Not all prehistoric mammals were vulnerable to dinosaurs. A mammal species discovered in China fed on dinosaurs in Early Cretaceous forests. It was named Repenomamus giganticus.

Repenomamus was a large mammal for the Cretaceous Period, approximately 145.5–65.5 million years ago. While most mammals of that time were small, nocturnal insectivores,  Repenomamus measured 1 metre in length and had a large body, short legs, powerful jaws and sharp teeth. The discovered fossils also suggest that it was either a marsupial or a monotreme, an egg-laying mammal such as the platypus or echidna.

The teeth and skull anatomy of the first fossils discovered in China suggested that Repenomamus was carnivorous. Direct evidence came in 2000 with the discovery of a Repenomamus giganticus skeleton containing the remains of a young Psittacosaurusin its stomach. The partially articulated remains of the young Psittacosaurus suggest that it had been consumed by Repenomamus. Although it is impossible to determine whether the young dinosaur was hunted or scavenged by Repenomamus, this is the first evidence that, while dinosaurs ate mammals, some mammals also ate dinosaurs.

Sources: Nature