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
Dr. Meng Jin, (left), Associate Curator, Division of Paleontology; his graduate student Hu Yaoming, (center), and Dr. Wang Yuanqing, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, today unveiled at the American Museum of Natural History in New York City a 130-million-year-old opossum-sized mammal, Repenomamus robustus, with a small dinosaur, a psittacosaur, preserved in its stomach area. The fossil is the first direct evidence that some primitive mammals fed on small vertebrates, including young dinosaurs. Credit: Roderick Mickens, American Museum of Natural History.
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 doglarger 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 mammalthe 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,