Sixty-five million years ago, an asteroid hit the earth in what is now the Gulf of Mexico and the resulting environmental changes killed off the dinosaurs, opening the way for the subsequent evolution of mammals.
Or not. This scenario has been taught to schoolchildren for decades, but Gerta Keller and Thierry Addate, geoscientists at Princeton University and the University of Lausanne, Switzerland, have reanalyzed old data and collected new and they suggest that the asteroid hit 300,000 years before the mass extinction. Theirs isn't a paradigm-shifting suggestion because they don't reject the environmental impact theory, proposing that rather than an asteroid, the environmental change could have been due to the explosion of massive volcanoes and the resulting dust in the atmosphere blocking out the sun and so on. Some scientists have replied in defense of the asteroid theory. But, it is a further reminder that every scientist before yesterday was wrong, at least to some extent. Is the instant death idea so fixed that if it wasn't an asteroid we need another cloudy explanation?
Ah, but everyone has in mind the Disney animated view of a herd of Brontosauruses grazing, then looking up curiously at the flaming meteor and its ensuing dust-cloud obscuring the sun then grimacing in terror (cartoon reptiles can fear in their eyes), then staggering, and plopping down dead (with a few farewell quivers from the tips of their tails, accompanied by ominous strains from the bass section).
So perhaps you can pick your favorite dust cloud. Whether or not any global dust-cloud can cause such extinctions (leaving countless species of all sorts, including reptiles, alive) is one of the debates that have accompanied the dust-cloud theory. But there probably has been a bit too much uncritical acceptance of the one-hit-killed-all (except those it didn't) theory.
Melodramatic global smudges provide a parable for what happened, since we certainly all know that the T. rex really is gone (unless it went under water to get away from the dust, and became the Loch Ness moster!), but may not be an accurate reflection of the actual day-to-day facts at that time deep in the mists of history.