The relative role of impacts in all of the extinctions remains uncertain.  The end-Cretaceous is best-studied and seems to have an abrupt spike in extinction right at the impact layer.  On the other hand, the large late Eocene Chesapeake Bay impact seems to have had little effect on the faunas in Alabama and Florida (the nearest molluscan faunas with adequate documentation).  Along with an even bigger crater in Siberia, it came during a time of ongoing extinction related to global cooling and so has no clear effect of its own.

It's an ongoing issue for all the other major extinctions, with specific claims of impact-related evidence coming up regularly, but no evidence as strong as for the K/T.

Mollusks have survived through all the big extinctions, though some molluscan taxa have not, and so they make an excellent group for studing extinction and recovery.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
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That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa