On a high ridge in Arizona’s Petrified Forest National Park, paleontologist Paul Olsen sits on the fallen trunk of a 215-million-year-old tree, now turned to stone. The tree once loomed 70 or 80 feet above a riverine landscape teeming with fish, turtles, giant crocodilians and tiny, early species of dinosaurs. From here, Olsen can survey the remnants of this lost world: miles and miles of surreal badlands, where sediments built up over millions of years have eroded back down to expose endless cross sections of brightly colored rocks. The layers represent tectonic movements, natural climate cycles, the growth and disappearance of lakes, buildups of river deltas. The petrified trees scattered across the landscape are only the most obvious fossils; others are bleeding out by the ton. It is perhaps the world’s richest trove of rocks from the late Triassic, when dinosaurs, and early mammals, got their evolutionary start. The Triassic was also a hothouse world: a time of high atmospheric carbon dioxide, rapid climate shifts, and fast-moving extinctions. Olsen thinks there may be much to learn from it for our own time.