For some 30 years, scientists have debated what sealed the fate of the dinosaurs. Was an asteroid impact more or less solely responsible for the catastrophic mass extinction at the end of the Cretaceous geological period, 65 million years ago? Or were the dinosaurs already undergoing a long-term decline, and the asteroid was merely the coup de grâce?

So three young researchers, led by Stephen L. Brusatte, a graduate student at Columbia University who is affiliated with the American Museum of Natural History, decided to test this hypothesis with a close examination of the fossil record over the 12 million years leading up to the mass extinction.

For the study, the researchers departed from the practice of focusing almost exclusively on raw counts of the number of species over time. Instead, they analyzed changes in the anatomies and body plans of seven large groups of late Cretaceous dinosaurs for insights into their evolutionary trajectory.

Groups that show an increase in variability, for example, might have been evolving into more species, giving them an ecological edge. But decreasing variability might be a warning sign of approaching doom.