotopes of carbon.
Erwin does his best to keep the text from bogging down, but many readers will need to shift to skim-reading for several chapters to stay with the story. Those readers may also be disappointed to discover that the "Denouement" chapter, in which Erwin has promised to finger the prime suspect, is not very conclusive.
"The short answer is that we do not know, or at least I do not know...," he admits. "I have so often been wrong about the cause of extinction that, in deference to my battered sense of scientific worth, I am tempted not to hazard an answer. But leaving the question hanging is hardly sporting, so here goes."
He then briefly reviews arguments for and against the most promising suspects and comes down slightly in favor of one of them. A mystery writer would be pilloried for such a wishy-washy conclusion, but a scientist deserves commendation. Much of the fun in science comes from tugging at the loose ends of well-established ideas.
In that spirit, Erwin closes with two chapters that follow the best of those dangling questions. He recognizes that there are insights to be gained into the Permian extinction if one examines the very slow recovery of life in the early Triassic. Additionally, there is much to learn by studying the mid-Triassic upsurge of species that rivaled the original Cambrian explosion of multi-cellular life more than a half-billion years ago.
In these closing chapters, the writing becomes particularly entertaining when Erwin introduces other scientists' ideas of "Lazarus taxa," species and genera that reappear in the fossil record after apparently dying out in the great extinction. He counters with his own "Elvis taxa," meaning new life forms that evolve to imitate the irreplaceable originals that have, indeed, left the fossil record.