I've often wondered if the Cretaceous Tertiary boundary might have been marked by more than one impact. It always seemed unlikely to me that the Chicxulub impact event could have done in the entire planet but there is evidence on the moon that craters are sometimes caused by multiple impact events in the from of strings of craters and Shoemaker-Levy gave us some evidence of a comet breaking up after a close encounter and coming back to cause a multiple impact event. Such an event, even if it was several small impactors, would be devastating to the planets ecology, far more so than a large single impact event.
I know, just me being a smart ass but in the pseudoscience forum it might be cool.