Five times in the last half a billion years, tremendous, global-scale extinctions have wiped out a significant fraction of life on Earth - and each of them presents a grand puzzle. The most recent and the most familiar is the extinction that wiped out the dinosaurs - between the Cretaceous and Tertiary periods, about 65 million years ago. But before that, 205 million years ago, was the "End-Triassic Event" - it set the stage for the Jurassic Period, which saw the rise to prominence of the dinosaurs. Just what happened that killed off half the species on the planet, though,...