Fossils suggest a greater role for mammals

Researchers have dubbed the new group of creatures 'reptile-mammals.'
SCRIPPS HOWARD NEWS SERVICE
Mammals have long been considered the Rodney Dangerfields of dinosaur times. Our distant ancestors have been seen as inconsequential wannabes, hiding out in burrows or scurrying about at night to avoid getting squished or eaten by the giant reptiles.
But 130 million-year-old fossils found in China, described Wednesday in the journal Nature, are suggesting that ancient mammals deserve more respect.
Although most mammals were diminutive compared to the biggest dinos, some were at least the size of a large dog, had the teeth and jaws of voracious carnivores -- and included smaller dinosaurs on their menu.
"Traditionally, any artistic representation of the Mesozoic Era [140 million to 65 million years ago] you see has large dinosaurs in the middle, the central figures of the environment, and tiny mammals off in the corner, if you see them at all," said Meng Jin, curator of paleontology at the American Museum of Natural History in New York City and senior author of the report.
"With these new specimens, we change that picture a little bit and challenge the view that mammals were small and did nothing but survive while waiting for the dinosaurs to get out of the way."
Standard view
The standard view has been that mammals were forced to stay small simply because the dinosaurs were so large.
However, the new findings at least raise the possibility that some dinosaur species faced evolutionary pressure to grow larger -- or to develop more birdlike features -- to thwart predatory mammals, writes Duke University paleobiologist Anne Weil in a perspective article in Nature.
"Maybe these small dinosaurs got larger, or got off the ground, to avoid the rapacious mammals," she said.
Meng and colleagues at Beijing's Institute of Vertebrate Paleontology and Paleoanthropology have dubbed the new group of creatures "reptile-mammals" -- Repenomamus -- because their teeth and even their short limbs somewhat resemble those of predatory dinosaurs, although they were mammals in other respects.
The fossils are "superbly preserved in three dimensions, a rare level of detail with a lot of ske
letal and skull material," Meng said. That's because they were buried soon after they died in muck comprised of river sand and volcanic ash that quickly hardened around the bones.
"It looks as though they may have been curled up asleep when they died, perhaps poisoned by volcanic fumes," the researcher said.
The resulting rocks from that slurry in what is now northeastern China's Liaoning Province have yielded numerous well-preserved fossils of a variety of dinosaurs, birds and other animals in recent years, including a skull, but no other remains, of the smaller of the two new specimens.
Direct evidence
That creature was a opossum-sized animal called Repenomamus robustus.
A complete robustus skeleton found by Meng and his colleagues was not only quite detailed itself, but it also had the remains of part of a juvenile dinosaur called a psttacosaur in its stomach. This is the first direct evidence that primitive mammals fed on other vertebrates and didn't just subsist on plants and insects.
The teeth of the parrot-sized dinosaur are slightly worn, indicating it was caught on the run, and not snatched from an egg or nest.
Another Chinese find by the researchers was the complete skeleton of a relative, Repenomamus giganticus.
It was not exactly a giant, but was nearly a yard long from nose to tail and had the bulk of a large modern dog -- far larger than mammals the size of today's mice and rats that until recently made up almost all the known Mesozoic mammal family.
Although giganticus resembled no currently living animal -- in fact, it has no direct descendants -- the creature is somewhat comparable to a Tasmanian devil, a squat, carnivorous marsupial that today lives only on the island of Tasmania, south of Australia.