More Evidence That Dinosaurs Suffered a One-Two Extinction Punch

What killed the dinosaurs? That was a mystery for decades; when I was a kid, there were tons of ideas but precious little evidence for any of them, making them little more than speculation. In the late 1970s and early '80s, though, the hypothesis was put forward that a giant asteroid or comet impact did the deed, and over the years evidence mounted. The impact idea gained wide acceptance, but some details remained stubbornly difficult to explain with a single catastrophic event. Another idea tha...