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.

In Arizona’s Petrified Forest National Park, researchers are scouring the fossil-rich surface and drilling deep into ancient rocks to learn what happened during the late Triassic, some 201 million to 235 million years ago.