Therian mammals, the ancestors of most modern mammals, began their massive diversification 10-20 million years before the extinction of dinosaurs, according to a new study.

The study, published in the Proceedings of the Royal Society B, questioned the familiar story that dinosaurs dominated their prehistoric environment, while ancient mammals took a backseat, until the dinosaurs went extinct 66 million years ago, allowing mammals to shine.

“The traditional view is that mammals were suppressed during the ‘age of the dinosaurs’ and underwent a rapid diversification immediately following the extinction of the dinosaurs,” said study co-author Elis Newham, a PhD student at the University of Southampton.

“However, our findings were that therian mammals were already diversifying considerably before the extinction event and the event also had a considerably negative impact on mammal diversity.”

The old hypothesis hinged upon the fact that many of the early mammal fossils that had been found were from small, insect-eating animals — there didn’t seem to be much in the way of diversity.

However, over the years, more and more early mammals have been found, including some hoofed animal predecessors the size of dogs. The animals’ teeth were varied too.

Newham and his colleague David Grossnickle, a PhD candidate at the University of Chicago, analyzed the molars of hundreds of early mammal specimens.

They found that the mammals that lived during the years leading up to the dinosaurs’ demise had widely varied tooth shapes, meaning that they had widely varied diets.

These different diets proved key to an unexpected finding regarding mammal species going extinct along with the dinosaurs.

Not only did mammals begin diversifying earlier than previously expected, but the mass extinction wasn’t the perfect opportunity for mammal evolution that it’s traditionally been painted as. Early mammals were hit by a selective extinction at the same time the dinosaurs died out.

The team was surprised to see that mammals were initially negatively impacted by the mass extinction event.

“I fully expected to see more diverse mammals immediately after the extinction,” Grossnickle said.

“I wasn’t expecting to see any sort of drop. It didn’t match the traditional view that after the extinction, mammals hit the ground running. It’s part of the reason why I went back to study it further – it seemed wrong.”

The reason behind the mammals’ pre-extinction diversification remains a mystery.

The scientists suggest a possible link between the rise of mammals and the rise of flowering plants, which diversified around the same time.

“We can’t know for sure, but flowering plants might have offered new seeds and fruits for the mammals. And, if the plants co-evolved with new insects to pollinate them, the insects could have also been a food source for early mammals,” Grossnickle said.

“The study is particularly relevant in light of the mass extinction the earth is currently undergoing.”

“The types of survivors that made it across the mass extinction 66 million years ago, mostly generalists, might be indicative of what will survive in the next hundred years, the next thousand.”

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Grossnickle D.M. & Newham E. Therian mammals experience an ecomorphological radiation during the Late Cretaceous and selective extinction at the K–Pg boundary. Proc. R. Soc. B, published online June 7, 2016; doi: 10.1098/rspb.2016.0256