THE JURASSIC PERIOD marks the beginning of the Age of Dinosaurs proper. Yes, the first true dinosaurs entered the scene at least 30 million years before the Jurassic began. But as we’ve seen, these earlier Triassic dinosaurs had not even a remote claim to being dominant. Then Pangea began to split, and the dinosaurs emerged from the ashes and found themselves with a new, much emptier world, which they proceeded to conquer.