Illustration showing Repenomamus robustus as it attacks Psittacosaurus lujiatunensis before a volcanic debris flow buries them both, ca. 125 million years ago. MICHAEL W. SKREPNICK COURTESY CANADIAN MUSEUM OF NATURE

A first-of-its-kind fossil of a mammal and a dinosaur from around 125 million years ago "locked in mortal combat" challenges the idea that dinosaurs ruled the land, researchers wrote in a study published Tuesday.

The new fossil, discovered on May 16, 2012, in China's Liaoning Province, shows a mammal attacking a dinosaur about three times its size. The mammal, a carnivorous Repenomamus robustus, was the clear aggressor, researchers wrote in the journal Scientific Reports.

"The mammal died while biting two of the dinosaur's left anterior dorsal ribs; its mandible plunges downward into the indurated sediment to firmly clasp the bones," the study's authors wrote.

The discovery of the two creatures is among the first evidence to show actual predatory behavior by a mammal on a dinosaur, Dr. Jordan Mallon, palaeobiologist with the Canadian Museum of Nature and co-author on the study, said in a press release.

Repenomamus robustus is a badger-like animal that was among the largest mammals living during the Cretaceous period.

The dinosaur was identified as a Psittacosaurus, an herbivore about the size of a large dog.

Paleontologists had previously surmised Repenomamus preyed on dinosaurs because of fossilized bones found in the mammal's stomach.

"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.

Experts believe the attack was preserved when the two animals got caught in a volcanic flow. The area where the fossil was discovered has become known as "China's Pompeii" because of the many fossils of animals that were buried en masse by mudslides and debris following one or more volcanic eruptions.

After the find, scientists worked to confirm the fossil was not a forgery. The researchers said the intertwined skeletons and the completeness of the skeletons suggest the find is legitimate and that the animals were not transported prior to burial.

Steve Brusatte, a paleontologist at the University of Edinburgh who was not involved in the research, tweeted about the find, suggesting it was like Wile E. Coyote catching the roadrunner. He said the find turns "the old story of dinosaur dominance on its head."

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