Rock layers in northeastern Montana. Arrow points to a coal bed that marks the main extinction level. Above this level, no dinosaurs survived. This image relates to a paper that appeared in the Feb. 8, 2013, issue of Science, published by AAAS. The paper, by Paul R. Renne at the Berkeley Geochronology Center in Berkeley, CA, and colleagues was titled, "Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary."