In 2012, Scott Richardson traveled to the site of the Tropic Shale, a geologic formation that stretches across Kane and Garfield counties and through a large swath of the Grand Staircase-Escalante National Monument. The shale is made up of mud deposited from an ancient sea (the Western Interior Seaway) active during the latter half Mesozoic era (251.9 to 66 million years ago), which is now called the “Age of Reptiles.” Richardson, a volunteer for numerous paleontology museums and organizations, had been fossil hunting in the Tropic Shale before—but that year, he found something special...