Despite years of intensive research about the extinction of non-avian dinosaurs about 65.5 million years ago, a fundamental question remains: were dinosaurs already undergoing a long-term decline before an asteroid（小行星） hit at the end of the Cretaceous? A study led by scientists at the American Museum of Natural History gives a multifaceted（多层面的） answer. The findings, published online May 1 in Nature Communications, suggest that in general, large-bodied, "bulk-feeding" herbivores were declining during the last 12 million years of the Cretaceous. But carnivorous（食肉的） dinosaurs and mid-sized herbivores（食草动物） were not. In some cases, geographic location might have been a factor in the animals' biological success.

"Few issues in the history of paleontology（古生物学） have fueled as much research and popular fascination as the extinction of non-avian dinosaurs," said lead author Steve Brusatte, a Columbia University graduate student affiliated with the Museum's Division of Paleontology. "Did sudden volcanic eruptions or an asteroid impact strike down dinosaurs during their prime? We found that it was probably much more complex than that, and maybe not the sudden catastrophe that is often portrayed."

The research team, which includes Brusatte; Mark Norell, chair of the Museum's Division of Paleontology; and scientists Richard Butler of Ludwig Maximilian University of Munich and Albert Prieto-M‡rquez from the Bavarian State Collection for Palaeontology, both in Germany, is the first to look at dinosaur extinction based on "morphological disparity"-the variability of body structure within particular groups of dinosaurs. Previous research was based almost exclusively on estimates of changes in the number of dinosaur species over time. However, it can be very difficult to do this accurately.

"By looking just at trends in taxonomic diversity, you get conflicting answers about the state of dinosaurs prior to extinction," Brusatte said. "This is because the results can be biased by uneven sampling of the fossil record. In places where more rock and fossils were formed, like in America's Great Plains, you'll find more species. We wanted to go beyond a simple species count for this study."

By looking at the change in biodiversity within a given dinosaur group over time, researchers can create a rough snapshot of the animals' overall well-being. This is because groups that show an increase in variability might have been evolving into more species, giving them an ecological edge. On the other hand, decreasing variability might be a warning sign of extinction in the long term.