Recent excavations
in China’s Liaoning province have uncovered a well-preserved complete skeleton
of a dog-sized mammal, alongside a cat-sized mammal that had the remains of
its last supper — a young dinosaur — fossilized in its stomach. The
findings not only are the first evidence of a mammal eating a dinosaur, but
also open the door for more speculation on the size and development of mammals.
These two fossils, of Repenomamus giganticus — a new species and
the larger of the two — and Repenomamus robustus, were excavated
out of the fossil-rich Yixian Formation by local residents in 2002 and 2003,
says Yaoming Hu, a paleontologist with the American Museum of Natural History
in New York City, and lead author of a paper describing the finds in the Jan.
13 Nature. Fossils of dinosaurs, mammals, frogs, prehistoric birds, plants,
insects and more have come out of the Yixian Formation. Radiometric dating puts
the site at 128 to 139 million years old, during the peak of the “Age of
Dinosaurs,” when mammals were thought to be rodent-sized insect-eating
critters that cowered in the shadows of dinosaurs and only came out at night
to forage.
Excavations in China led to the discovery of two important 130-million-year-old
fossilized mammals: Repenomamus giganticus, the largest mammal found
to date from the Cretaceous, and Repenomamus robustus (left), a cat-sized
mammal found with a small dinosaur preserved in its stomach area (the tangled
mass in the middle of the fossil), thus providing the first direct evidence
that mammals preyed on dinosaurs. Photos courtesy of Yaoming Hu, AMNH.
Traditionally, paleontologists have thought that mammals were “forced to
remain small by a combination of heavy predation pressure from dinosaurs and
the saturation of ecological niches by large reptiles,” wrote Anne Weil,
a paleontologist at Duke University in North Carolina, in an accompanying commentary
in Nature. Now it appears that mammals may have produced predation pressure
on dinosaurs as well, she says: Perhaps mammals might have affected how quickly
dinosaurs grew and evolved instead of the other way around.
For animals, “body size is one of the most important factors in influencing
life history strategy,” Hu and colleagues wrote. These large Mesozoic mammals
likely competed with dinosaurs for food and territory, he says.
“In the Mesozoic, the rule was that no matter what size you were, there
was always someone ready to eat you,” says Kevin Padian, a paleontologist
at the University of California, Berkeley. And because mammals are carnivorous
by default, it shouldn’t be a surprise, he says, to find that larger mammals
would have preyed on anything smaller than themselves, including dinosaurs.
R. giganticus, the new species of Repenomamus, was a 26- to 31-pound
mammal about 3 feet in length, with proportionally shorter legs than most of
today’s mammals, and likely most resembled a Tasmanian devil. The new R.
robustus fossil represents a mammal about 9 to 13 pounds and less than 1.5
feet in length and was found in a position curled up like a sleeping dog. In
the place where the animal’s stomach would have been was a 5.5-inch-long
juvenile Psittacosaurus that seemed to have been ripped apart and swallowed
in large pieces.
R. robustus fossils have been found before, though not as well-preserved
and certainly not with dinosaur bones inside, Hu says. Finding a dinosaur in
the new specimen’s stomach is “as strong an indication as we’re
going to get that mammals preyed on dinosaurs,” Weil says.
“This is a good ‘man-bites-dog’ story,” Padian says, “and
I hope this find makes people more aware of the diversity of mammals during
the Mesozoic, but I’m not sure it will overturn any paradigms.”
Megan Sever
Back to top
 |
Geotimes Home | AGI Home | Information Services | Geoscience Education | Public Policy | Programs | Publications | Careers  |