The earth guards its secrets. Each of the Big Five extinctions, when examined in detail, turns out to be complicated and different. Take, for example, the late Devonian extinction. The iridium there, not high to begin with, appears to be strongly associated with the remains of the bacterium Frutexites. Some experts believe that Frutex-ites was able to extract and concentrate iridium from seawater, indicating, they say, that iridium is not a reliable marker of impact after all. Pro-impactors respond that Frutexites was able to concentrate iridium just because an impact inserted excess iridium into the oceans in the first place.
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