"The picture is not what we thought before; there is a new direction, a new possibility," said Meng Jin, one of the paper's authors and a paleontologist at the museum. The announcement raises questions about a long-held evolutionary theory that assumes mammals during the dinosaur era were small because it was the only size that allowed them to survive amid the dangerous world of predatory dinosaurs.

Scientists have long thought mammals did not begin a true growth spurt until after dinosaurs were wiped out 65 million years ago. The discovery raises the possibility that mammals influenced the dinosaurs' evolution, not just the other way around. "We've had a very simplified view where mammals are prey and dinosaurs are predators," said Anne Weil, a paleontologist at Duke University, North Carolina, who wrote an accompanying article in the magazine. So unexpected was the finding, that the scientists did not realise something was in the animal's stomach until they examined it under a microscope almost two years after discovering the specimen.

Even then they assumed it was the remains of an embryo, a partially formed baby mammal that was killed with its mother probably under mountains of volcanic ash. More research revealed it was a juvenile Psittacosaurus dinosaur 12.7 centimetres long, with the potential to grow to nearly two metres. Scientists do not know how old the dinosaur was when it died, although it had apparently used its teeth. The mammals lived during the Mesozoic era from about 280 million to about 65 million years ago, largely known as the age of the dinosaurs.

The mammal that swallowed the dinosaur is called Repenomamus robustus, and its sharp teeth suggest it was a particularly nasty character. Scientists say it probably tore the juvenile dinosaur apart and gulped it down in chunks. Related to that animal is the far bigger Repenomamus giganticus found nearby, a squat, toothy animal. Probably weighing about 14 kilograms, it may have been able to stand its ground against at least some of the dinosaurs of the period. The findings mean there may have been even bigger creatures that nursed their young during this period.

The Boston Globe