Huge Asteroid Is Still the Central Villain in Dinosaurs’ Extinction

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.

In science, alas, not all projects fulfill researchers’ ambitions. The findings of this one were mixed and generally inconclusive, Mr. Brusatte’s team reported in an article published online last week by the journal Nature Communications. At best, striking a positive note, the team wrote that the “calculations paint a more nuanced picture of the last 12 million years of dinosaur history.”

As Mr. Brusatte explained, the late Cretaceous “wasn’t a static ‘lost world’ that was violently interrupted by an asteroid impact.” Some dinosaurs, he noted, “were undergoing dramatic changes during this time, and large herbivores seem to have been mired in a long-term decline, at least in North America.”

The findings showed that duck-billed hadrosaurs and horned ceratopsids, two groups of large-bodied, bulk-feeding plant eaters (meaning they ate just about anything and everything) might have declined in diversity at this time. In contrast, small herbivores like the ankylosaurs and pachycephalosaurs, and the meat-eating tyrannosaurs and coelurosaurs, appeared to be holding steady or perhaps increasing in diversity. So, too, were the enormous herbivorous sauropods like apatosaurs.

The results were not uniform on different continents. While hadrosaurs declined in North America, their diversity seemed to have been increasing in parts of Asia. The fossil record in many regions was insufficient for reliable analysis, meaning that the extinction debate will continue.

Besides Mr. Brusatte, the other authors were Richard J. Butler of the University of Munich, Albert Prieto-Márquez of the Bavarian State Collection for Paleontology and Geology in Munich, and Mark A. Norell, an American Museum paleontologist who is Mr. Brusatte’s doctoral adviser.

Dr. Norell said the study of skeletal changes in groups of species over time was “a novel way” to assess their prospects for survival over the long haul. “It would be nice to have more fossils to see how much these results are real,” he said.

Paul C. Sereno, a paleontologist at the University of Chicago who had no part in the study, agreed that such investigations of life at the end of the Cretaceous had been “limited by the coarseness of the data where you really need it.” He questioned whether the research technique, though useful in studying simpler invertebrates, could be applied successfully to dinosaurs.

“It’s an interesting study, and they are quality researchers,” Dr. Sereno said, “but I don’t think it changes the picture over all: Extinctions aren’t a simple process, but ultimately the asteroid was the major factor at the end of the Cretaceous.”

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