The Miocene mammal Necrolestes patagonensis ventures out of its burrow 16 million years ago in Patagonia, present-day Argentina. Necrolestes is now recognized as a member of a group long thought to have become extinct shortly after the extinction of the large dinosaurs at the end of the Cretaceous period. Credit: Reconstruction by Jorge Gonzalez, copyright Guillermo W. Rougier for PNAS use as needed