Fossil shows early mammals fed on dinosaurs

Scientists have found the remains of a dinosaur in a fossilized mammal's stomach, a discovery that upends decades of scientific belief about the relationship between dinosaurs and mammals.

A team led by scientists from the American Museum of Natural History in New York City discovered bones of a beaked, two-legged dinosaur in the stomach of a cat-sized animal that died about 130 million years ago in China's northeastern Liaoning region. At the same site the team also unearthed the remains of the largest primitive mammal ever found, a creature roughly as big as a beagle.

The two findings, reported yesterday by the journal Nature, suggest the two types of creatures may have competed fiercely for food and habitat.

''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 key 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 reasoned mammals didn't 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 who was not part of the research but wrote an accompanying article in Nature about it.

So unexpected was the finding, Meng and other scientists didn't even realize something was in the animal's stomach until they examined it under a microscope this past summer, 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 suddenly along with its mother probably under mountains of volcanic ash. It was only after they did more research that they realized it was a five-inch long juvenile Psittacosaurus with the potential to grow to nearly six feet long. Scientists don't know how old the dinosaur was when it died, although it apparently had used its teeth.

The mammals lived during the Mesozoic era from about 280 million to about 65 million years ago that is largely known as the age of the dinosaurs. Although scientists have found teeth and other evidence suggesting that mammals might have been bigger than first believed during this period, it was sparse and incomplete. The team's findings are by far the most extensive skeletons ever found of larger mammals.

''What this gives us is that this . . . ecosystem is much more nuanced," Weil said.

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. The mammal has no modern relatives. And although it probably couldn't run very fast, its skeleton suggests it could stand on its hind legs and stalk small prey.

Related to that animal is the far bigger Repenomamus giganticus found nearby, a squat, toothy animal that most closely resembles the Tasmanian devil. Probably weighing in at around 30 pounds, the mammal might have been able to stand its ground against at least some of the dinosaurs of the period. Its jaw length is about that of a fox and while it didn't have a dinosaur in its stomach area, scientists believe it too was capable of attacking and eating dinosaurs, probably even bigger ones than its smaller cousin. Because of the teeth of both specimens, scientists suspect they went after dinosaurs -- and didn't just happen to eat a dead or weak one they found scavenging for any kind of food.

The findings also leave open the possibility that there might have been even bigger creatures that nursed their young during the age of the dinosaurs.

''It really is possible there were larger mammals," Weil said.

Beth Daley can be reached by email at bdaley@globe.com