Fierce mammal and its dinosaur prey fossilized in volcanic eruption

A badger-like mammal died while chomping into the ribs of a hapless horned plant-eating dinosaur struggling to escape more than 100 million years ago. The pair were perfectly preserved, still locked in combat, in "China's dinosaur Pompeii,"researchers report.

Dating to the Cretaceous Period, the dramatic fossil unearthed in northeastern China shows thefour-leggedmammal Repenomamus robustus the size of a domestic cat ferociously entangled with the beaked two-legged dinosaurPsittacosaurus lujiatunensis, as big as a medium-sized dog.

Scientists suspect they were suddenly engulfed in a volcanicmudflow and buried alive during mortal combat.

"Dinosaurs nearly always outsized their mammalcontemporaries, so traditional belief has been that theirinteractions were unilateral the bigger dinosaurs always atethe smaller mammals," said paleobiologist Jordan Mallon of theCanadian Museum of Nature in Ottawa, who helped lead the studypublished in the journal Scientific Reports.

"Here, we have good evidence for a smaller mammal preying ona larger dinosaur, which is not something we would have guessedwithout this fossil," Mallon said.

A complex food web

Most mammals during the Mesozoic Era, the age of dinosaurs, were shrew-sized bit players in the larger theatre of life, doing well to avoid becoming someone else's lunch. Repenomamusshows that at least some mammals gave as good as they got.

"I think what's key here is that Mesozoic food webs weremore complex than we had imagined," Mallon said.

The area in Liaoning Province where the virtually completefossil was found is called the "Chinese Pompeii," owing tovarious fossils of animals buried in volcanic eruptions.

Examining the fossil was like a crime scene analysis.Repenomamus is perched atop the prone Psittacosaurus, grippingthe jaw and hind leg while biting into the ribcage. Repenomamus measures 47 centimetres (18 inches)long.Psittacosaurus is 120 centimetres(four feet)long. Both are thought to be not quite full adults.

"There have been specimens of carnivorous dinosaurs preyingon plant-eating dinosaurs before, but there has never been anexample of a mammal preying on a dinosaur," said Canadian Museumof Nature paleontologist and study co-author Xiao-Chun Wu.

Preservedin battle

It is rare to find fossils showing animals interacting.Another fossil found in the 1970s in Mongolia shows twodinosaurs predator Velociraptor and plant-eater Protoceratopsfighting about 80 million years ago before being buried alive,perhaps in a collapsing sand dune.

The researchers discounted the idea that the Repenomamus andPsittacosaurus fossil showed a mammal merely scavenging acarcass.

"For one, the mammal is on top of the dinosaur as though itwas trying to subdue it, which the scavenging hypothesis doesn'taccount for," Mallon said.

"Second, there are no bite marks on the bones of thedinosaur, which we would expect if it had been sitting out forlong, exposed to scavengers. Lastly, the hind foot of the mammalis trapped by the folded hind leg of the dinosaur, which isunlikely to have happened if the dinosaur had already been deadwhen the mammal came across it," he said.

While Psittacosaurus was an early relative of the horneddinosaur lineage, it lacked facial horns and a head crest. Itpossessed a parrot-like beak to crop plant material.

Repenomamus, one of the dinosaur age's largest mammals, hadshort and sprawling limbs, a long tail, a sinuous body, a robustskull and shearing teeth. Mallon compared its appearance to theliving Chinese ferret-badger.

There was previous evidence of the dino-eating habits ofRepenomamus. One Repenomamus fossil from the same area had babyPsittacosaurus bones in its stomach.

"What's unique about our fossil is the fact that itdemonstrates that Repenomamus was capable of tackling largerdinosaur prey," Mallon said.