Mammals began to flourish ten to twenty million years before dinosaurs became extinct about 66 million years ago, contradicting the view that mammals were suppressed by success of the giant animals, a new study has claimed.

The traditional view is that mammals were suppressed by the dinosaurs’ success, and that they did not really take off until after the dinosaurs went extinct, said David Grossnickle from University of Chicago in the US.

This study shows that therein mammals, the ancestors of most modern mammals, were already diversifying before the dinosaurs died out, said Grossnickle. The old hypothesis hinged upon the fact that many of the early mammal fossils that had been found were from small, insecteating animals there did not seem to be much in the way of diversity, researchers said. But over the years, more and more early mammals have been found, including some hoofed animal predecessors the size of dogs, they said. The animals‘ teeth were varied, too.

Grossnickle, along with Elis Newham at the University of Southampton in the UK analysed the molars of hundreds of early mammal specimens in museum fossil collections. 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, researchers said. Not only did mammals begin diversifying earlier than previously expected, but the mass extinction was not the perfect opportunity for mammal evolution that it has traditionally been painted as, they said. Early mammals were hit by a selective extinction at the same time the dinosaurs died out generalists that could live off of a wide variety of foods seemed more apt to survive, but many mammals with specialised diets went extinct.

Scientists were 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.