Rare Fossil May Show a Small Mammal Attacking a Dinosaur

The well-preserved bones could upend the traditional view of dinosaurs as predators and mammals as prey, if the findings are confirmed

fossil of a dinosaur and mammal
The dinosaur and mammal engaged in a final tussle. Gang Han via the Canadian Museum of Nature

A remarkably well-preserved fossil discovered in China suggests that millions of years ago, some mammals actively hunted dinosaurs that were several times their size.

The 125-million-year-old bones, uncovered in 2012, consist of a cat-sized creature called Repenomamus robustus, whose skeleton is entangled in a final tussle with a beaked dinosaur known as Psittacosaurus lujiatunensis. The two animals’ fight was immortalized when a nearby volcanic eruption entombed them in a fast-moving wave of ash and mud.

“We’ve always had this picture of mammals as the literal underdogs,” Elsa Panciroli, a paleontologist at the Oxford University Museum of Natural History, tells NPR’s Ari Daniel. “They’re being trampled. They’re cowering in the darkness at night, just trying to avoid being eaten.”

But in a new paper published Tuesday in Scientific Reports, researchers upend that traditional view. They suggest the positioning of the mammal above the dinosaur, its teeth chomping down on the reptile’s ribcage and its paw clutching the dinosaur’s beak are all signs that indicate predation. And the lack of tooth marks on the bones of the dinosaur suggests the mammal was actively hunting—not merely scavenging—its meal.

“We already knew that mammals did occasionally prey on at least baby dinosaurs,”  Jordan Mallon, a study co-author and paleobiologist at the Canadian Museum of Nature, tells NPR. “What’s new here is even a fully grown Psittacosaurus wasn’t necessarily safe from these smaller mammalian predators.”But not all researchers agree with the predation interpretation.

“The mammal’s hand inside the mouth of the dinosaur, which had a high bite force, suggests at least the dinosaur was dead at the time of burial, or it would have easily sliced off the hand,” Hans Larsson, a paleontologist at McGill University, tells New Scientist’s Chen Ly. “The awkward interlocking legs between the two suggests both were dead or tumbled while being buried. In this case, I think there is not enough evidence to say with full confidence the mammal was the predator caught in the act of subduing its prey.”

The research team estimated that at their time of death, the badger-like mammal weighed about 7.5 pounds, while P. lujiatunensis weighed about three times that, at 23.4 pounds.

The fossils were uncovered in the Lujiatun beds in China’s Liaoning province—a site sometimes called “Dinosaur Pompeii.” The area holds a cache of life-like remains of animals including dinosaurs, pterosaurs, early birds and mammals, turtles, lizards, freshwater fish, frogs, plants and insects that were buried by airborne volcanic debris and ash. But the region has also seen a few instances of fossil forgeries in recent years, writes Science’s Celina ZhaoThe team acknowledged that forgeries occurred in the area, but they say their analysis of the fossil and its surrounding sediment verified its legitimacy, and they are confident it is genuine, writes Maddie Burakoff for the Associated Press.

It’s rare for fossils to display the interactions of animals in this way. For a fossil to be preserved in the first place, the conditions must be exactly right. When the bones do fossilize, they’re often scattered or incomplete—they don’t typically capture details of animal behavior.

“This is the kind of specimen that paleontologists dream of—a pristine snapshot of ancient behavior and ecology,” Raymond Rogers, a geologist at Macalester College who was not involved in the study, tells Science. “If this remarkable specimen is the real deal, it is a one-in-a-million find.”