By now, most people are familiar with the theory that an asteroid that smacked into our planet 66 million years ago caused the extinction of the dinosaurs. But it turns out that dinosaurs might not have been the only casualty of that cataclysmic event. New analysis of the fossil record indicates that a full 93 percent of mammals living at the time also went extinct, a number significantly higher than previously thought.

Researchers at the Milner Centre for Evolution at the University of Bath looked back over mammalian fossil reports and research that had been published for about the last 100 years, specifically focusing on the time frame on both sides of the meteor strike – about 68 million years ago to 65.7 million years ago. They also concentrated on mammals in North America.

Not only did they find that more mammals went extinct during this time than the roughly 75 percent previously thought, but also that they rebounded extremely quickly, with the number of species doubling those found before the impact in just 300,000 years – a relatively short time in evolutionary terms.

"Because mammals did so well after the extinction, we have tended to assume that it didn't hit them as hard," says Nick Longrich from the Milner Center. "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." Longrich is the lead author on a paper regarding the find published in the Journal of Evolutionary Biology.

Longrich and his team also discovered a high degree of separation in terms of the way species developed geographically.

"You might expect to see the same few survivors all across the continent. But that's not what we found," he says. "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. This may have helped drive the recovery. With so many different species evolving in different directions in different parts of the world, evolution was more likely to stumble across new evolutionary paths."

For example, Longrich told Gizmag that there was a mammal in Wyoming called the Eoconodon, which was considered giant at the time, being about the size of a cat. But that same species wasn't found in nearby Montana. "It's almost like you're looking at separate islands, but there are no physical barriers between these localities," he says.

With 100 years of fossil data lying around, it seemed surprising to us that no other researchers had arrived at the same conclusion as Longrich and his team before now. So we asked him about that, and it turns out there's been a bit of a mammal-dino-asteroid debate raging in academia.

"It may just be a matter of perspective," he told us. "Historically the asteroid impact hypothesis has been really controversial – and the paleontologists working on fossil mammals have been bitter opponents of the asteroid-impact hypothesis. So they've probably tended to overlook evidence that supports it, and haven't really been interested in looking at the issue carefully. We don't specialize on mammals, so we're able to look at the evidence in a different way – and in fact, the fossil mammal data strongly support the asteroid hypothesis; it's some of the best data out there in support of it, in fact."