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News Item: Fifth Worst Mass Extinction Linked to Asteroid Impact



http://www.space.com/scienceastronomy/solarsystem/tj_extinction_010511-1.html

Fifth Worst Mass
                                       Extinction Linked to
                                       Asteroid Impact

They call them the Big Five -- a handful of unfathomable mass
                  extinctions over the past 500 million years, each
estimated to
                  have obliterated somewhere between 50 and 96 percent
of all
                  species on the planet.

                  That much we know, because Earth recorded the mass
deaths
                  in layers of ancient soil, where crowds of miniscule
corpses
                  and other evidence show wholesale destruction of the
smallest
                  critters, on which larger animals depend.

                  What we don't know, except in one case, is what caused
these
                  five mass extinctions. Nor is there solid evidence
showing how
                  rapidly the catastrophes occurred.

The balance is at the URL.

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