On a rise with a sweeping view of the Indian Creek valley in southern Utah, skirts of red earth unfurling for kilometers in all directions, Adam Huttenlocker crouches to examine a knee-high nub of Cedar Mesa sandstone. Embedded in the rock is an ivory oval with a smoky center. The paleontologist, from the University of Southern California in Los Angeles, leans in for a closer look. Other researchers gather round, and soon they identify the mysterious eyelike fragment: It is a cross section of limb bone, probably from a synapsid—the group of reptiles that gave rise to mammals—that lived here more than 300 million years ago.