Charting Earth's (Many) Mass Extinctions

How many mass extinctions has the Earth had, really? Most people talk today as if it’s five, but where one draws the line determines everything, and some say over twenty.

However many it might be, new mass extinctions seem to reveal themselves with shocking frequency. Just last year researchers argued for another giant die-off just preceding the Earth’s worst, the brutal end-Permian.

Now, one more has come into focus as stratigraphy improves. With that, four of the most awful things that life has ever suffered all came down (in many cases, literally, as volcanic ash) within 60 million years. Not a great span of time with which to spin the wheel, in case your time machine is imprecise…

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