The published fossil record from western North America from two million years before the Cretaceous-Paleogene boundary until 300,000 years after the asteroid hit shows species diversity before and after this extinction event, allowing researchers to estimate the severity of the event and how quickly the mammals recovered. The extinction rates were much higher than previous estimates based on more limited data sets.
...over 93 percent (mammals) became extinct across the Cretaceous-Paleogene (K-Pg) boundary, due to the same asteroid that killed the dinosaurs in the Cretaceous period 66 million years ago.
Dr Longrich added, "Because mammals did so well after the extinction, we have tended to assume that it didn't hit them as hard. However our analysis shows that the mammals were hit harder than most groups of animals, such as lizards, turtles, crocodilians, but they proved to be far more adaptable in the aftermath.
"It wasn't low extinction rates, but the ability to recover and adapt in the aftermath that led the mammals to take over."
After this extinction event, there was an explosion of diversity, and it was driven by having different evolutionary experiments going on simultaneously in different locations.