The extinction event at the end of the Permian period is considered the largest mass extinction of all time. While the one that ended the dinosaurs, among other creatures, killed roughly half the species at that time, the Permian extinction is estimated to have wiped out 90% of the total living species at the time. While there's some agreement that a meteorite impact helped end the dinosaurs, the question of what caused the Permian event is less well-known. By me, anyway, which is why I bought this.
Benton doesn't get to his actual theory of what caused it until the second-to-last chapter. Because first he wants to talk about fossils, about paleontologists trying to figure out the relative ages of rocks in different parts of the world, about the argument over whether "mass extinctions" were even a thing that happened. That delves into catastrophism versus uniformitariansim, and Charles Lyell again, who we discussed in Raup's The Nemesis Affair. Benton also gets into the iridium layer and the Cretaceous extinction, since that whole thing was what helped get catastrophism back on its feet as a more widely accepted possibility.
If the popular concept of the change in life over time is that it is gradual, that species die out one or two at a time because "better" species come along and shove them into oblivion (and Lyell didn't even believe that, he thought nothing truly went extinct, they just went away somewhere for awhile and would reemerge later), then the idea of mass extinctions, some huge disaster that wipes out dozens or hundreds of species at once is absurd. So, if it looks like a bunch of species just vanish abruptly from the rocks, well, that's actually probably the equivalent of dozens of millions of years you're looking at, not a few thousand. Or you just have an incomplete fossil record (not unlikely, since lots of organisms don't die in places that lend to their remains being fossilized.)
Which does raise an interesting point Benton touches on at times: How quickly does it have to happen to be considered a single mass extinction? The notion in the book seems to be that the Permo-Triassic mass extinction took place in maybe 800,000 years, maybe a bit less. From a human perspective, that's a long time, but from the perspective of the length of time all life has been on earth, it's an eyeblink. Even relative to the average length of time a single species might persist (which I think is ~2 million years), it's not that long. There are certainly species that went extinct due to slow changes in climate or the introduction of new competitors or predators. But maybe uniformitarianism versus catastrophism is a matter of how large the time scale you're looking at is.
For the most part, Benton's an engaging writer. Some of the discussion of rock layers or mapping differences in oxygen isotopes is dry, but he's able to bring it around to explain why that's important. If you can't decide where the Permian ends and the Triassic begins, how can you decide whether a mass extinction happened then?
'The point is that good fortune is a characteristic of mass extinctions. The survivors are more lucky than specially adapted. The most advanced, intelligent, fast-breeding animal species may be wiped out by the chance calamity of an extinction event when it is obliged to face challenges that have never been encountered before.'
No comments:
Post a Comment