The specific extinction being discussed in the end Permian version, the largest mass extinction event we're aware of in the planet's history. Erwin takes a methodical approach, breaking down the scale of the extinction based on fossil evidence. From there, he spends a chapter laying out the major hypotheses, ranging from meteorite impact to volcanism, to anoxic oceans, to sea level drop and several others besides. In an approach I really appreciated, he discusses what kinds of evidence scientists would need to find to support each hypothesis.
After that, there's chapters spent on what's actually been discovered. Some of this is more interesting than others. The part about their managing to get a fairly brief timespan for the extinction - brief in geologic time, we're still talking maybe a half-million years - was kind of cool. So were the parts about the difficulty in doing comparative ages between marine and land rock formations, because you may not be able to age them by radioactive decay, and their fossil records rarely overlap. However, Chapter 7 focuses on the levels of various minerals in the oceans around the end-Permian, and when Erwin got deep into carbonate levels and what the different levels tell them about where the carbonate was at a given time, I was struggling not to fall asleep. I didn't really retain any of that chapter.
Still, the evidence laid out - and this is what they had available when the book was published in 2006 - Erwin returns to the hypotheses described earlier and goes through them, discussing which ones lack evidence, or which ones appear outright refuted by the evidence. For example, the oceans apparently rose during the Permian mass extinction, which kind of chucks the notion sea level drop caused the problem out the window.
He also acknowledges the spots where they just don't have enough evidence, or are working with too small a data set. So far, no one had found an iridium layer in the rocks like the one that marks the meteor that hit near the end of the dinosaurs. Or any of the shocked quartz or other geologic markers they think distinguish such impacts. But Erwin points out that those may just have been unique to that particular meteor, that if a meteor did hit near the end of the Permian (and there are some possibilities, but no definite impact site as of 2006), there's no reason to assume it was also high in iridium.
(He does, if you're wondering, offer his own perspective on what he think caused the extinction. It's not quite what he calls his "Murder on the Orient Express" hypothesis, but it's not a simple, one cause thing, either.)
For the most part, Erwin tries to keep things from getting too technical. He includes graphs and charts to highlight things like the extent of the basalt floods that appear to be evidence for massive volcanism (albeit the cause is unclear), so the book only sometimes falls into a wall of text. He describes the part he's taken in the research, and the times he was wrong (he apparently jumped with both feet onto the "sea level drop" bandwagon when it was first proposed.) I don't think he's slanted towards one hypothesis or another in his presentation, but I don't know all the data, so I don't know what he's not saying. He mentions some of his colleagues may feel he's giving their sides short shrift, but it at least feels like he's laying the data out for the reader, but also discussing what it doesn't say for certain. There may be a huge drop in sedimentation for a brief period, but the cause of it could be many things.
'This may seem paradoxical, but the destruction of brachipods or ammonoids was so complete that it makes it difficult to say anything useful about the actual cause of the extinction. The essence of solving the problem of differential extinction is being able to compare similar winners and losers. A clade where all species survived is not particularly edifying; nor is a group that almost completely disappeared.'
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