93% of mammals were likely killed off with the dinosaurs

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93% of mammals were likely killed off with the dinosaurs

Way to go, asteroid.

A new study suggests that a whopping 93 percent of mammals died alongside the dinosaurs 66 million years ago, when a colossal asteroid struck Earth.

Contrary to the popular assumption that mammals fared way better than the dinosaurs during this cataclysmic event - which is supported by the fact that they're so prevalent in the fossil record shortly after the event - new research shows there's more to the story than we thought.

Researchers from the University of Bath in the UK analysed the fossil record 2 million years before the Cretaceous-Paleogene boundary - the point in time when a suspected asteroid collided with Earth, wiping out non-avian dinosaurs like T. rex and Triceratops, and pretty much everything else.

They then looked at the fossil record up to 300,000 years afterwards, to see how many mammals died out and how quickly they rebounded.

The evidence suggests that 93 percent of mammals died out after the asteroid struck, says the team, though those that did survive were able to adapt after the disaster, and as a result, managed to rebound quickly.

In fact, merely 300,000 years after the collision - an extremely short amount of time, evolutionarily speaking - the amount of mammal species had doubled, compared to how many were around before the asteroid.

The team’s findings help explain why we have this misconception that many mammals went unscathed by the asteroid. Instead, says the team, it was quite the opposite, because mammals were hit harder than other types of animals, such as lizards, turtles, and crocodiles.

The problem largely stems from the fact that mammals were so adaptable that they appeared - at first glance - to have never vanished.

"The fossil record is biased in favour of the species that survived. As bad as things looked before, including more data shows the extinction was more severe than previously believed," said team member Nick Longrich. "It wasn't low extinction rates, but the ability to recover and adapt in the aftermath that led the mammals to take over."

So how did the mammals pull off such an amazing comeback, while most other land-based creatures did not?

Well, the biggest advantage that mammals had over non-avian dinosaurs was size. Around 66 million years ago, mammals were extremely small, compared to today’s standards. In fact, the ones that survived the asteroid were likely the same size or smaller than an average house cat, the team reports.

Their small size meant that they could scavenge for food and hide from other predators. Basically, with a good chunk of the dinosaurs gone - or at least on their way out - the few mammals that survived were able to hide and hunt in a new world that they adapted to so fast, the fossil record makes it seem like they never left at all.

"Because mammals did so well after the extinction, we have tended to assume that it didn't hit them as hard," Longrich said. "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."

The team also reports that the surviving mammals adapted differently in different parts of the world, producing a variety of different species.

"You might expect to see the same few survivors all across the continent. But that's not what we found," said Longrich. "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 path," he added.

The new study comes at a good time, too, because researchers from Curtin University in Australia are currently digging inside the Chicxulub Impact Crater in the Gulf of Mexico - the site where the deadly asteroid fell 66 million years ago, releasing the same amount of energy as 100 million nuclear bombs in the process - to see how life evolved in the moments afterwards.

Hopefully, with new research shining more light on the subject, we'll one day understand how the asteroid strike led to the end of the dinosaurs, and how life across the globe rebounded, especially since researchers are still debating if the asteroid is singlehandedly to blame or not.

The study was published in the Journal of Evolutionary Biology.