Dinosaur vs. mammal: 'Jaw-dropping' fossil reveals prehistoric battle 

Compared to the massive, magnificent dinosaurs of the Mesozoic era, ancient mammals are often thought of as little more than beady-eyed pipsqueaks scurrying around in the larger reptiles’ shadows. But now an exquisitely preserved fossil from northeastern China shows that sometimes, the mammals bit back.

The fossil consists of two, nearly complete fossilized skeletons that have been intertwined for approximately 125 million years. The larger skeleton belongs to a dog-size, plant-eating dinosaur known as Psittacosaurus. The smaller skeleton atop the dinosaur belongs to Repenomamus robustus—a meat-eating mammal that would have been about the size of a badger.

“The mammal is gripping the lower jaw of the dinosaur, and it’s kind of holding the dinosaur down and biting into its ribs,” explains Jordan Mallon, a paleobiologist at the Canadian Museum of Nature and co-author of the new study describing the find, published in the journal Scientific Reports.

“I remember when I first looked at it,” he says. “My jaw dropped and my eyes bulged out of my head, because it’s such a fantastic fossil.”

The mammal-vs-dino fossil hails from a rocky outcrop known as the Lujiatun Member of the Lower Cretaceous Yixian Formation—or what some scientists refer to as “Chinese Dinosaur Pompeii,” due to the heavy amount of volcanic activity in the area at the time. With ash build up everywhere and seasonal rains, it’s thought that mudslides were common—and extremely sudden.

“They sort of come out of nowhere,” says Mallon, who has experienced similarly slippery conditions during field work in Alberta, Canada. “These volcanic mudflows would habitually wipe out the animals living in the environment, including this new fossil we described.”

The dinosaur and mammal were instantaneously covered in ash and mud, preserving the details of their mortal struggle.

“I’ve never seen anything like it before,” says Stephen Brusatte, a University of Edinburgh vertebrate paleontologist and author of The Rise and Reign of the Mammals.

“Preserving any part of an animal for 125 million years is hard enough,” says Brusatte, who was not involved in the new study. “But capturing two animals locked in combat, it seems like a miracle.”

Mammal vs. dino: Anatomy of an attack 

While the experts assumed from the start that the fossil depicted a mammal attacking a dinosaur, they wanted to be sure.

“It looks for all the world like a predation event,” says Mallon. “But we didn’t want to take that for granted. We’re scientists, right?”

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The fossil could show a mammal scavenging at the carcass of a recently deceased dinosaur. To rule that scenario out, the experts point to three factors.

First, the Psittacosaurus bones showed no signs of scraping or gnawing. “Mammals today, when they’re scavenging a carcass, let’s say on the plains of the Serengeti, they’ll very often leave bite marks on the bones,” says Mallon. “And there’s just no signs of any bite marks on the bones.”

Second, Repenomamus is perched atop the dinosaur, as though it is trying to hold it down and finish it off. This positioning is much less likely for a scavenging mammal, reason the scientists.

“And lastly, the sort of clincher for me is the fact that the hind leg of the mammal is sort of trapped within the folded hind leg of the dinosaur, which could only have happened if the dinosaur had collapsed down onto the mammal,” says Mallon. “And that can’t happen unless the dinosaur is still alive.”

Brusatte agrees with the authors’ conclusion. “The two animals are intertwined. The meat-eater has its teeth and claws sunk into the plant-eater. I mean, these critters aren't giving each other a hug or dancing a waltz. What else could they be doing?” he says.

Rethinking Mesozoic mammals

While it may seem unusual that a six-pound mammal would attack a dinosaur around three times its size, it’s also not as unlikely as you might think.

“Modern-day carnivores can successfully hunt animals much larger than them by themselves,” says Nuria Melisa Morales Garcia, a vertebrate paleontologist at the University of Bristol in the United Kingdom.

For instance, weasels are known to take down prey up to 10 times their own size, and wolverines sometimes hunt animals as large as moose and caribou.

“We don’t really know if this individual of Repenomamus would’ve successfully killed the dinosaur if it weren’t from the volcanic flow that caused their deaths, or if it was common for Repenonamus to successfully hunt and kill adult Psittacosaurus,” says Morales Garcia, who was not affiliated with the new study. “But it is certainly convincing that these two animals were locked in a struggle.”

This is also not the first time these animals have shown up in a fossil together. A 2005 study published in Nature described a Repenomamus fossil with a juvenile Psittacosaurus in its belly.

“Our understanding of Mesozoic mammals has changed so much in the past few decades,” says Morales Garcia. “There’s already a large number of studies out there that have shown us that mammals were thriving during the Mesozoic. They were swimming, they were gliding, they were climbing—they were eating dinosaurs!”