New fossil finds from China are painting a different picture of the mammals that lived alongside the dinosaurs during the Mesozoic era. Most mammals known from this time are thought of as relatively small, nocturnal creatures--the hunted rather than the hunters. Findings published today in the journal Nature provide the first direct evidence that some mammals dined on their dinosaur contemporaries.

Researchers from the American Museum of Natural History (AMNH) in New York City and the Chinese Academy of Sciences in Beijing recovered the 130-million-year-old remains of an opossum-size mammal from China's fossil-rich Liaoning province. While cleaning the fossil of Repenomamus robustus, the team discovered a small patch of bones within the rib cage, where the stomach of similarly sized living mammals would be. The stomach contents included the limbs, fingers and teeth of a juvenile herbivorous dinosaur known as a psittacosaur. Although adult psittacosaurs grew to a height of around six feet, the baby prey was just five inches long, about a third the size of Repenomamus robustus. From wear marks on the dinosaur's teeth, the researchers inferred that it was not an embryo. (This supports the notion that the mammal hunted the dinosaur, rather than snatching an unhatched egg out of a nest.) In addition, they surmise that it was swallowed in chunks because some of its bones were still connected.

The scientists also found remains of a larger mammal, about the size of a small dog, that was a close relative of the dinosaur eater. This fairly complete skeleton, Repenomamus giganticus, represents the largest mammal known so far from the Mesozoic era (spanning from about 250 million to 65 million years ago). Both creatures belong to a lineage that has no extant descendants. Remarks study co-author Jin Meng of AMNH, "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."