For 185 million years, dinosaurs evolved into a panoply of predators and prey that fill the record books for size and shapes. Mammals, meanwhile, were nocturnal, shrewlike nobodies that snatched insects and stole the occasional egg. Only after dinosaurs went extinct 65 million years ago could mammals escape from the shadows and begin to thrive. Or so the story goes.
In this week's issue of Nature, Chinese paleontologists describe the largest mammal skeleton--more than a meter long--ever found from the Mesozoic era. And this furry Goliath wasn't content just to eat bugs: A smaller relative was discovered nearby with the bones of a baby dinosaur in its stomach.
The new fossils, each about 130 million years old, come from northeastern China. Paleontologists had already discovered skulls of the smaller animal, called Repenomamus robustus, but could get only a vague estimate of its body size. Now the same team has found a fairly complete specimen of an adult that probably weighed about 4 to 6 kilograms. "It looked something like a Tasmanian devil," says team member Yaoming Hu, a graduate student at the City University of New York. Collaborators include his adviser Jin Meng of the American Museum of Natural History in New York City and colleagues at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.
While removing rock from the specimen, preparators made a rare discovery: teeth and bones strewn about inside the ribcage, in the likely position of the animal's stomach. The jumble included the remains of a herbivorous dinosaur hatchling, a 14-centimeter-long Psittacosaurus. One leg appears mostly intact, suggesting that the mammal dismembered and wolfed down its food.
R. robustus wasn't the only mammal that dinosaurs had to worry about. Another skeleton, better preserved, was even larger. Named Repenomamus giganticus, it was 1 meter long and weighed roughly 12 to 14 kg, as much as a modern coyote. "It was probably competing with carnivorous dinosaurs for food and territory," Hu says.
And that raises interesting questions, notes Anne Weil of Duke University in Durham, North Carolina. "What these finds really allow us to do--at least speculatively--is ask how mammals might have influenced dinosaur evolution," she says. In other words, Mesozoic mammals may have cast a shadow of their own.