When dinosaurs gave way to the birds
Stunning feathered fossils unearthed from a 'Mesozoic Pompeii' in Liaoning offer insights into the dawn of bird life
In the autumn if 1995, I spent several days in Beijing's Institute of Vertebrate Palaeontology and Paleoanthropology, working on a story about dinosaur hunting. Though I heard tales of finding dinosaur bone beds, and saw fossils including a stegosaur tail club casually left on a sofa, my greatest surprise came as a professor offered to show me some new discoveries.
Weeks earlier, a new bird fossil from China had become world news. Known as the "holy Confucius bird", it was said to be almost as old as the first known bird, Archaeopteryx, but differed partly as its beak had no teeth. Just eight specimens of Archaeopteryx were known since the first was discovered in 1861, and this was only the second kind of bird from the Jurassic.
So I was startled when the professor took me to his room, opened a small box, and showed me another Confucius bird. Then he reached onto his shelves, and took down two new Jurassic bird fossils - one of which was the size of a warbler with a ridge along its breastbone indicating it was a far better flier than Archaeopteryx or the Confucius bird. Elsewhere in the institute, a technician was preparing another Confucius bird specimen, meticulously removing fine-grained rock to expose tiny bones, and feathers.
The professor, Hou Lianhai, had previously studied thousands of recent bird fossils from Zhoukoudian , the Peking Man site. With these new discoveries, he was shifting focus to what has proved to be one of the world's greatest fossil areas, yielding specimens including feathered dinosaurs and early birds.
The fossil beds are in Liaoning province in the mainland's northeast. Here, during the age of dinosaurs, the climate was temperate, and the landscape was dotted with lakes: excellent conditions for a rich variety of creatures. Volcanoes in Inner Mongolia sometimes erupted, showering the area with fine ash that preserved plants and animals in astonishing detail.
Although fossils tend to preserve only hard, durable shells and bones, this "Mesozoic Pompeii" features insects like water bugs, dragonflies and ants. Farmers have discovered they could make more money selling outstanding fossils than growing crops - the first Confucius bird specimen was bought in a market by an amateur palaeontologist who showed it to Hou and colleagues.
Although the fossils may be stunningly detailed, studying them has not always proved straightforward. A reassessment of the age of the Confucius bird and the other fossils that Hou showed me revealed that they are not from the Jurassic, but belong to the early Cretaceous - making them still among the earliest birds, but around 30 million years younger than Archaeopteryx.
But the biggest blooper with the Liaoning fossils arose in November 1999, when National Geographic loudly and proudly unveiled Archaeoraptor, billed as "a missing link between terrestrial dinosaurs and birds that could actually fly". National Geographic had published despite some scientists' reservations, and the next year had early bird's egg on its face, retracting the story with an acknowledgement that the specimen was a "composite". Its top half came from one species, the bottom from another.
Creationists seized on the debacle to claim it was evidence against the theory of evolution; one website absurdly and unscientifically concluded, "God's word never changes and must therefore be the basis for all our thinking". But for anyone really thinking, the Liaoning fossils offer a wealth of insight into the origins of birds, albeit without simplistic missing links.
The most important fossils include feathered dinosaurs, which are clearly not birds but strongly substantiate the theory that birds evolved from dinosaurs. This notion was first proposed by British biologist Thomas Huxley in the late 19th century, as he observed similarities between Archaeopteryx and a small theropod. Theropods are two-legged dinosaurs that include Tyrannosaurus rex, as well as a host of smaller species.
Arguments have raged over whether such similarities are merely coincidental, with some scientists believing birds were derived from ancient reptiles. But the relative wealth of feathered theropods found in Liaoning has made the reptile origin look increasingly unlikely. These feathers may have first evolved to regulate temperature, as well as for colourful plumages to attract mates.
By far the largest of the feathered dinosaurs was Yutyrannus huali, a nine-metre- long carnivore weighing over a tonne, whose name means "beautiful feathered tyrant". No one is suggesting this is a bird ancestor, though it may be allied to the younger T. rex, which went extinct along with other dinosaurs at the end of the Cretaceous, 65 million years ago. Professor Xu Xing of the Beijing palaeontology institute led the study of Yutyrannus, and suggested its feathers had a display function.
Feathers did, however, help small dinosaurs fly - such as the four-winged dinosaur Microraptor. Less than a metre long and weighing under a kilogram, this had extensive feathering along both its front and hind legs, and may have glided between trees rather like today's flying squirrels.
Then there are the bird fossils found alongside the dinosaurs. Confucius birds are the most abundant, with hundreds of specimens now in museums. They are the first known birds with true beaks, though perhaps belong to a side branch of birds that became extinct.
At times, it is hard to tell if a fossil should be classed as a dinosaur or a bird. In 2011, Nature reported that the discovery of a new Liaoning dinosaur had, "knocked Archaeopteryx off its perch as the first bird". This was partly because the skull shape of Archaeopteryx was similar to the new dinosaur, but differed from early birds. Soon, counter-arguments appeared.
And last month, Nature published a study by researchers including Pascal Godefroit of the Royal Belgian Institute of Natural Sciences, which restored Archaeopteryx to the bird lineage, and described a new species they considered to be an even earlier bird, Aurornis xui. "In my opinion, it's a bird," Godefroit told Nature. "But these sorts of hypotheses are very controversial. We're at the origins of a group. The differences between birds and [non-avian] dinosaurs are very thin."
Given the slender differences and the abundance of Liaoning fossils, expect more remarkable insights into the dawn of the birds. And as you walk around and see black kites soaring over the city, you might wonder: since dinosaurs evolved into birds, what might today's birds evolve into in future?
Martin Williams is a Hong Kong-based photographer and writer specialising in conservation and the environment. He holds a PhD in physical chemistry from Cambridge University