An asteroid impact may have signaled the death knell for the non-avian dinosaurs, but according to a new study published in Proceedings of the National Academy of Sciences, they were already in a spiral of evolutionary decline long before the arrival of the 66-million-year-old space-borne apocalypse.

Researchers from the Universities of Reading and Bristol used a cutting-edge statistical model to show that dinosaurian species were already going extinct faster than new ones were emerging up to 50 million years before the asteroid arrived. This suggests that the asteroid impact, far from being the only reason for their demise, was more akin to the final straw for the iconic creatures after they had ruled the world for 150 million years.

At the time, the epic fragmentation of the world’s continents Laurasia and Gondwana began to isolate dinosaur populations from each other, the sea level was changing, and prolonged volcanism was profoundly altering the climate. However, none of these adequately explain the observed decline, hinting that another factor – possibly the rise of mammals – was to blame.

“Other studies keep pushing the rise of mammals further back in time,” co-author Chris Venditti, a lecturer in evolutionary biology at the University of Reading, told IFLScience. “Perhaps the early mammals were causing havoc to the dinosaurs, eating their eggs, encroaching on territory, and limiting their capacity to thrive.”

^{The speciation rate of dinosaurs, from their initial rise to their demise. The Sauropodomorpha (blue), Theropods (red) and Ornithischia (green) all experienced a dramatic mid-Early Cretaceous decline. The nested graph shows that the Hadrosauriforms (light green) and the Ceratopsians (light blue) were thriving, against all other trends, towards the end of the Cretaceous. Sakamoto et al./PNAS}

Ever since the 180-kilometer-wide (112-mile-wide), 20-kilometer-deep (12-mile-deep) Chicxulub crater was discovered in the Gulf of Mexico in the late 1970s, a debate within the paleontological community has raged: Was it just the asteroid that killed the dinosaurs, or were there other antagonists? This study definitely suggests that other factors were already at play, even if it cannot be certain what they were.

Previous research has suggested that global ecosystems during the Cretaceous period had been weakened by a major loss of herbivore biodiversity. With a lack of prey to eat, the carnivorous predators began to die out as a result, which is when the asteroid hit.

This previous study actually suggested that if the asteroid impacted a few million years earlier, the dinosaurs may not have gone extinct. In this sense, the appearance of the asteroid was a colossal stroke of bad luck.

Although the new study generally agrees that the dinosaurian extinction was a product of a series of misfortunes, it disagrees that certain herbivores also saw a decline. In fact, the duck-billed Hadrosauriforms and the horned Ceratopsidae were two notable plant-eating groups to have been diversifying at the time of the apocalypse.

“They possess a jaw adaptation that allows them to process food very efficiently, and I think that gave them the edge,” Manabu Sakamoto, a paleontologist from the University of Reading and lead author of the study, told IFLScience.

The end-Cretaceous mass extinction, although not the worst the Earth has experienced, was terrible by any measure. It’s generally agreed that huge, continental-scale volcanism conspired with the cataclysmic asteroid impact to darken the sky within a few years.

This so-called “impact winter” rapidly turned the Earth into a cold world, and with only limited photosynthesis happening, food chains began to collapse. Up to 75 percent of all life perished; this included up to a staggering 95 percent of marine life, sustained only by a trickle of edible algae down to the sheltered aquatic depths.