When Yale paleontologist John H. Ostrom first suggested in 1973 that birds are directly descended from dinosaurs, the idea was greeted with skepticism and even amusement. However, the recent discovery in China of a fossilized, chicken-sized dinosaur sporting something that looks like downy fluff along its spine has rekindled interest in Professor Ostrom's theory.
The scientist -- who returned in late March from a trip to rival museums in Beijing and Nanjing where mirror-image halves of the newly discovered fossil are housed -- still seems stunned by what he saw. Although he stops just short of calling it a feathered dinosaur, he says the fossil is the most exciting evidence yet supporting the dinosaur origin of birds and explaining their evolution.
"I never expected to see anything like this in my lifetime," Professor Ostrom says. "I literally got weak in the knees when I first saw photos last October. The apparent covering on this dinosaur is unlike anything we have seen anywhere in the world before -- quite different from modern feathers or hair, but also different from the skin of other dinosaurs."
A team of world-renowned scientists was assembled for the recent trip to China by the Academy of Natural Sciences in Philadelphia, where a news conference was held April 24 to announce the team's findings. In addition to Professor Ostrom, the team included Professor Larry Martin of the University of Kansas, a skeptic of the dinosaur-bird link; Alan H. Brush of the University of Connecticut, who is an authority on bird feathers; and Dr. Peter Wellnhofer of the Bavarian State Museum in Munich, Germany, an authority on the earliest bird, Archaeopteryx, and the dinosaur Compsognathus, both discovered in Bavaria.
Thought to be between 120 million and 140 million years old, the small Chinese dinosaur was probably carnivorous, like all theropod -- two-legged -- dinosaurs from which Professor Ostrom believes birds are descended. The fossil, which Beijing scientists Ji Qiang and Ji Shuan of the Chinese Geology Museum recently named Sinosauropteryx prima, has stubby forelimbs and probably was incapable of flight. Its downy back covering may have served primarily as insulation or ornamentation, says Professor Ostrom, who noted that the Chinese fossil is very similar to Compsognathus longipes, a Bavarian dinosaur with stubby forelimbs that he spent months studying in 1978 in a vain attempt to find signs of feathers.
"An insulating covering might indicate the Chinese fossil is that of a warm-blooded animal, as would its upright stance. Cold-blooded animals are sprawlers -- they are low to the ground -- while all upright animals are warm-blooded," says Professor Ostrom, who in 1969 first hypothesized that some dinosaur species were warm- blooded.
The bird-dinosaur proposal of 1868
Professor Ostrom is quick to note that he was not the first scientist to suggest a direct link between dinosaurs and birds, but merely the first to revive a theory proposed more than 100 years earlier -- in 1868 -- by British biologist Thomas Henry Huxley, who noted that a fossil unearthed from a limestone quarry near Solnhofen, Germany, had characteristics of both reptiles and birds. Named Archaeopteryx lithographica -- Archaeopteryx means "ancient wing" -- the 150-million-year-old feathered animal with teeth is the missing link between reptiles and birds, Huxley argued, an affirmation of Charles Darwin's evolution theories.
In 1926, respected Danish paleontologist Gerhard Heilmann noted that so-called primitive birds lacked clavicles, the theorized evolutionary precursor of the wishbone found in modern birds that emerged about 70 million years ago. He argued that any link between dinosaurs and birds was indirect, tracing back to a common ancestor -- a reptile -- 200 million years ago or more. Meanwhile, the search has continued for small, bipedal reptiles from the Jurassic period with bird-like characteristics that could prove a direct link between dinosaurs and birds.
After viewing the Chinese fossil, the team of scientists agreed that the question of avian origins remains unsettled, despite this new evidence. But the answer could come soon, they say, because the fossil bed in the Liaoning Province where a farmer found the fossil last summer is "a global treasure." It contains fossils from a critical period between 120 million and 150 million years ago when rapid climate change gave rise not only to the first birds, but also the first flowering plants and many of the first insect species.
'New month' in geological calendar
"Nowhere else in the world are fossils from such a critical time so well preserved," says Professor Ostrom, noting that the volcanic ash in which the fossils are encased should yield accurate radiometric dates. "We are terribly excited because the site almost certainly will yield other extremely important fossils. It appears to be an entirely new page, a new month in our calendar of the known geological record -- a month that was a burgeoning springtime for life on earth."
While scientists at the Nanjing Paleontology Institute have not yet published findings about their half of the fossil, scientists at the Chinese Geology Museum in Beijing reported in the latest issue of the journal Chinese Geology that the "primitive feature of the fossil suggests it is the earliest bird in the world" and that it represents the transition from small bipedal dinosaurs to birds.
The Beijing scientists also supported Professor Ostrom's claim that the fossil was a carnivore or omnivore that could not fly, but ran like the modern ostrich, possibly replacing Archaeopteryx as the oldest ancestor of birds. They argued that the fossil proved Professor Ostrom's theory that birds evolved from small bipedal dinosaurs, and that the western Liaoning Province was the origin and evolutionary center of birds in the world during the late Jurassic and early Cretaceous periods.
Professor Ostrom hypothesizes that the upright stance of bipedal dinosaurs freed their forelimbs "to do something different," such as becoming flapping wings that eventually gave birds powered flight. "Animals that can either run or fly have a huge evolutionary advantage," he says, "which explains why there are twice as many bird species today as there are species of all other air-breathing vertebrates combined." He likens the evolutionary success of birds to that of insects, many of which can both run and fly.
While scientists now agree that a far greater variety of early birds lived alongside dinosaurs 120 million years ago than had been thought, they are far from agreement about the origin of birds and the link between dinosaurs and birds. The answer may well lie beneath the ground in China's Liaoning Province, says Professor Ostrom, and in further study of the recently unearthed fossil.