This is the Permian/Triassic extinction, the one which separated the time of domination of mammal-like reptiles from the time of the dinosaurs, I guess. Isn’t there a really ‘gradualist’ theory that tries to blame the extinction on the existence of the Pangaea supercontinent, and the likely extreme continental climate? That one is really hard to credit.
There’s a real impact-denial (’: movement among some dimwits in the UK and various (mostly east coast) US camp-followers. Also, beginning with Dewey McLean, quite a number of nimrods have saddled on the lowly volcano as a vehicle for mass extinctions — that is, now that they’ve finally accepted the idea that there were mass extinctions. :’)