You've heard how an asteroid strike 66 million years ago wiped out the dinosaurs. But there's more to the story, according to a new study, which suggests non-avian dinosaurs weren't doing so hot before sulfates and dust filled the atmosphere—some 10 million years before, in fact. "The alternative scenario is that dinosaur diversity was ... lower just before the asteroid impact than millions of years before," Fabien Condamine, lead author of the study published in Nature Communications, tells New Scientist. Condamine and colleagues developed a computer model to assess species diversity and extinction rates, factoring in 1,600 fossils from 247 dinosaur species dated from 150 million to 66 million years ago. They determined species were going extinct faster than they were being replaced ahead of the asteroid impact.
The dinosaurs' "sudden" troubles began 76 million years ago, when "rates of extinction rose and in some cases, the rate of origin of new species dropped off," Condamine says in a release, per CNN. Co-author Mike Benton notes "the longer-lived dinosaur species were more liable to extinction, perhaps reflecting that they could not adapt to the new conditions on Earth." This coincided with a cooling of the global climate, says Benton. "Then, the loss of herbivores made the ecosystems unstable and prone to [an] extinction cascade." In this view, the asteroid impact was "a coup de grâce," Condamine tells New Scientist. However, some fault the study, which contradicts another that found dinosaurs would've thrived had the asteroid missed, for its basis on an incomplete fossil record. (There's a new theory on the asteroid itself.)