for National Geographic News
Early mammals were not only bigger than previously thoughtsome 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.
One incredibly well-preserved fossilof an early mammal known as Repenomamus robustushad the remains of a small dinosaur in its belly. It is the first evidence that mammals dined on dinosaurs.
The second fossil find, named Repenomamus giganticus, is 50 percent larger than R. robustus. Until now R. robustus was the largest known mammal of the Mesozoic era.
The discoveries, uncovered in the rich dinosaur fossil beds of China's Liaoning Province, provide the first hard evidence that Mesozoic mammals were more than the rat-size plant- and insect-eating creatures that they have long been portrayed to be.
"The size of R. giganticus challenges the conventional idea that mammals during the Mesozoic were very small, probably nocturnal, and living in the shadow of the dinosaurs and other reptiles," said Yaoming Hu. Hu is a researcher affiliated with the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China, and the American Museum of Natural History in New York City.
"This new evidence of larger size and predatory, carnivorous behavior in early mammals is giving us a drastically new picture of many of the animals that lived in the age of dinosaurs," said Jin Meng, associate curator in the division of paleontology at the American Museum of Natural History.
Hu, Meng, and colleagues published their findings in the January 13 issue of the journal Nature.
Bigger, Badder Carnivorous Mammals
The R. giganticus fossil is that of a young adult that weighed about 30 pounds (14 kilograms) and was more than 3 feet (1 meter) long. It had a chunky body, relatively short legs, front teeth that were both large and sharp, and a long tail.
At roughly the size and shape of a small- to medium-size modern-day dog, this early mammal might not sound terribly big or threatening.
"That's because people have a skewed view of what size a mammal is because we're bigger than most living mammals," said Anne Weil, a paleontologist at Duke University. "At 30 pounds, R. giganticus was larger than most species of living mammals."
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