The dinosaurs came, ruled, and went—but so too did many mammals. Researchers announced last Monday one such (previously unknown) species from the northwestern highlands of New Mexico. Kimbetopsalis simmonsae was a small, post-apocalyptic, leaf-eating, beaver-resembling, 100-kg creature whose name literally means “Simmon’s cutting shears of Kimbeto Wash,” after the Navajo name of the dig site, the creature’s dietary habits, and Nancy Simmons, a paleontologist who worked on similar species.

To give us a picture of what this land looked like a few hundred thousand years after the asteroid impact that killed off the dinosaurs, The Daily Beast talked to Thomas Williamson, curator of Paleontology at the New Mexico Museum of Natural History & Science and co-author of the paper published Monday in the Zoological Journal of the Linnean Society. “When this animal was alive, New Mexico was a very tropical place,” Williamson says. “It was basically covered with rainforest—things like alligators and palm trees. Very, very tropical.”

After the asteroid impact 66 million years ago, all non-bird dinosaurs went extinct. A few mammals survived across this cataclysm, and they went absolutely rampant. They increased in diversity and evolved very quickly in the following few million years.

This beaver-like creature was not a placental mammal (like humans or dogs), nor was it a marsupial (like a kangaroo or opossum), nor was it a monotreme (like an egg-laying platypus or echidna). Rather, it was part of a now extinct order of mammals called the “multituberculates.” While the most striking differences between orders of mammals is their life cycles, paleontologists can differentiate these mammalian orders just from looking at their teeth. “They get their name because their teeth have these tubercles,” Williamson says.

Multituberculates go farther back than either the placentals or the marsupials. The common ancestral species of all the mammals living today, which was probably a species resembling a Dimetrodon, lived in the late Triassic about 230 million years ago. “Some evidence puts the split of multituberculates soon after that origin,” Williamson explains. “Multituberculates were really important through the rest of the Mesozoic,” which ended with the asteroid impact.

“[Multituberculates] were one of the most successful mammalian groups during the late Cretaceous [before the asteroid impact]”, Williamson says. Throughout the Cretaceous of Western North America, there were mostly multituberculates and marsupials.

As for placental mammals (or rather, their common ancestor), they were less important. When the asteroid hit, it wiped out almost all the North American dinosaurs and marsupials. “After the dinosaurs went extinct they actually increased in diversity,” says Williamson. “They were one of the first groups of mammals to take advantage of this ‘new world’ that had been opened up to them.”

Not much is known about the life cycle of multituberculates. “It’s one of those things we’d really like to know, but just really don’t know anything about,” Williamson laments. “We don’t think multituberculates laid eggs, because in the very few skeletons we have, it looks like the birth canal wouldn’t have been large enough.”

Teeth tell researchers most of what they know about Kimbetopsalis simmonsae. “Superficially, they look a lot like beavers because they have this big chompers in the front,” Williamson explains. Of course, beavers are placental mammals so they aren’t at all related. “This is one of the first groups of mammals that eats primarily vegetation. During the Cretaceous most mammals were eating insects.”

Not much else is known about the habits of the species. “Mostly what we find are teeth and jaws and parts of the skull,” Williamson says. “We don’t really know much about the skeleton. What we do know suggests they might have been digging burrows. Digging mammals have exaggerated muscles in their forelimbs, and their close relatives had these.”

Ripe for the taking as Western North American was, it was not without its dangers. Kimbetopsalis simmonsae spent time avoiding alligators and larger placental predators resembling later species of bear-dog, which at this point “approached the size of a grey wolves,” Williamson says.

Yet, it wasn’t predators that wiped all the multituberculates out. “They began to drop in diversity and dwindle away and they all went extinct about 35 million years ago,” Williamson explains. “A likely possibility is they couldn’t compete with [placental] rodents.”

According to Williamson these findings came out of a “big, NSF-funded, collaborative effort” to understand “how these ecologies recover from a devastating mass extinction, and how things evolved in a time like this. It’s amazing how fast things evolve,” he says. “We can see it right there in the rock record.”

People have been collecting fossils from that area for over 130 years and are still finding new things. The first Kimbetopsalis simmonsae fossil was actually discovered by an undergraduate. “I work on dinosaurs too, but the nice thing about mammals is we can identify them by their teeth. You can’t do that very easily with dinosaurs,” Williamson says. “We knew that this was new the moment that the student brought it to us, [saying] ‘I’ve never seen this before—this is new!’ That’s very satisfying.”