In a pine forest in rural northeastern China, a rugged shale slope is packed with the remains of extinct creatures from 125 million years ago, when this part of Liaoning province was covered with freshwater lakes. Volcanic eruptions regularly convulsed the area at the time, entombing untold millions of reptiles, fish, snails and insects in ash. I step gingerly among the myriad fossils, pick up a shale slab not much larger than my hand and smack its edge with a rock hammer. A seam splits a russet-colored fish in half, producing mirror impressions of delicate fins and bones as thin as human hairs.
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One of China's star paleontologists, Zhou Zhonghe, smiles. "Amazing place, isn't it?" he says.
It was in 1995 that Zhou and colleagues announced the discovery of a fossil from this prehistoric disaster zone that heralded a new age of paleontology. The fossil was a primitive bird the size of a crow that may have been asphyxiated by volcanic fumes as it wheeled above the lakes all those millions of years ago. They named the new species Confuciusornis, after the Chinese philosopher.
Until then, only a handful of prehistoric bird fossils had been unearthed anywhere in the world. That's partly because birds, then as now, were far less common than fish and invertebrates, and partly because birds more readily evaded mudslides, tar pits, volcanic eruptions and other geological phenomena that captured animals and preserved traces of them for the ages. Scientists have located only ten intact fossilized skeletons of the earliest known bird, Archaeopteryx, which lived at the end of the Jurassic period, about 145 million years ago.
Zhou, who works at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences in Beijing, believed that the extraordinary bone beds in Liaoning might fill in some of the many blanks in the fossil record of the earliest birds. He couldn't have been more prophetic. In the past 15 years, thousands of exquisitely preserved fossil birds have emerged from the ancient lakebed, called the Yixian Formation. The region has also yielded stunning dinosaur specimens, the likes of which had never been seen before. As a result, China has been the key to solving one of the biggest questions in dinosaur science in the past 150 years: the real relationship between birds and dinosaurs.
The idea that birds—the most diverse group of land vertebrates, with nearly 10,000 living species—descended directly from dinosaurs isn't new. It was raised by the English biologist Thomas Henry Huxley in his 1870 treatise, Further Evidence of the Affinity between the Dinosaurian Reptiles and Birds. Huxley, a renowned anatomist perhaps best remembered for his ardent defense of Charles Darwin's theory of evolution, saw little difference between the bone structure of Compsognathus, a dinosaur no bigger than a turkey, and Archaeopteryx, which was discovered in Germany and described in 1861. When Huxley looked at ostriches and other modern birds, he saw smallish dinosaurs. If a baby chicken's leg bones were enlarged and fossilized, he noted, "there would be nothing in their characters to prevent us from referring them to the Dinosauria."
Still, over the decades researchers who doubted the dinosaur-bird link also made good anatomical arguments. They said dinosaurs lack a number of features that are distinctly avian, including wishbones, or fused clavicles; bones riddled with air pockets; flexible wrist joints; and three-toed feet. Moreover, the posited link seemed contrary to what everyone thought they knew: that birds are small, intelligent, speedy, warmblooded sprites, whereas dinosaurs—from the Greek for "fearfully great lizard"—were coldblooded, dull, plodding, reptile-like creatures.
In the late 1960s, a fossilized dinosaur skeleton from Montana began to undermine that assumption. Deinonychus, or "terrible claw" after the sickle-shaped talon on each hind foot, stood about 11 feet from head to tail and was a lithe predator. Moreover, its bone structure was similar to that of Archaeopteryx. Soon scientists were gathering other intriguing physical evidence, finding that fused clavicles were common in dinosaurs after all. Deinonychus and Velociraptor bones had air pockets and flexible wrist joints. Dinosaur traits were looking more birdlike all the time. "All those things were yanked out of the definition of being a bird," says paleontologist Matthew Carrano of the Smithsonian National Museum of Natural History.
But there was one important feature that had not been found in dinosaurs, and few experts would feel entirely comfortable asserting that chickadees and triceratops were kin until they had evidence for this missing anatomical link: feathers.
A poor Chinese farmer, Li Yingfang, made one of the greatest fossil finds of all time, in August 1996 in Sihetun village, an hour's drive from the site where I'd prospected for fossil fish. "I was digging holes for planting trees," recalls Li, who now has a full-time job at a dinosaur museum built at that very site. From a hole he unearthed a two-foot-long shale slab. An experienced fossil hunter, Li split the slab and beheld a creature unlike any he had seen. The skeleton had a birdlike skull, a long tail and impressions of what appeared to be feather-like structures.