Dinosaurs were dying off tens of millions of years before an asteroid strike hastened their extinction, according to a new study published Monday in the Proceedings of the National Academy of Sciences.
The research challenges recent theories that dinosaurs were flourishing 66 million years ago, when a massive asteroid collided with an area in present-day Mexico, resulting in the Chicxulub crater.
About 50 million years before that cataclysmic event, however, dinosaurs entered a long period of decline, with species dying off faster than new ones emerged, according to researchers from the University of Reading and the University of Bristol, both in England.
"While the asteroid impact is still the prime candidate for the dinosaurs’ final disappearance, it is clear that they were already past their prime in an evolutionary sense," Manabu Sakamoto, lead paleontologist from Reading, said in a statement.
Scientists have long sought to settle a debate about the dinosaurs' vitality prior to obliteration.
Volcanic activity, the shifting of the continents and cooler global temperatures might have have been responsible for this earlier threat to the dinosaurs' survival, the researchers said.
The sauropods, a group of dinosaurs that included long-necked species like Apatosaurus, were dying out the quickest, researchers said -- though there was not an across-the-board decline. Triceratops, however, were among dinosaurs that thrived.
The researchers reached their findings by conducting a statistical analysis and examination of the fossil record from when dinosaurs appeared 231 million years ago to the time when they ceased to exist.
A study from two years ago concluded that the dinosaurs were at their most vulnerable to environmental change when the asteroid struck, in what is now the Yucatan peninsula -- and may have survived the asteroid if it had occurred a few million years sooner or later, according to the BBC.
The newer study conflicts with this 2014 research, which had found no evidence of long-term decline.
Stephen Brusatte, a paleontologist at the University of Edinburgh in Scotland who was not involved in the new study, told The Guardian that such a decline may be "very, very plausible," but that doesn’t mean dinosaurs were "wasting away" before their demise.
Rather, he said, they could have bounced back.
"It may be that the effects of the asteroid were a bit worse because you had dinosaurs that maybe weren’t as strong in an evolutionary sense as they once had been," Brusatte said. "But I think if there was no asteroid you would still have dinosaurs around today."