Ask Mark Norell about the heyday of dinosaurs, and he'll tell you that it isn't over yet - that today, dinosaurs thrive and are more diverse than at any time in the planet's history.
Look no farther than your bird feeder for proof.
This widely held view of how birds have extended the dinosaurs' lineage has long been a controversial one. But that theory received some of its strongest support yet as the National Geographic Society in Washington unveiled detailed fossils of two feathered creatures on June 23.
At least 120 million years old, the two dinosaurs are filling crucial gaps in the evolutionary history of modern birds. In the process, they are challenging paleontologists' notions of what is a bird and what is a dinosaur and providing important clues about the development of feathers and flight.
"These are the sort of fossils you dream about," says Norell, chairman of vertebrate paleontology at the American Museum of Natural History in New York and one of four scientists involved in analyzing the specimens. "At this point, the evidence indicating that birds are derived from dinosaurs is absolutely convincing."
The creatures were unearthed from deposits in China's Liaoning Province, the source for more fossils that bear on the dinosaur-bird kinship than anywhere else in the world, researchers say.
The more primitive of the two animals is about the size of a turkey, with inch-long, plume-like feathers along its legs and spine. A clump of feathers also appears near its chest. Because it resembles Archaeopteryx, the earliest-known bird, the research team named the creature Protarchaeopteryx robusta.
The second animal, Caudipteryx zoui, is a closer relative of Archaeopteryx and modern birds. It sports a fan of plumes at the end of its tail. Other portions of Caudipteryx include the remains of down-like feathers, suggesting that most of its body was cloaked.
They're still dinosaurs
After scrutinizing 90 different physical traits in the fossils, the researchers concluded that the specimens had not crossed the threshold from two-legged, meat-eating dinosaurs to birds. Their analysis suggested that Protarchaeopteryx may be closely related to Velociraptors, while Caudipteryx was closer to, but still more primitive than, Archaeopteryx.
Yet the feathers on both new specimens are decidedly birdlike, says Philip Currie, curator of dinosaurs at the Royal Tyrrel Museum of Paleontology in Drumheller, Alberta. Although the feathers are too symmetrical to allow flight, he says, images of the feathers taken by a scanning-electron microscope clearly show features found in feathers today, such as shafts, barbs, and the tinier interlocking barbules that hold the barbs together.
Such features never would have survived the fossilization process were it not for the unique geology of the area, the researchers say. Ordinarily, organic material such as feathers would have rotted away long before the skeletal remains became stone. But the grains of sediment were so small and tightly packed that the layer was devoid of oxygen. Bacteria that ordinarily would break down organic material couldn't survive there.
Interestingly, the Caudipteryx also contains tiny pebbles in its stomach, ingested to help the creature digest its food - a technique birds use today.
The official scientific report detailing the latest finds will appear in the June 25 issue of the journal Nature.
Ironically, the same clues that are tightening the bonds between birds and small theropod dinosaurs "are blurring the definition of what precisely constitutes a bird," according to Ji Qiang, a geologist at the National Geological Museum of China in Beijing and head of the team.
Until now, some reference books on fossils view feathers as a feature exclusive to birds. That relationship no longer holds, Norell notes.
The evolution of flight
These discoveries also help answer questions about why feathers evolved in the first place, notes Jacques Gauthier, a curator at Yale University's Peabody Museum in New Haven, Conn.
"There was a little battle going one about whether feathers arose for flight, since they seem so perfectly suited to it," he says, "Now we're seeing these things on animals that did not fly; they're too large and their arms are too short."
Instead, feathers probably arose to keep in body heat, he continues.
The emergence of feathers on land-runners also could help settle another long-running debate: whether flying developed from the ground up or from the trees down, or something in between.
Coupled with other recent fossil finds from China as well as Argentina, these latest discoveries are giving researchers new insights into "how this whole flight apparatus self-assembled," Dr. Gauthier says.
From the skeletal remains of theropod arms, "we known that they had already evolved flight strokes" and used the movements to catch prey, he says. "You have a flight stroke evolving in a context having nothing whatever to do with flight. You have feathers appear having nothing to do with flight. Then both of those adaptations collide in one beast, and lo and behold, it's off the ground."
"It's really spectacular stuff that they're getting out of these fossils," he says.