Dinosaur diversity before the asteroid.

Few phenomena in Earth's history have been as widely popularized as the mass extinction of non-avian dinosaurs. Yet, the disappearance of such awe-inspiring animals has roused more than public fascination—it has spurred enduring scientific debate. Paleontologists largely agree that an asteroid dealt the final blow to non-avian dinosaurs, but their susceptibility to annihilation when the asteroid plummeted to Earth remains an open question. On page 400 of this issue, Flynn et al. (1) reveal that a well-known rock layer in northwestern New Mexico captures a rare glimpse into the last days of the dinosaurs. Their findings markedly increase the number of dinosaur species known to have roamed Earth ~400,000 years before the asteroid's impact, challenging the idea that weakened ecosystems played a role in their demise. [ABSTRACT FROM AUTHOR]