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Question for someone who may know:

Obviously "mass extinction" does not mean "total extinction of all life", or else we wouldn't be here. Do we have any idea to what degree the bio-diversity of the planet was reduced in prior occurrences?

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From Wikipedia[1], "In the past 540 million years there have been five major events when over 50% of animal species died." So if your measure for bio-diversity is the number of unique animal species, it dropped by half at each mass extinction event. The events only reflect a drop in the diversity of animal species as this is an "easily observed, biologically complex component of the biosphere", so the overall change in bio-diversity could be lesser or greater.

1: http://en.wikipedia.org/wiki/Extinction_event

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