Canadian and Chinese scientists have described an unusual fossil from around 125 million years ago that shows a dramatic moment in time when a meat-eating mammal attacked a larger, plant-eating dinosaur.
“The two animals are locked in mortal combat, intimately intertwined, and this is some of the first evidence showing actual mammalian predatory behavior on a dinosaur,” explains Dr. Jordan Mallon, a paleobiologist at the Canadian Museum of Nature and co-author of the study published today in the journal Scientific Reports.
The fossil’s presence challenges the view that dinosaurs faced few threats from their contemporaneous mammals during the Cretaceous, when dinosaurs were the dominant animals. The rare fossil is now in the collections of the Weihai Ziguang Shi Yan School Museum in China’s Shandong province.
The dinosaur in the well-preserved fossil is identified as a species of Psittacosaurus, which is about the size of a large dog. Plant-eating psittacosaurs are among the earliest known horned dinosaurs and lived in Asia during the Early Cretaceous, from about 125 to 105 million years ago. The mammal in the fossil pair is a badger-like animal, called Repenomamus robustus. Although not large by dinosaur standards, it was one of the largest mammals during the Cretaceous, at a time when mammals had not yet come to dominate the Earth.
Prior to this discovery, paleontologists knew that Repenomamus fed on dinosaurs, including Psittacosaurus, due to the herbivore’s fossilized baby bones found in the mammal’s stomach.
“The coexistence of these two animals is not new, but what is new to science through this incredible fossil is the predatory behavior it displays,” says Mallon.
The fossil was collected in the Chinese province of Liaoning in 2012, and both skeletons are almost complete. Their integrity is due to the fact that they come from an area known as the Liujitun fossil beds, which have been dubbed “China’s Pompeii dinosaur.”
The name refers to the many fossils of dinosaurs, small mammals, lizards, and amphibians in the area, animals that were suddenly buried en masse by landslides and debris after one or more volcanic eruptions. The existence of volcanic material in the rocky matrix of the study fossil was confirmed after analysis by Canadian Museum of Nature mineralogist Dr. Aaron Lussier.
The Psittacosaurus-Repenomamus fossil was in the care of study co-author Dr. Gang Han in China, which came to the attention of Canadian Museum of Nature paleobiologist Xiao-Chun Wu. Doctor Wu has worked with researchers in China for decades and knew it was special when he saw it.
Close examination of the fossil pair shows that the Psittacosaurus is lying down, with its hind limbs folded on either side of its body. The Repenomamus’s body coils to the right and it sits on top of its prey, with the mammal grasping the larger dinosaur’s jaw. The mammal is also biting into some of the ribs, and Repenomamus’ hind foot latches on to the dino’s hind leg. “The weight of the evidence suggests that an active attack was taking place,” says Dr. Mallon.
Mallon, Wu and their colleagues ruled out the possibility that the mammal was simply searching for a dead dinosaur. The dinosaur’s bones bear no tooth marks, for example, suggesting that it wasn’t being unleashed, but rather being preyed on. And it’s unlikely the two animals would have gotten so entangled if the dinosaur had been dead before the mammal found it. The position of the Repenomamus on top of the Psittacosaurus suggests that it was also the aggressor.
Analogies of smaller animals attacking their larger prey are known in the modern world. Mallon and Wu note that some solitary wolverines have been known to hunt larger animals, including caribou and domestic sheep. And in the African savannah, wild dogs, jackals and hyenas will attack prey that is still alive, with the prey collapsing, often in shock.
“This could be the case as depicted in the fossil, with the Repenomamus actually eating the Psittacosaurus while it was still alive, before both were killed in turbulent circumstances,” Mallon explains.
The research team speculates in their paper that volcanically derived deposits from the Lujiatun fossil beds in China will continue to yield new evidence for species interactions otherwise unknown in the rest of the fossil record.