Your mental folder of dinosaur facts may need to be updated soon. Most people are taught at a young age that dinosaurs went extinct when a large asteroid struck Earth 66 million years ago, but according to new research, that may have been the last chapter of a 40-million-year-long story. As Discovery reports, a study published recently in the journal Proceedings of the National Academy of Sciences suggests that dinosaurs were already on the decline for tens of millions of years before the asteroid delivered the final blow.

The researchers from the University of Bristol and the University of Reading used a statistical approach and data from the fossil record to determine that as far back as 50 million years before the asteroid, species from the three major dinosaur groups (Ornithischia, Sauropodomorpha, and Theropoda) were going extinct faster than new ones were emerging. "We were not expecting this result," lead author Manabu Sakamoto said. "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."

The researchers told Discovery that there were other global factors contributing to the demise of the dinos, including changes in climate, rising seas, and the evolution of early mammals, who may have "outcompeted dinosaurs for resources, eaten their eggs, spread diseases or caused other problems for the once mighty dinos." While their numbers may have been dropping, University of Edinburg paleontologist Steve Brusatte believes that dinosaurs would have held on for some time had the asteroid not struck. "The asteroid hit at a time when dinosaurs had already been around for a long time, and had already endured their really prolific periods of evolution," he said. "But the way I see it, the dinosaur extinction still came down to the asteroid. No asteroid, no extinction."

[h/t Discovery]