The fossils of a dinosaur and a small mammal have been discovered locked in combat when they apparently died together mid-fight, revealing new insights into the relationship between ancient reptiles and mammals.

It's often thought that during the reign of the dinosaurs, mammals were mostly small critters scurrying around under those giant scaly feet. But new studies are showing our ancestors were more than just dino chow – sometimes they fought back.

That's the story presented by a newly described fossil from the Yixian Formation in China, dating back around 125 million years. It features a Psittacosaurus, a dinosaur about the size of a Labrador, and a Repenomamus, a mammal about the size of a badger, immaculately preserved together in a life-or-death struggle.

The dinosaur is lying on its right side, with its neck and tail curled inwards to the left. The mammal is sitting on top of it, with its teeth plunged between Psittacosaurus' ribs. Repenomamus' left front paw is grabbing hold of the dinosaur's lower jaw, slightly displacing it, while its left hind paw is gripping its opponent's left shin, with its own leg trapped under the dinosaur's.

Given how entangled they are, and the lack of other bite marks on the dinosaur's bones, the fossil indicates that they weren't just washed together after they'd died separately, or that the mammal was scavenging the dead dinosaur's remains. Instead, the pair was frozen in this incredible action pose for millions of years, with the team hypothesizing that they were buried under a volcanic mudslide mid-mortal combat.

This isn't the first time paleontologists have been treated to a fossilized fight scene. In the 1970s a Velociraptor and a Protoceratops were found in the Gobi Desert, in what's come to be called (unimaginatively) the Fighting Dinosaurs. The raptor's razor-sharp claw is embedded in the Protoceratops' neck, which has meanwhile locked its beaked jaws onto its foe's arm, breaking it. The similarly named Dueling Dinosaurs captures two of the most iconic species ever – a T-rex and a Triceratops – apparently duking it out for all eternity.