In the early 1980s, paleontologists Jeff Eaton and Rich Cifelli started digging for fossils in one of the most inaccessible regions of the United States: the Kaiparowits Plateau of southern Utah. They were looking not for dinosaurs, but for ancestral mammals. Mammals almost litter the fossil record after dinosaurs became extinct 65 million years ago, but they were rare before then. Eaton and Cifelli ventured onto the Kaiparowits to comb its rocks for mammals’ tiny teeth and bones.