A Double Whammy?

Dinosaurs have been extinct for 65 million years, but the controversy over what killed them goes on, having apparently developed a life of its own. Last week the Great Dinosaur Debate was in the news again, as scientists sparred over two of the newer theories about the prehistoric doomsday.

While both proposals acknowledge that the demise of the fabled beasts -- and of many other species -- was triggered by the impact of a huge comet or asteroid, they present different views of the blast's calamitous aftermath. One holds that clouds of sulfuric acid, formed from the debris thrown skyward by...