The specimen belongs to a primitive mammal about 130 million years old and its stomach contents show that it ate young dinosaurs called psittacosaurs. A US-Chinese team of researchers has described the find in Nature magazine. In the same issue, the group reports discovering the largest known primitive mammal from the same locality. The team found the Early Cretaceous specimens in the famous fossil beds of Liaoning Province in north-eastern China.

The mammal with the dinosaur in its stomach belongs to a carnivorous mammal called Repenomamus robustus, which was about the size of an opossum. "At first, we thought it was a placental mammal carrying an embryo. But then we looked more closely and saw it was a dinosaur," said co-author Dr Meng Jin, curator of palaeontology at the American Museum of Natural History. "The position was also interesting; it was located in the lower left side of the fossil - exactly the position where the stomach is located in extant mammals." Dog-sized predator The new species of mammal, also found by the researchers in Liaoning, was probably about 50% larger - weighing about 13kg (30lbs). It has been named Repenomamus gigantus.

But fragmentary evidence from Liaoning suggests even bigger mammals may prowled the region during the Cretaceous.

"This find has helped to break a stereotype about early mammals," said Dr Zhe-Xi Luo, a palaeontologist at the Carnegie Museum of Natural History in Pittsburgh, US, who also studies early mammals.

Most mammal fossils from the time of the dinosaurs are about the size of mice and rats. As such, they were at a distinct size disadvantage compared with predatory dinosaurs.

The combined discovery of a dinosaur in the stomach of R. robustus and the dog-sized R. gigantus suggests mammals were not the timid insect-eaters they have been portrayed as in the past.

"Mammals at this time were thought to have lived in the shadow of the dinosaurs. But the picture is quite different now," Dr Jin told the BBC News website.

Dr Jin and Dr Luo both agree that the general picture that primitive mammals were small, nocturnal prey animals still holds true.

But, said Dr Luo: "We have always suspected the feeding niches of early mammals were more diverse, but we never had the proof."

Interestingly, many small dinosaur fossils have been found in the same beds as the new mammals. The researchers cannot yet say whether mammals dominated their reptilian counterparts at this location.

Big mammals like Repenomamus could have been prey for larger dinosaurs that have not yet been seen here. But broadly speaking, carnivores usually reside at the top of food chains.

The wonderfully preserved specimens were pulled from the Yixian Formation, a class of fossil beds in the Liaoning Formation.

This formation has produced an abundance of amazing fossils, including feathered dinosaurs, early birds, fish and mammals.

Dr Jin thinks the astounding preservation of these fossils may be down to how the animals died.

"The bottom section of the Yixian Formation is sandstone with a lot of volcanic ash in it. Many of the fossils are preserved in a resting position. Some of them look as if they are sleeping.

"It could be that poisonous gas produced by volcanism killed many animals while they were asleep."

Fossil Overturns Ideas of Jurassic Mammals
Randolph E. Schmid, AP Science Writer
February 23, 2006

The discovery of a furry, beaver-like animal that lived at the time of dinosaurs has overturned more than a century of scientific thinking about Jurassic mammals.

The find shows that the ecological role of mammals in the time of dinosaurs was far greater than previously thought, said Zhe-Xi Luo, curator of vertebrate paleontology at Carnegie Museum of Natural History in Pittsburgh.

The animal is the earliest swimming mammal to have been found and was the most primitive mammal to be preserved with fur, which is important to helping keep a constant body temperature, Luo said in a telephone interview.

For over a century, the stereotype of mammals living in that era has been of tiny, shrew-like creatures scurrying about in the underbrush trying to avoid the giant creatures that dominated the planet, Luo commented.

Now, a research team that included Luo has found that 164 million years ago, the newly discovered mammal with a flat, scaly tail like a beaver, vertebra like an otter and teeth like a seal was swimming in lakes and eating fish.

The team, led by Qiang Ji of the Chinese Academy of Geological Sciences in Beijing, discovered the remains in the Inner Mongolia region of China. They report their findings in Friday's issue of the journal Science.

Matthew Carrano, curator of dinosaurs at the Smithsonian's National Museum of Natural History, called the find "a big deal."

An important factor is how specialized the creature was, said Carrano, who was not part of the research group.

"It gives a hint that early mammals were not just these shadowy creatures at the time of dinosaurs" but were having their own evolution. There have been hints of such animals in the past but nothing equal to the remains found by Luo and colleagues, he said.

Thomas Martin of the Research Institute Senckenberg in Frankfurt, Germany, said the discovery pushes back the mammal conquest of the waters by more than 100 million years.

"This exciting fossil is a further jigsaw puzzle piece in a series of recent discoveries," commented Martin, who was not part of Luo's team.

It's the first evidence that some ancient mammals were semi-aquatic, indicating a greater diversification than previously thought, the researchers said.

Modern semi-aquatic mammals such as beavers and otters and aquatic mammals like whales did not appear until between 55 million years ago and 25 million years ago, according to the researchers.

The new animal is not related to modern beavers or otters but has features similar to them. Thus the researchers named it Castorocauda lutrasimilis. Castoro from the Latin for beaver, cauda for tail, lutra for river otter and similis meaning similar.

The researchers found imprints of the fur, both guard hairs and short, dense under fur that would have kept water from the skin.

Weighing in at between 1.1 and 1.7 pounds, about the size of a small female platypus, Castorocauda is also the largest known Jurassic early mammal.