Early mammals were not only bigger than previously thought—some were carnivores and hunted small dinosaurs for dinner.
Scientists in China have uncovered the fossil remains of two mammals that lived around 130 million years ago. The finds will revolutionize current thinking about life during the Mesozoic era (248 million to 65 million years ago), a time when both dinosaurs and mammals arose.
It contradicts conventional evolutionary theory that early mammals couldn't possibly attack and eat a dinosaur because they were timid, chipmunk-sized creatures that scurried in the looming shadow of the giant reptiles.
A second mammal fossil found at the same site claims the distinction of being the largest early mammal ever found. It's about the size of a modern dog, a breathtaking 20 times larger than most mammals living in the early Cretaceous Period.
Considering the specimens in tandem, scientists suggest the period in which these animals lived may have been much different than is commonly understood as the Age of Dinosaurs -- a time dominated by long-necked, 85-ton plant-eaters and the emergence of terrifying hunters with bladelike teeth and sickle claws.
It appears that at least some smaller dinosaurs had to look over their shoulders for snarling, meat-eating mammals claiming the same turf.
"This new evidence gives us a drastically new picture," said paleontologist Meng Jin of the American Museum of Natural History in New York City, a co-author of the study in Thursday's issue of the journal Nature.
Other scientists who did not work on the bones described the discoveries as "exhilarating."
"This size range really has surprised everybody," said Zhexi Luo of the Carnegie Museum of Natural History in Pittsburgh, who digs in the same area of northeast China. "It dispels the conventional wisdom."
The fossils were found more than two years ago by villagers in the rich fossil beds of Liaoning province. The specimens were taken to a Beijing lab, where they were cleaned and analyzed by Chinese and American scientists.
The dinosaur-eater belongs to a species called Repenomamus robustus, known previously from skull fragments. It has no modern relatives.
The squat, toothy specimen measures a little less than 2 feet long, and probably weighed about 15 pounds. On R. robustus' left side and under the ribs in the area of its stomach are the fragmented remains of a very young Psittacosaurus.
This common, fast-moving plant-eater is known as the "parrot dinosaur" because it had a small head with a curved, horny beak. Its arms were much shorter than its legs. Adults grew to be 6 feet long, but the one that was devoured was just 5 inches.
"It must have swallowed food in large hunks," Meng said.
The larger, second fossil also is a Repenomamus, but considerably larger -- more than 3 feet long with a likely weight of more than 30 pounds. Dubbed R. giganticus, it weighed 20 times more than most of the 290 known early mammals, Meng said.
A larger mammal could roam and hunt aggressively, preying on young dinosaurs.
"Giganticus is in a league by itself," Luo said. "It's the world champion so far for body mass in any Mesozoic mammal."
This new class of predatory mammals has set off new speculation.
Originally, scientists believed that mammals remained small because larger dinosaurs were hunting them. Only after dinosaurs went extinct by 65 million years ago did surviving mammals begin to grow larger, they reasoned.
Now, the discovery of larger mammals is reversing some of the speculation. The Liaoning region already is famous for its trove of small feathered dinosaurs and early birds.
"Maybe small dinosaurs got larger -- or got off the ground -- to avoid rapacious mammals,'' wonders Duke University paleontologist Anne Weil.
Equally mysterious is how these specimens died. Neither shows evidence of being hunted itself.
The Yixian rock formation in which their bones were encased is a combination of river sediments and volcanic ash. The formation also includes the fossils of insects, frogs and other creatures, suggesting a mass die-off.
"It's possible that poisonous volcanic gas killed the animals when they were sleeping,'' Meng said. ''Then there was a catastrophic explosion that buried the whole thing."
The above story is based on materials provided by National Geographic Society.