Scientists working in a rich fossil field in China have unearthed the skeleton of an opossum-sized mammal whose stomach contained the remains of a baby dinosaur, a startling finding that sheds new light on the struggle for survival 130 million years ago.
Until now, science had viewed the first mammals solely as tiny shrewlike creatures that lived in the shadow of the dinosaurs. By day, these early mammals hid in caves; under cover of darkness they scurried forth to hunt insects.
Only when the dinosaurs died off was it safe for the original mammals to go outdoors, whereupon they enjoyed an explosion of biodiversity that ultimately advanced to primates and human beings, or so most scientists believed.
The new discovery, reported Wednesday in the British journal Nature, challenges much of that. It provides the first direct evidence that some primitive mammals were carnivores that competed with the dinosaurs for food and land.
The dinosaur-eating mammal belonged to a species dubbed Repenomamus robustus. But it was a runt compared with a nearby fossil of an even larger new species the size of a 30-pound dog, Repenomamus giganticus. Both went extinct long ago.
R. giganticus is the largest known mammal with fairly complete fossil remains ever found from the Mesozoic era, 280 to 65 million years ago, when dinosaurs and other animals faced extinction.
"We think these fossils are pretty cool. Mammals are never in the spotlight of Mesozoic life. Dinosaurs are the big guys," said team leader Jin Meng, curator in the division of paleontology at the American Museum of Natural History in New York.
"Now, we've found relatively large mammals and they were carnivores, which always are at the top of the food chain. This gives us a drastically new picture of many of the animals that lived in the age of dinosaurs," Meng said.
Scientists combined the words for reptile and mammal to name Repenomamus. The resemblance to reptiles can be seen in their large, pointy teeth and short limbs that emerge from the body at an angle.
But their limb joints allowed more flexibility of movement than is found in reptiles, making the animals more mobile than even such modern relics of antiquity as the egg-laying mammals known as platypuses.
Such freedom of movement, found among marsupials and other more advanced mammals, helped carnivores compete for prey and elude predators, Meng said.
"Although Repenomamus probably wasn't very fast, it could stand on its hind limbs and walk effectively enough to stalk small prey, as the new robustus fossil indicates."
Meng and a graduate student, Hu Yaoming, worked with colleagues from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.
Both species are thought to have no living descendants, though Meng said the closest modern animal in appearance -- "Admittedly, I'm stretching things a bit" -- would be the squat, toothy, predatory marsupial known as the Tasmanian devil, which is confined to the island of Tasmania, southeast of Australia.
He doesn't think the fossilized creatures chewed their food.
"The baby dinosaur was broken down into large pieces, suggesting the animal was swallowed in big chunks," he said.
Careful study revealed the meal to be a young psittacosaur, a primitive horned dinosaur that grew to be a meter or two long. The site where it was found, in the northeastern China province of Liaoning, is an area with a wealth of feathered dinosaurs and other exquisitely preserved fossils of Mesozoic creatures, including insects.
These latest finds "should trigger another avalanche of questions and speculations," Duke University paleontologist Anne Weil predicted in a commentary for Nature.
"For years, all we had of early mammals were very tiny fossils," she said in an interview. "It got to the point in the 1970s that scientists were explaining in the literature why it was that early mammals had to be small."