One theory that has lost a lot of ground in recent times is that the dinosaurs and other Cretaceous life-forms were wiped out by a massive dose of radiation from a nearby supernova.
Perhaps the weakest point of this theory is simply that it kills the wrong creatures. Experiments show that simple organisms like plankton can typically withstand radiation doses about a ten times higher than what can be tolerated by small mammals such as mice, so why would the primitive mammals have survived when most species of plankton were wiped out? When you consider that most plankton would have in any case have been protected from much of the radiation by surface water, it seems even less likely that such an event, if powerful enough to wipe out most plankton species, could have resulted in any mammals surviving.
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