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The documented presence of two large (???100-km diameter), possibly coeval impact craters of late Eocene age, requires modification of the impact-kill curve proposed by David M. Raup. Though the estimated meteorite size for each crater alone is large enough to have produced considerable global environmental stress, no horizons of mass mortality or pulsed extinction are known to be associated with either crater or their ejecta deposits. Thus, either there is no fixed relationship between extinction magnitude and crater diameter, or a meteorite that would produce a crater of >100-km diameter is required to raise extinction rates significantly above a ???5% background level. Both impacts took place ???1-2 m.y. before the "Terminal Eocene Event"( =early Oligocene pulsed extinction). Their collective long-term environmental effects, however, may have either delayed that extinction pulse or produced threshold conditions necessary for it to take place.
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Roadblocks on the kill curve: Testing the Raup hypothesis