Early mammals conjure up images of rat- or shrew-size creatures that skulked in the shadows of dinosaurs, trying to avoid being ripped limb from limb by the terrible lizards. Now it seems that the hunted sometimes became the hunter. Newfound fossils reveal a baby dinosaur inside a mammal's gut--the first direct evidence of such predation.

An international team based its conclusions on a species called Repenomamus robustus, a mammal about as big as a Virginia opossum that lived during the Mesozoic, 130 million or so years ago. Within the rib cage of R. robustus was the skeleton of a young dinosaur whose serrated teeth, limbs and toes mark it as Psittacosaurus, a hornless relative of Triceratops that reached cow proportions in adulthood. Whereas the mammal's bones are preserved in their anatomical position, the dinosaur's are mostly fragmented and packed together where the stomach lies in living mammals. "The most likely explanation is that it was eaten," concludes Jin Meng, a paleontologist at the American Museum of Natural History in New York City. The findings were scheduled to have appeared in the January 13 issue of Nature.