Dinosaur Fossil Found in Mammal's Stomach
By Joseph B. Verrengia
AP Science Writer
January 12, 2005

In China, scientists have identified the fossilized remains of a tiny dinosaur in the stomach of a mammal. Scientists say the animal's last meal probably is the first proof that mammals hunted small dinosaurs some 130 million years ago.
It contradicts conventional evolutionary theory that early mammals couldn't possibly attack and eat a dinosaur because they were timid, chipmunk-sized creatures that scurried in the looming shadow of the giant reptiles.

In this case, the mammal was about the size of a large cat, and the victim was a very young "parrot dinosaur" that measured about 5 inches long.

A second mammal fossil found at the same site claims the distinction of being the largest early mammal ever found. It's about the size of a modern dog, a breathtaking 20 times larger than most mammals living in the early Cretaceous Period.
Considering the specimens in tandem, scientists suggest the period in which these animals lived may have been much different than is commonly understood as the Age of Dinosaurs -- a time dominated by long-necked, 85-ton plant-eaters and the emergence of terrifying hunters with bladelike teeth and sickle claws. 

It appears that at least some smaller dinosaurs had to look over their shoulders for snarling, meat-eating mammals claiming the same turf.
"This new evidence gives us a drastically new picture," said paleontologist Meng Jin of the American Museum of Natural History in New York City, a co-author of the study in Thursday's issue of the journal Nature.

Other scientists who did not work on the bones described the discoveries as "exhilarating."
"This size range really has surprised everybody," said Zhexi Luo of the Carnegie Museum of Natural History in Pittsburgh, who digs in the same area of northeast China. "It dispels the conventional wisdom."
The fossils were found more than two years ago by villagers in the rich fossil beds of Liaoning province. The specimens were taken to a Beijing lab, where they were cleaned and analyzed by Chinese and American scientists.
The dinosaur-eater belongs to a species called Repenomamus robustus, known previously from skull fragments. It has no modern relatives.

The squat, toothy specimen measures a little less than 2 feet long, and probably weighed about 15 pounds. On R. robustus' left side and under the ribs in the area of its stomach are the fragmented remains of a very young Psittacosaurus.
This common, fast-moving plant-eater is known as the "parrot dinosaur" because it had a small head with a curved, horny beak. Its arms were much shorter than its legs. Adults grew to be 6 feet long, but the one that was devoured was just 5 inches.
The remains still are recognizable, indicating that R. robustus ripped its prey like a crocodile, but probably had not developed the ability to chew food like more advanced mammals.
"It must have swallowed food in large hunks," Meng said.
The larger, second fossil also is a Repenomamus, but considerably larger -- more than 3 feet long with a likely weight of more than 30 pounds. Dubbed R. giganticus, it weighed 20 times more than most of the 290 known early mammals, Meng said.
A larger mammal could roam and hunt aggressively, preying on young dinosaurs.
"Giganticus is in a league by itself," Luo said. "It's the world champion so far for body mass in any Mesozoic mammal."
This new class of predatory mammals has set off new speculation.
Originally, scientists believed that mammals remained small because larger dinosaurs were hunting them. Only after dinosaurs went extinct by 65 million years ago did surviving mammals begin to grow larger, they reasoned.
Now, the discovery of larger mammals is reversing some of the speculation. The Liaoning region already is famous for its trove of small feathered dinosaurs and early birds.
"Maybe small dinosaurs got larger -- or got off the ground -- to avoid rapacious mammals,'' wonders Duke University paleontologist Anne Weil.
Equally mysterious is how these specimens died. Neither shows evidence of being hunted itself.

The Yixian rock formation in which their bones were encased is a combination of river sediments and volcanic ash. The formation also includes the fossils of insects, frogs and other creatures, suggesting a mass die-off.
"It's possible that poisonous volcanic gas killed the animals when they were sleeping,'' Meng said. ``Then there was a catastrophic explosion that buried the whole thing."

An astonishing new fossil unearthed in China has overturned the accepted view about the relationship between dinosaurs and early mammals.

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.

The research was funded by the U.S. National Science Foundation, National Natural Science Foundation of China, Ministry of Science and Technology of China, Chinese Ministry of Land Resources, National Geographic Society and Carnegie Museum