The extinct turtle was about 2 feet long from head to tail. Its streamlined shell was adapted for living in a riverine environment. When it was alive, 76 million years ago during the Cretaceous Period, Southern Utah looked more like present-day Louisiana. The climate was wet and hot, and the landscape was dominated by rivers, bayous and lowland flood plains. It lived alongside tyrannosaurs, armored ankylosaurs, giant duck-billed dinosaurs such as Gryposaurus and Parasaurolophus, and other dinosaurs that left abundant fossil remains in the Upper Cretaceous Kaiparowits Formation of Southern Utah. But those fossil beds also hold the remains of...