Caught in the act: Mammal found with teeth sunk in a much larger dinosaur

A new fossil described this week captures two intertwined animals caught in a life-or-death struggle right before both were entombed in a volcanic event. Published in Scientific Reports this Tuesday, the fossil doesn’t capture one dinosaur attacking another—rather, the predator in this case is a smaller mammal known as Repenomamus robustus, and it died with its teeth clamped upon the herbivorous Psittacosaurus lujiatunensis, a dinosaur three times its size.

Gut contents from a Repenomamus fossil described in 2005 prove this same mammalian species ate very young and considerably smaller Psittacosaurus. But the remarkable fossil revealed today is the first evidence of any Cretaceous mammal attacking a larger dinosaur. It’s an astounding snapshot of ancient behavior, challenging previous assumptions of predator/prey dynamics millions of years ago.

A final struggle

These two species in the fossil couldn’t be more different. Psittacosaurus is a type of bipedal ceratopsian dinosaur—an early relative of dinosaurs such as Triceratops—with a large beak-like snout and spiky tail bristles. This was a herd animal, and it's the most commonly found fossil in the Lujiatun Member of the Lower Cretaceous Yixian Formation of China. This particular Psittacosaurus was approximately 6.5–10 years old when it died.

Repenomamus robustus was an early mammal approximately the size and weight of a Virginia opossum. Its left paw is seen grabbing the dislocated lower jaw of the dinosaur, and the two animal’s hindlimbs are tangled. Because Repenomamus died while biting the dinosaur, the authors couldn’t use its teeth to determine its age. However, they concluded that it was almost an adult based on its fused long bones. This voracious little mammal was a mere 1.42–3.43 kg (approximately three to seven lbs.), Yet its prey was 6–10.6 kg (13–23 lbs).

Two of Repenomamus’ ribs may be broken, but whether this occurred in its struggle with the dinosaur or through the fossilization process is unknown.

This extraordinary fossil was discovered in the aforementioned Lujiatun Member of the Lower Cretaceous Yixian Formation of China in 2012 by a local farmer. It’s an area known as the “Chinese Pompeii,” a nod to the Italian archaeological site, for its level of exquisite fossil flora and fauna that were preserved due to volcanic activity.

The authors propose that the dinosaur and mammal were quickly overcome by a lahar—a “volcanic debris flow, which occurs after an eruption, via hydraulic reworking of the deposited ash,” co-author Jordan Mallon explained in an email to Ars.

But wouldn’t such a debris flow not only impact the positioning of the animals but also transport them from where they died? At the very least, wouldn’t the animals have noticed something that large and dangerous racing toward them? Not according to a 2007 paper describing a cluster of young Psittacosauruses preserved in the same manner and from the same rock formation in China.

“These kinds of flows still happen today and can bury entire villages quite rapidly,” Mallon offered. “We know of other dinosaurs from the Lujiatun beds, including the sleeping Mei long, which indicates that such animals were not [awakened] by the debris flow(s). In the case of the fossil we describe, I would argue that the animals were otherwise [too] occupied to notice the incoming mudflow.”

Is this for real?

Mallon is a palaeobiologist with the Canadian Museum of Nature. He described his initial reaction to this fossil as “pure awe,” but then said he experienced “sober skepticism."

"I asked myself, ‘Can this be real?’” he said.

It’s a fair question, given some notable forgeries in paleontology. But it’s one that Mallon and his authors address in the paper.

Forgeries are often the combination of two separate species. Perhaps the most well-known forgery is “Archaeoraptor,” in which two or more fossil species, including bird and feathered theropods, were presented as one. Another is the Piltdown Man in the UK. Forgeries tend to be considerably less complex and tangled than the two species described in this paper, the team argues. The authors went further by exposing more of the mammal’s left dentary and revealed that it “plunges into the matrix to clasp the dinosaur’s ribs,” something that would be difficult to fake.

The fossil was donated in 2020 by lead author Gang Han of the Hainan Vocational University of Science and Technology to the Weihai Ziguang Shi Yan School Museum. Mallon explained that Han “was a key player in the development of the school museum and wanted them to have this as a feature fossil.” It is thanks to “Dr. Hans' intimate knowledge of the Lujiatun beds and the people who dig there and the additional preparation that we were able to do on the fossil [that] alleviated my concerns,” Mallon added.

Fion Waisum Ma is a vertebrate paleontologist at the Smithsonian’s National Museum of Natural History who was not involved in the research. “I appreciate the authors' effort in checking the fossil's authenticity by conducting additional preparation work,” she wrote in an email to Ars. “This is important, as fossils not directly excavated by professionals may have a risk of forgery.”

“This fossil reminds me of the iconic ‘Fighting Dinosaurs’ specimen from Mongolia,” she continued, referencing another extraordinary fossil. “When the preservation conditions are favorable, we can occasionally obtain snapshots of the prehistoric world.”

Going big

But is it truly a snapshot of predation, or could this be the last moments of a mammal scavenging a dead dinosaur? The authors point to important clues that favor the former, including a lack of any other bite marks on the body of the Psittacosaurus, the intimately intertwined position of the animals, and the location of the mammal on top of the dinosaur. If the mammal were scavenging, it could have done so from the ground.

Smaller mammals today are known to sometimes attack much larger prey, including wolverines attacking moose or caribou. The team conducted analyses to ensure the size of this dinosaur would not preclude a mammal the size of Repenomamus from being able to tackle it. And the prone position of the Psittacosaurus, the authors note, is similar to today’s prey that simply give up after being exhausted by their attempts to survive.

This aggressive predation from a mammal “comes as a bit of a shock” to Mallon. “Dinosaurs and mammals did not overlap in body size during the Mesozoic Era, so traditional knowledge holds that dinosaur-mammal interactions were unilateral, which is to say that the dinosaurs ate the mammals,” he said. “We never would have guessed that small mammals were capable of eating larger dinosaurs were it not for this unique fossil.”

Ma agrees. “We know that smaller animals sometimes prey on larger animals,” she said. “But what comes as a surprise is such behavior being captured as a fossil. Understanding the interaction between extinct animals is challenging, as most fossils do not preserve the real-life scenario. Exceptional fossils like this one are key to reconstructing the food chain of ancient ecosystems.”

Co-author Xiao-Chun Wu is a palaeobiologist at the Canadian Museum of Nature who collaborates with Han. “There have been specimens of carnivorous dinosaurs preying on plant-eating dinosaurs before,” he wrote to Ars, “but there has never been an example of mammals preying on dinosaurs.” Although gut contents prove mammals ate dinosaurs in the Cretaceous, he feels “it is more important and meaningful to discover a concrete [specimen] of this mammal preying on the dinosaurs.”

Scientific Reports, 2023. DOI: 10.1038/s41598-023-37545-8