Repenomamus was the largest mammal of the Cretaceous. It was the first mammal that showed there were mammals living alongside dinosaurs that were bigger than mice. Repenomamus was the size of a small dog. It was not just the size of Repenomamus that shocked paleontologists: Inside the stomach cavity of Repenomamus, there were remains of a young Psittacosaurus. This was clear evidence that Repenomamus hunted dinosaurs.