Of the approximately 4,600 living mammal species currently identified, more than a third are rodents, and more than 23 percent are bats.
"R. giganticus was a squat, powerful mammal with large fangy front teeth," Weil said. "And believe me, it's not something you'd like to have hold of your leg."
The two early mammal species were probably predators, not scavengers, say the scientists. Their teeth were large and pointy, and their jaw musculature was strong. This suggests that they were capable of capturing, holding, and ripping apart their prey. The juvenile Psittacosaurus dinosaur found in the stomach of the R. robustus fossil looked to have been dismembered and swallowed in chunks, rather than chewed.
The Psittacosaurus dinosaur was a two-legged plant eater with a beak like a parrot's and four grasping fingers. R. robustus almost certainly preyed on only the young, injured, or old Psittacosauruses. That's because Psittacosauruses grew to be as tall as six feet (two meters).
Whether small dinosaurs were a special treat, a ready snack, or a mainstay of the R. robustus's diet is unknown.
"There's no evidence to indicate whether these larger mammals were occasional hunters or habitual hunters," said Hu, who is the lead author of the study.
The fossil of the mammal that ate the dinosaur is so well preserved that scientists were able to examine the teeth of the Psittacosaurus found in the mammal's belly. The fact that there were teeth, and that they showed wear, indicated that the dinosaur was not eaten as an embryo in the egg. Measurements suggest the dinosaur was about one-third the size of the R. robustus that ate it.
"One presumes the larger species [R. giganticus] could eat larger, although still not huge animals," said Weil, who wrote a commentary on the findings in this week's Nature. "Modern mammals that weigh that amount prey on animals about half their size."
Rethinking the Mesozoic
As little as two decades ago, schools taught that the dinosaurs ruled the Earth from 248 million years ago until some still-unknown catastrophic event caused their extinction around 65 million years ago. Mammals also evolved during this period. But since they were unable to compete for food or territory, they stayed tiny.
A catastrophesuch as an asteroid collisionis widely believed to have wiped out the supersize dinosaurs and ushered in the "age of the mammals." Mammals were not thought to have grown much bigger than rats until after the dinosaurs were gone.
Over the past several years, though, sketchy evidence has emerged that suggests that some mammal species may have grown larger during the dinosaur age.
"Up until now the evidence was extremely fragmentary; a tooth here, part of a skull there," Weil said. "Of course you can tell by a tooth that the animal was larger than a rat, but you can't really estimate body weight, which is the more important factor."
The finds in the geological strata known as the Yixian formation in Liaoning provide the first clear-cut confirmation that some mammal species grew larger than their cousins elsewhere. Why is another question.
"People have speculated as to why mammals in this locality are so much larger," Hu said. "Some people say it is probably because the dinosaurs here are relatively small. In any particular ecological system, there is competition for food and territory between some members of the system. If the dinosaurs are not so large, the mammals might have a chance to evolve and become larger. It's all just speculation at this point."
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