Everyone knows that the dinosaurs went extinct after a giant meteorite smashed into earth 65 million years ago, creating a huge dent in the planet’s surface just off the Yucatan peninsula, but did you know that there was an earlier, even more dramatic, mass extinction event? That apocalyptic event occurred approximately 252 million years ago, at the Permian-Triassic (PT) boundary, and it wiped out nearly 95% of all living species on earth, but unlike the Cretaceous-Tertiary (KT) mass extinction, the PT mass extinction cause(s) remain mysterious. In his book, Extinction: How Life Nearly Ended 250 Million Years Ago (New Jersey: Princeton University Press, 2006), Smithsonian paleontologist and curator, Douglas Erwin, investigates the current data and the resulting hypotheses that seek to explain what happened.
This book starts with a peculiar confession, particularly coming from a paleontologist: “Early Triassic rocks are boring. It doesn’t matter where you are, China, Europe or here in Utah; there is a certain similarity to them, and a dreadful monotony. A kind of austere beauty, but monotonous nonetheless.”
Of course, after 95% of all marine life and at least 70% of all terrestrial life on the planet had been extinguished, it only stands to reason that the rock layers corresponding to those times after this event would be boring because they hold few, if any, fossils to fascinate scientists.
After gaining the reader’s attention with that unusual admission, Erwin then focuses on the reason these rocks are so boring by describing the nature of the end-Permian mass extinction event that will be investigated throughout the book and providing a glimpse of the available evidence. Chapter two, “a cacophony of causes”, enumerates a variety of causes for this mass extinction before describing the six major hypotheses for which there is at least some supporting data: (1) an extraterrestrial impact of the same sort that caused the Cretaceous-Tertiary mass extinction that killed off the dinosaurs 65 million years ago; (2) extensive volcanism that produced the Siberian flood basalts (possibly triggered by an extraterrestrial impact), that radically changed the global climate and geochemistry; (3) continental drift (plate tectonics) with the formation of Pangaea that caused a sharp reduction in biome types; (4) extensive glaciation that caused a combination of global cooling and a drop in sea levels; (5) a decrease in oxygen in shallow and deep seas due to one of several possible causes; and (6) the Murder on the Orient Express hypothesis positing that a combination of several or all of the other already described events occurred nearly simultaneously, thereby causing this, the greatest extinction that the planet has ever known.
The next five chapters are devoted to investigating each of these hypotheses and several alternate hypotheses, detailing the data that support them, and exploring the strengths and limitations of the methods used to obtain these data. In these five chapters, the reader will become familiar with the patterns of extinction – who lived, who died and what ecological or physiological characteristics do they share? Erwin also discusses important evidence gathered from both terrestrial and marine sediments, and along the way, the reader will learn the significance of a variety of studies, including; using ammonoids, conodonts and other ancient species to define the structure of the PT boundary; the presence of a putative fungal spike in the early Triassic, soon after the PT boundary, also; obtaining radiometric dates using zircons obtained from volcanic ash beds; and the shifts in carbon isotope ratios, along with sulfur and strontium isotope data.
Chapter eight, “denouement”, explores the intersections between all these data and investigates the possibility that there were multiple causes to the end-Permian extinction. Finally, the author names his favorite hypothesis for the PT mass extinction (I won’t reveal that here; you’ll just have to read the book to find out which it is) but he is careful to state that he really does not know the answer, that the evidence is incomplete so far and more data could come to light that could change everything.
Erwin then says that learning about what occurred after the PT mass extinction is more scientifically compelling for him than understanding the cause of mass extinctions themselves. In “Resurrection and Recovery”, the author describes patterns of biotic recovery, noting that early Triassic fossils were dominated by a few opportunistic (“weedy”) species and states that it took more than 4 million years before there was a demonstrable increase in speciation in the fossil record. He speculates that this long recovery phase could be due to the necessity of reconstructing entire ecosystems rather than simply repopulating empty habitats after this global disaster. Erwin also discusses the curious abundance of “Lazarus taxa”; taxa that appeared to go extinct, only to reappear much later in the fossil record after their post-apocalyptic populations had built up to significant levels once more.
The last chapter of the book explores long-term implications and modern-day effects of mass extinctions within a broad context. Erwin reveals that, contrary to popular belief, the end-Permian extinction event probably did not significantly affect the long-term outcome of evolutionary history. Indeed, the author mentions that his colleagues think those taxonomic groups that dominate the oceans today were already expanding during the Permian, that the PT mass extinction simply accelerated that process. But Erwin also goes on to say that labeling the current biodiversity crisis as “the sixth extinction” is to invite inappropriate comparisons to historic mass extinctions. Our current biodiversity crisis is very different because so far, it is primarily an event where we are losing localized or specialized species rather than common, widespread species, as was the case in the mass extinctions. However, we are not “off the hook”, so to speak, because Erwin cautions that “biotic recoveries take far longer than the recorded history of Homo sapiens, and occasionally far longer than the entire recorded history of hominids.” Do we really want to cause an extinction that will take longer than our own evolutionary history to recover from?
This book is an engagingly-written paleontological mystery, neatly summarizing the latest information that we have regarding the end-Permian mass extinction. It is extensively and meticulously researched and is filled with citations and explanatory material that will tantalize the interested reader. Each chapter is carefully argued as it sifts through the available data in an even-handed way. I found the writing to be somewhat uneven in places but overall, it was engaging and generally accessible to the average reader, nicely reflecting the author’s wit at times and focused on the science rather than the drama surrounding the science. Further, this book does more than simply offering an account of the latest knowledge surrounding this mysterious mass extinction, it does a great job revealing how scientists think about complicated issues and the evidence that supports differing hypotheses.
Douglas H. Erwin is Senior Scientist and Curator in the Department of Paleobiology at the Smithsonian National Museum of Natural History and an External Faculty Member of the Santa Fe Institute. He began researching the end-Permian mass extinction in the early 1980s and has traveled many times to China, South Africa, and Europe seeking its causes.