A Taste for Dinosaurs

By MICHAEL D. LEMONICK

Conventional wisdom has long held that mammals spent millions of years in Darwinian limbo. As long as dinosaurs roamed the earth, our distant ancestors never got to be much more than cringing, shrewlike creatures that slinked out at night to nibble timorously on plants and insects when the terrible lizards were asleep. Only when a rogue comet wiped the dinosaurs out, went the story, did mammals begin to earn a little evolutionary respect.

But that picture changed dramatically last week with the announcement in Nature of two impressive fossils. One, of a brand-new species dubbed Repenomamus giganticus, demolishes the notion that most dinosaur-age mammals were never larger than squirrels. The animal, which lived some 130 million years ago, had the dimensions of a midsize dog or large badger--by far the biggest dinosaur-age mammal ever found. And the second, a new specimen of a previously discovered species called Repenomamus robustus, refutes the notion that it was always the mammals that got eaten. Inside the skeleton where the animal's stomach would have been are the fossilized remains of a baby dino. "This discovery was the chance of a lifetime," says Jin Meng, a paleontologist at the American Museum of Natural History in New York City and a co-author of the paper. "We can't expect to find things like this again."

Indeed, Meng and his colleagues didn't expect to find things like this at all. The smaller skeleton was discovered about two years ago by villagers in China's Liaoning province, site of some of the richest fossil beds in the world. They brought it to the attention of scientists, who took it to the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing for examination. "We didn't see the stomach contents at first," says graduate student Yaoming Hu, who is affiliated with the institute and is the lead author of the Nature paper.

After they did, however, it didn't take them long to realize they had struck scientific gold. "The teeth showed that [the animal's last meal] was a dinosaur, not a mammal," says Hu. On closer examination, the scientists determined that the remains were those of a juvenile psittacosaur, a herbivore known to inhabit the region. Some of the arm and leg bones were still attached to each other, suggesting that R. robustus didn't chew its food thoroughly but wolfed it down in large chunks.

That proves that at least one mammal from the age of dinosaurs was carnivorous--and since R. giganticus is a close cousin of R. robustus, it's reasonable to assume that the larger species ate meat as well. Moreover, the size and anatomy of R. giganticus, found in the same fossil beds as its smaller relative, suggest that it was an active predator rather than simply a scavenger. "It had a robust body, with short legs that splay out to the side, similar to a Tasmanian devil," says Meng. "It could walk fast and probably run. It could certainly move better than a crocodile."

Taken together, the finds overturn the idea that early mammals were tiny and timid. That had been eroding anyway with occasional discoveries of teeth and bone fragments that hinted at larger creatures. Now paleontologists can stop cooking up theories to explain why mammals were so little--that they had to be small to avoid being found, for example, or they couldn't grow larger because dinosaurs already occupied those ecological niches.

But it's now clear that mammals did fill some of the niches reserved for larger animals. "It's quite possible," says Anne Weil, a Duke University paleontologist who wrote a commentary accompanying the Nature report, "that they competed with dinosaurs for the same prey." And because they ate dinosaurs, she says, they may even have had an influence on dinosaur evolution. What sort of influence? "We don't know," she says. "That's how it is with the best finds. They leave you with more questions than answers." Those answers may be lurking under the barely scratched surface of Liaoning province. --Reported by Andrea Dorfman/New York

With reporting by Andrea Dorfman/New York