Researchers working in China's fossil beds have found the remains of a tiny dinosaur inside the belly of a mammal, a discovery that could change perceptions about early mammals.

The mammal, about the size of a raccoon, had a bird-like dinosaur about 12 centimetres long for its last meal. Scientists say it's the first evidence that mammals hunted small dinosaurs about 130 million years ago.

Conventional wisdom holds that early mammals were timid, rodent-like creatures, feeding on insects and seeds. The newly discovered fossils suggest some were meat eaters, occasionally preying on dinosaurs.

Meng Jin of the American Museum of Natural History, a co-author of the study, said the discovery gives researchers a drastically different picture of early mammal life.

A second mammal fossil found at the same site is the largest early mammal ever found, about the size of a modern dog and 20 times larger than most mammals living in the Cretaceous Period.

The dinosaur-eating mammal is a member of the species Repenomamus robustus, previously seen only in skull fragments. The skeleton is about 60 centimetres long and scientists think the animal weighed about seven kilograms.

Its dinosaur meal, a very young Psittacosaurus, appears as fragmented remains under the ribs on the left side, the location of its stomach.

Its larger cousin was given a new species name, Repenomamus giganticus.

The fossils were found more than two years ago in China's Liaoning province. Chinese and American researchers cleaned and analysed the remains in a lab in Beijing.

The study appears in Thursday's issue of the journal Nature.