(CN) — New research challenges any notion that all dinosaurs must have been higher up in the food chain than smaller mammals that lived concurrently.
A study published in Science Reports Tuesday describes a team of Chinese and Canadian scientists' examination of an unusual fossil formation from the Yixian Formation in northeast China — a site sometimes referred to as "China’s Dinosaur Pompeii" due to its abundance of fossilized creatures suddenly buried under lahar flow, a mudslide of volcanic debris.
It is here, just west of the Lujiatun Village in China’s Liaoning Provence in 2012, where local farmers discovered the remains of a carnivorous mammal and a larger plant-eating dinosaur, which were examined by lead author Gang Han, a professor at the Hainan Vocational University of Science and Technology in China.
Co-author Jordan Mallon, a paleontologist from the Canadian Museum of Nature in Ontario, told Courthouse News that the fossils suggest the smaller, badger-like, carnivorous Repenomamus robustus mammal had been attacking a larger plant-eating dinosaur called a Psittacosaurus — about the size of a large dog — when a lahar flow swept them to their demise.
“These animals lived in a seasonal environment. There was colder seasons, warmer seasons, drier seasons, wetter seasons,” Mallon explained. “And it was probably during one of these wetter seasons — seasonal flows happened or rains or what have you — that reworked that ash and created these mud flows"
Mallon said such lahar flows still happen today in places like Indonesia, and can wipe out entire villages.
“The two animals are locked in mortal combat, intimately intertwined, and it’s among the first evidence to show actual predatory behavior by a mammal on a dinosaur,” Mallon said in a press release.
Dinosaurs of the Psittacosaurus species were among the earliest known horned dinosaurs that lived in Asia during the Early Cretaceous about 125 to 105 million years ago. Meanwhile, the mammal, though smaller than its contemporaries at the size of a house cat, was among the largest mammals of its time.
Researchers say the remains suggest the dinosaur died lying prone with its hindlimbs folded on either side of its body. The mammal, coiled slightly on top of the dinosaur’s left side, appears to have died while gripping it and biting two of its left anterior dorsal ribs.
While researchers are uncertain whether the dinosaur’s ribs broke in life or during taphonomic or fossilization processes, they say it’s unlikely that the mammal had been scavenging a dead dinosaur. Researchers point out that the dinosaur’s bones are devoid of tooth marks — a trademark of carnivorous scavengers — and that the two animals wouldn’t have become so entangled if not for a struggle. They also say the scavenging scenario does not account for the position of the mammal, which could have just as easily eaten the dinosaur from ground level.
“We propose instead that the two animals were buried in an act of predation on the part of the mammal, only for both to have been entombed by a sudden lahar-type volcanic debris flow,” the researchers wrote. “This hypothesis would explain the entwined nature of the skeletons, wherein the left hindfoot of the mammal became trapped within the folded leg of the dinosaur when it collapsed to the ground.”
It’s also worth noting that this evidence of the mammal’s predatory behavior is not unfounded, as prior paleontological findings revealed fossilized baby Psittacosaurus remains in the same mammal’s stomach before.
“We make no claims that we have found the first evidence of a mammal eating a dinosaur,” Mallon said, explaining how that discovery had been made in 2005 from the same fossil beds their specimens came from.
What’s unique about this fossil discovery, Mallon said, is that it reveals how small mammals attacked nearly full-grown dinosaurs, and not just baby dinosaurs.
“It's quite shocking when you look at the fossil,” Mallon said. “The dinosaur, we estimate, is probably about three times larger by body mass than the mammal. And that's not typical that you would see that usually animals will eat other animals that are maybe the same size or even smaller, but there is precedent for that in the animal kingdom today.”
The study cites modern examples of mammals hunting larger prey. For example, lone wolverines occasionally hunt larger animals like moose, caribou and domestic sheep, while weasels — the smallest carnivore in the world — sometimes prey upon capercaillie, hazelhen and hare. Another point of comparison is that on the African savanna, it’s common for wild dogs, jackals and hyenas to hunt prey that collapse, seemingly forfeiting themselves in shock.
“Our inherited knowledge has always been that it was the larger dinosaurs that ate the smaller mammals,” Mallon said. “But this new find sort of cements the idea that those interactions, those ecological interactions, went both ways and that the mammals were able to eat dinosaurs at least occasionally, and that dinosaurs had something to fear from the mammals in spite of their smaller body size.”
Another question begged by the discovery is whether the fossils are authentic — an issue previously found with specimens reported from the Jehol Group of China. This set, however, is likely real, researchers say, as forged remains often involve the juxtaposition of two or more independent fossils and fail to replicate the tangled nature of the skeletons here.
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