The Badass Beaver-Like Mammal That Outlived the Dinosaurs

When we think of the extinction event that occurred 66 million years ago, we tend to focus on the staggering death toll. Half of Earth's species were killed off by this cataclysmic disaster, including the non-avian dinosaurs.

But there were also some animals that positively thrived in the ashes of the old ecological world, eagerly occupying the niches that had been opened in the wake of the mass die-off. Among them was the newly discovered species Kimbetopsalis simmonsae, a beaver-like mammal that lived about 500,000 years after the notorious asteroid impact that killed the dinosaurs.

The remains of this intrepid mammalian survivor were found in the Nacimiento Formation of New Mexico, and are described in detail in a new study from the Zoological Journal of the Linnean Society.

Kimbetopsalis simmonsae

belonged to a larger family of early mammals called multituberculates, which is notable for being

among the most long-lived and successful

mammalian lineages in the fossil record. The specimen's well-preserved set of teeth reveal powerful molars clearly adapted to chewing on plants and leaves, a diet that proved to be a major asset for Kimbetosalis during this tumultuous period in evolutionary history.

"Though the area that we are collecting the specimen is now a high desert, during the early Paleocene it was a rainforest and near sea level," paleontologist Thomas Williamson, the lead author of the paper, told me.

"Kimbetopsalis lived among palm trees and dodged crocodilians," he explained. "As a rainforest, the area was highly productive with lots of fruit and seed-bearing trees. Flowering plants had risen to dominate tropical forests during the Late Cretaceous and multituberculates really increased in diversity at about that time, possibly due to the new food sources that these plants provided."

It's poignant to imagine this small rodent-like animal flourishing on abundant plant life while so many other species were edged out by the punishing post-impact conditions. Indeed, animals like Kimbetopsalis simmonsae are especially interesting for paleontologists, because they provide concrete models of evolutionary shakeups during global ecological disasters.

"I think the coolest thing here is what this new mammal tells us about the extinction at the end of the Cretaceous, and the subsequent recovery," study co-author and paleontologist Steve Brusatte added.

"This was one of the worst mass extinctions in Earth's history," he said. "An asteroid fell out of the sky one day and the world changed in an instant. Dinosaurs had been dominant for over 100 million years and they quickly disappeared. Mammals—which had been living in the shadows of the dinosaurs for many tens of millions of years—made it through the extinction and suddenly had the opportunity to diversify in the brave new world. And they did this quickly, very quickly."

In the case of Kimbetopsalis, diversification meant taking advantage of the delicious botanical yields of the early Paleocene. "There weren't any mammals like [Kimbetopsalis] that lived with the dinosaurs—maybe a handful of species that were beaver-sized and few, if any, that exclusively ate plants," Brusatte said. "So it looks like mammals rapidly began to get bigger, evolve new diets, and colonize new environments very soon after the dinosaurs bit the dust."

"That's a neat story," he added, "because that is the story of how we got here. That explosive diversification of mammals led to primates, which led to us."

To that point, studying animals like Kimbetopsalis not only contextualizes the aftermath of past extinction events, it can also help us mitigate the effects of future ecological disasters—such as the one we are entering now.

"The reason that I study fossils is that they are our only evidence for how evolution works over long time scales, and how real organisms and ecosystems have changed in concert with a planet that has always been changing," Brusatte said.

"So how have real animals and ecosystems responded to these mass extinctions in the past?" he continued. "That can give us insight into how our world may change in the future, and hopefully give us information that may help us mitigate things."

Kimbetopsalis simmonsae eventually joined the dinosaurs in extinction, and multituberculates as a whole died out about 35 million years ago. But the fossils these animals left behind provide a fascinating glimpse into their lives and world. Hopefully, they will even help us preserve ours.