According to a new study, this artist rendering depicts four dinosaurs and dinosaur precursors from the Hayden Quarry of New Mexico. Dinosaurs arose in the Late Triassic, between 235 million and 200 million years ago, and came to dominate the planet in the Jurassic, 200 million to 120 million years ago. Scientists had thought the dinosaurs rapidly replaced their ancestor species. Indeed, until 2003, when a creature called Silesaurus was discovered in Poland, no dinosaur precursors had been found from the Late Triassic.Now, researchers report in the journal Science they have evidence from northern