n the time of dinosaurs, mammals were the meek that had yet to inherit the earth. They were small creatures, many no bigger than mice, and essentially nocturnal, feeding mainly on insects and cowering in holes and underbrush from the terrible tread of the reptilian lords of the land.
Two newly discovered fossils show that this lowly image of early mammals, long the reigning view of science, did not do them justice. A few of these animals were as large as a dog and spunky enough to devour dinosaurs, at least juvenile dinosaurs.
The 130-million-year-old fossils, announced yesterday by Chinese paleontologists, challenge conventional thinking and lead to a new and more diverse perception of mammal life in the Mesozoic era, 280 million to 65 million years ago.
In interviews and a report being published today in the journal Nature, the researchers described finding the skull and most of the bones of what they say is the largest mammal known to live in the age of dinosaurs. The animal's skull was half again the length of the next largest mammal of the period. The entire body probably weighed 30 pounds and stretched more than three feet, longer than a good-size basset hound's.
From the same fossil beds in northern China, the paleontologists also uncovered the remains of a related species about 15 inches long, the size of an opossum, and made a striking observation. The mammal's last meal had been a juvenile dinosaur. Its limbs, fingers and teeth were lodged within the mammal's rib cage where its stomach had been.
The dog-size animal has been named Repenomamus giganticus. The smaller one is a specimen of Repenomamus robustus.
"Our discoveries," the scientists wrote in the journal report, "constitute the first direct evidence that some of these mammals were carnivorous and fed on small vertebrates, including young dinosaurs, and also show that Mesozoic mammals had a much greater range of body sizes than previously known."
The scientists further concluded that "Mesozoic mammals occupied diverse niches and that some large mammals probably competed with dinosaurs for food and territory."
Dr. Anne Weil, a paleontologist at Duke University who was not involved in the research, agreed that the fossils were a fascinating discovery and certain to shake up the field of early mammal studies.
In an accompanying article in Nature, Dr. Weil said, "These latest finds should trigger another avalanche of questions and speculation."
She and other paleontologists said several recent discoveries had yielded clues suggesting that some species were at least as large as robustus and might have been as large as giganticus. But the new fossil skeletons are more complete and definitive, they said.
Dr. Jin Meng, a paleontologist at the American Museum of Natural History in Manhattan and a member of the discovery team, said that any Mesozoic mammal remains were rare and that these were "giving us a drastically new picture" of many of the animals of the age of dinosaurs.
Standing in the museum laboratory with the carefully cleaned bones of the mammal that ate the little dinosaur, Dr. Meng said, "Now we have to see how common was the phenomenon of these larger, carnivorous species."
But in one respect, he added, the general pattern of Mesozoic life remained unchanged: although other large early specimens may be found, most of the mammals were still small and no match for the dominant reptiles. Primitive mammals presumably had little chance to evolve in stature because the dinosaurs and other reptiles were stronger, lived longer and moved faster than the mammals in the competition for food and favorable habitats.
Only with the extinction of dinosaurs 65 million years ago could the mammals begin to assert themselves and flourish in size and diversity, evolving into lions and tigers and bears and, in time, humans who are curious about life when the world was younger.
The two skeletons were collected in 2003 by farmers at the abundant fossil deposits in Liaoning Province in China, where many dinosaurs and a sprinkling of their mammalian contemporaries have been uncovered in recent years. The fossils caught the eye of visiting scientists, who bought them and took them to the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing for detailed examination.
Yaoming Hu, a researcher at the institute and a graduate student the City University of New York, is the lead author of the journal report of the discovery. The co-authors are Dr. Meng, Mr. Hu's dissertation adviser; and Dr. Yuanqing Wang and Chuankui Li, both of the Beijing institute.
An artist's rendering of the two species, giganticus and robustus, showed animals with low-slung bodies with short legs and long tails, as suggested by the fossils. They are covered in dark, short-haired fur, for which there is no direct evidence; if the animals were primarily nocturnal, it is assumed that evolution would have favored those with dark fur for concealment.
The researchers said giganticus, though it resembled no animal living today and has no living descendants, was somewhat comparable to a Tasmanian devil, a squat, carnivorous marsupial living on the Australian island of Tasmania. Analysis of the giganticus teeth indicated that the specimen was an adult. Its combined head and body was 60 percent longer than that of its robustus cousin.
The well-preserved remains of giganticus are at the paleontology institute in Beijing. A full-scale replica is being prepared for a new dinosaur exhibition at the American museum, opening in May.
Geological dating of sediments in the fossil beds showed that both species lived at approximately the same time, about 130 million years ago. The animals that were fossilized in the region probably died in volcanic eruptions.
The first fossils of robustus were excavated in 2000, when it was recognized as the largest known Mesozoic mammal represented by substantially complete remains. Dr. Wang was one of the discoverers. The specimen was assigned the genus Repenomamus, combining words for reptiles and mammals to reflect the animal's reptile-mammal attributes.
The big surprise about the new robustus fossils being reported now was not revealed until researchers had a close look in the Beijing laboratory. Mr. Hu pointed to the skeleton, in the American museum lab, and showed the patch of small bones in the rib cage. They were the tiny limbs, fingers and teeth of a juvenile psittacosaur, a two-legged herbivorous dinosaur common in the Chinese fossil beds.
The baby dinosaur, Mr. Hu said, was only five inches long, a third the size of the animal that ate it. An adult psittacosaur was often six feet tall. Judging by the mammal's teeth and jaws, it did not chew its food, but swallowed the dinosaur in chunks.
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