JUVENILE PSITTACOSAUR FOUND IN BELLY OF PRIMITIVE MAMMAL FOSSIL SHOWS EARLY MAMMALS FED ON YOUNG DINOSAURS

ADDITIONAL DISCOVERY OF EVEN BIGGER, DOG-SIZED RELATIVE SUGGESTS MESOZOIC MAMMALS GREW TO BE MUCH LARGER THAN PREVIOUSLY THOUGHT

Two paleontologists from the American Museum of Natural History and their colleagues have studied a fossil of a 130-million-year-old opossum-sized mammal called Repenomamus robustus and found the remains of a psittacosaur in its stomach area. This fossil, discovered in China and described in a new paper in the journal Nature, is the first direct evidence that some primitive mammals fed on small vertebrates, including young dinosaurs. In the same paper, the team also has described the fossil of a much larger and very close relative of the psittacosaur eater, Repenomamus giganticus, which was the size of a small dog—larger than some dinosaurs that lived in the same region of China at this time. Together, these two fossil findings on mammals in the genus Repenomamus show that some Mesozoic mammals were carnivores, could grow to be much larger than previously thought, and competed with smaller dinosaurs for food and land. The dog-sized animal, R. giganticus, is the largest known mammal ever found with fairly complete fossil remains from the Mesozoic era (280 to 65 million years ago, just before dinosaurs and numerous other animals faced extinction). Although it resembled no animal living today, it is somewhat comparable in size and shape to a Tasmanian devil, a squat, carnivorous marsupial that today lives only on the island of Tasmania, southeast of Australia.

Model of a juvenile Psittacosaurus dinosaur

© AMNH / Denis Finnin

The authors of the paper are Meng Jin, Associate Curator in the Division of Paleontology at the American Museum of Natural History; and colleagues, including Wang Yuanqing, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing; and Dr. Meng's graduate student Hu Yaoming who studies at the Museum, is enrolled at the City University of New York, and also is a researcher at the IVPP. The fossil was discovered in rock composed of volcanic and former riverbed sediments in northeastern China's Liaoning Province, where numerous well-preserved non-avian dinosaur and bird fossils have been found in recent years.

Most Mesozoic mammals were the size of today's mice and rats, weighing a few pounds at most and at a distinct size disadvantage in the face of predatory dinosaurs. However, a full-grown R. giganticus probably weighed close to 30 pounds and could hold its own against small dinosaurs. The discovery of R. giganticus's size combined with the discovery of the R. robustus fossil with the psittacosaur in its stomach recasts scientists' understanding of Mesozoic animals. It is now clear that an adult Repenomamus could successfully take on a small or juvenile vertebrate.

"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," Dr. Meng said.

A full-scale model of how scientists think Repenomamus might have looked in life is being created for a 700-square-foot diorama depicting a prehistoric forest and its inhabitants to be featured in a groundbreaking new exhibition, Dinosaurs: Ancient Fossils, New Discoveries, opening at the Museum on May 14, 2005.

An Extremely Rare Finding of Stomach Contents

Fossil mammal R. robustus with dinosaur remains in its stomach

The last meal of the R. robustus was discovered in the fossil's preparation process, which involved painstakingly removing sediment surrounding the fossil. As the work was under way, preparators made an extremely rare discovery, possibly the first of its kind in a Mesozoic mammal—the animal's stomach contents. They were revealed as a patch of small bones within the adult mammal's ribcage near the vertebrae where the stomach would be in living mammals. The bones turned out to be the limbs, fingers, and teeth of a juvenile psittacosaur, a two-legged, parrot-beaked herbivorous dinosaur that was common in the area of China where R. robustus was found. Adult psittacosaurs, with short, deep heads and parrot-like beaks, grew to be nearly six feet tall and had four-fingered grasping hands. The baby psittacosaur found in this fossil was only five inches long, one-third the size of the animal that ate it. Wear marks on its teeth indicate it was not an embryo. Some of the psittacosaur's long bones were still connected to one another in the belly of the mammal, suggesting that Repenomamus swallowed the psittacosaur in chunks.

R. robustus had large, pointy incisors, canines, and premolars useful for catching, holding, and ripping prey, further evidence that this group of primitive mammals ate meat as well as plants. Its robust jawbones and deep pits on nearby bones suggest that large muscles powered this mammal's jaws. But its molars were small and blunt. Along with the evidence of the psittacosaur chunks, the teeth suggest that Repenomamus did not chew its food and that chewing evolved further up the family tree of mammals. The teeth and jaw muscles also suggest the animal was an aggressive predator, rather than a scavenger.

Discovery Shows Mesozoic Mammals Got Around

Model of R. giganticus,
an extinct badger-sized mammal

© AMNH / Roderick Mickens

Prior to the discovery of R. giganticus, Mesozoic mammals were thought to be primarily nocturnal insectivores, limited to tiny meals due to their miniscule size. Scientists hypothesized that these primitive mammals remained small because carnivorous and herbivorous reptiles, already on the evolutionary scene, prevented mammals from competing for the food and territory and evolving to become larger. Mammals never got a good chance to grow larger, scientists thought, because larger reptiles, including dinosaurs, lived longer, could move faster, and could travel further to find food due to their size advantage. The discovery of R. giganticus calls these hypotheses into question and shows that these large Mesozoic mammals probably roamed just as widely as some of the small dinosaurs with which they competed for food and territory.

When initially naming Repenomamus, scientists combined the words reptile and mammal to refer to the curious morphology of these "reptile mammals." The resemblance to reptiles can be seen in their large, sharp, pointy teeth and short limbs that stick out from their bodies at an angle. But their limb joints allowed more freedom of movement than is typical in reptiles. In fact, their limbs were more mobile than those of mammals such as the waddling platypuses. This additional freedom of movement, such as is found among marsupials and other more advanced mammals, helps carnivores compete for prey and elude predators. Although Repenomamus probably could not run fast, it could stand on its hindlimbs and walk effectively enough to stalk small prey-as this new fossil of R. robustus indicates.

The Liaoning Fossil Beds in China

Consisting of layers of volcanic and sedimentary rock, the Yixian Formation in China's Liaoning Province has yielded an enormous variety of fossil fish, birds, insects, reptiles, shrimp, flowers, mammals, and dinosaurs dating back to the late Jurassic and early Cretaceous periods-more than 128 million years ago. At that time, the region was dotted with freshwater lakes, streams, rivers, and volcanoes. Volcanic explosions rained fine ash into the lakes, and animals that died or fell into the water were quickly buried in the fine-grained sediment at the bottom where they were preserved with remarkable detail.

The work on R. giganticus and R. robustus was funded by the Major Basic Research Projects of Ministry of Science and Technology, China; National Natural Science Foundation of China; and the National Science Foundation.

More about the Upcoming Exhibition and New Dinosaur Discoveries

The new exhibition, Dinosaurs: Ancient Fossils, New Discoveries, curated by Mark Norell, Curator and Chairman of the Division of Paleontology at the American Museum of Natural History, will open in May and be on view through January 8, 2006. The exhibition will reveal how current thinking about dinosaur biology has changed dramatically over the past two decades, and will highlight ongoing cutting-edge research by Museum scientists and other leading paleontologists around the world. Dinosaurs will present the most up-to-date look at how scientists are reinterpreting many of the most persistent and puzzling mysteries of the dinosaurs-how they looked, how they behaved, how they moved-including, ultimately, the complex and hotly debated theories of why they became extinct.

Along with the Repenomamus model and dozens of other models of Mesozoic birds, mammals, fish, insects, plants, and reptiles, Dinosaurs will feature a full-size model of a psittacosaur and a remarkably preserved, 130-million-year-old fossil of a three-foot-long psittacosaur, the same kind of dinosaur found in the belly of the R. robustus fossil. Other exhibition highlights will include a new full-size cast of a skeleton of a Tyrannosaurus rex in a dynamic pose; a six-foot-long mechanical T. rex skeleton that will show how the dinosaur walked; recently uncovered fossils of a partial skeleton of a juvenile albertosaur; enormous apatosaur bones including vertebrae and tail portions; a Protoceratops skull; preserved cicadas, crayfish, and other arthropods; and many other specimens from the rich fossil beds of the Liaoning Province of China.

In the past decade, Dr. Norell has also been making annual visits to China to confer with paleontology colleagues at Beijing University; the Chinese Academy of Geological Sciences; and the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. During these visits, he studies the newest fossils collected from Liaoning Province and other recently discovered rich fossil beds in China. These visits also enhance a strong and highly productive informal exchange of scientists and research that has developed in recent decades between these Chinese institutions and the American Museum of Natural History.

Media Inquiries: Department of Communications, 212-769-5800

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