Extraordinary 125 Million-Year-Old Fossil Shows Dinosaur in 'Mortal Combat'

An extraordinary fossil has preserved the dramatic moment when a carnivorous mammal attacked a larger plant-eating dinosaur around 125 million years ago.

For a study published in the journal Scientific Reports, a team of Canadian and Chinese scientists described the fossil, which was found in China's northeastern Liaoning Province.

"The two animals are locked in mortal combat, intimately intertwined," Jordan Mallon, a paleobiologist with the Canadian Museum of Nature and co-author on the study, said in a statement.

Fossils that record an attack in progress are "very rare"—particularly when the incident involved predatory behavior by a mammal on a dinosaur.

"The famous 'fighting dinosaurs' fossil comes to mind, which preserves a Velociraptor and Protoceratops locked in a battle to the death," Mallon said. "I'm also aware of a pair of mammoths that died in ritualistic combat, with their enormous tusks in a bind. Ours is the first fossil showing a mammal fighting a dinosaur, as far as I'm aware."

The unusual fossil challenges the perception that dinosaurs faced few threats from their mammal contemporaries during the Cretaceous period (145-66 million years ago) when the former were the dominant creatures on Earth. The Cretaceous is the third and final period of the Mesozoic Era.

"I think what's most significant is that this fossil shows that dinosaur-mammal interactions weren't simply unilateral during the Mesozoic, which is to say that the larger dinosaurs didn't simply feed on the smaller mammals," Mallon said. "In some cases, mammals were able to prey upon dinosaurs, too—even nearly fully grown ones."

The mammal represented in the fossil is from the genus Repenomamus. It was an extinct, carnivorous, badger-like animal weighing between 6 and 9 pounds.

The dinosaur, meanwhile, is of the genus Psittacosaurus. This herbivorous, bipedal dinosaur was considerably larger than Repenomamus—roughly similar in size to a large dog. It weighed in at approximately 22 pounds.

Psittacosaurs were among the earliest known horned dinosaurs and lived in Asia between around 125 to 105 million years ago.

Repenomamus was among the largest mammals roaming the Earth during the Cretaceous, but it was significantly smaller than many of its dinosaurs contemporaries.

"Given the offset in size between the two, we questioned how likely it would be that a small mammal would attack a dinosaur nearly three times its own body mass," Mallon said.

"However, some living mammals, like wolverines and weasels, are known to attack prey much larger than themselves—the former are able to take down caribou and moose. So, we think it is entirely reasonable that Repenomamus might have occasionally attacked even fully grown Psittacosaurus."

Prior to the latest study, there was some evidence to suggest that Repenomamus hunted dinosaurs, including Psittacosaurus. Researchers had found fossilized baby bones of the plant-eater where the stomach of the mammal would have been.

"The co-existence of these two animals is not new, but what's new to science through this amazing fossil is the predatory behavior it shows," Mallon said in the statement.

The weight of the evidence regarding the manner in which the mammal and dinosaur were preserved suggests that the Repenomamus was most likely attacking the dinosaur when the two animals perished, researchers said.

"Given the intimately entwined nature of the skeletons, it seems unlikely that they were preserved together by accident," Mallon said.

"We entertained the possibility that the mammal was scavenging the dinosaur when the pair was buried, but there are several aspects of the fossil that don't make sense in this light. For one, the position of the mammal atop the dinosaur suggests that it was trying to subdue the dinosaur, which doesn't make much sense on the scavenging hypothesis."

In addition, the mammal appears to be be biting into the ribcage area of the dinosaur's skeleton.

"The dinosaur's skeleton is also bereft of tooth marks, which we would expect if it had been subject to extended scavenging," Mallon said. "Finally, the hind foot of the mammal is trapped by the folded hind leg of the dinosaur, which is unlikely to have happened if the dinosaur was already dead. These factors combined make us think that this fossil represents a failed act of predation on the part of the mammal."

But what killed both of the animals, burying them simultaneously and preserving this dramatic moment in time for millions of years?

"Such fossilized 'moments in time' only come about through rapid burial," Mallon said. "In this case, the animals were buried and preserved by a sudden volcanic mudslide, also called a lahar. Such lahar flows are still seen today, and can wipe out entire villages."

The fossil was uncovered in an area known as the Liujitun fossil beds, which have been dubbed "China's dinosaur Pompeii".

This region is home to the fossils of numerous dinosaurs, small mammals, lizards, and amphibians that were buried en masse millions of years ago by mudslides and debris from one or more volcanic eruptions.

The manner in which the Repenomamus and Psittacosaurus were killed has meant that the fossilized skeletons are well-preserved and both nearly complete, providing a fascinating glimpse into this Cretaceous-era battle.

The fossil was originally discovered in 2012 but only recently have researchers examined it in detail. It is now being kept in the collections of the Weihai Ziguang Shi Yan School Museum in China's Shandong Province.