Zhou Zhonghe was a recently graduated palaeontology student when he went on a trip to western Liaoning Province in Northeast China in the summer of 1990.

He was part of a research team from the Beijing-based Institute of Vertebrate Palaeontology and Palaeoanthropology in search of fish fossils in the Chaoyang region, which is known for its rich animal and plant fossils.

The sunlight was scorching. Zhou and his colleagues scouted around by day, hoping for some exciting discoveries. He did not have to wait too long before finding something valuable by a creek running through a valley - a paddlefish fossil.

When the field work was over, Zhou and the team returned to Beijing with a good collection of fossils. But Zhou was not very satisfied - the small valley still lingered in his mind and he sensed that something big might still lie out there.

When it was getting cooler in September that year, Zhou returned alone to the valley to try his luck. He did not find another paddlefish fossil this time, but two bird fossils from a hard marlite (marlstone) rock.

The two fossilized birds were later identified to be of two distinct categories. One of them, with the whole of its cranium almost intact, was identified as the most complete bird fossil of the early Cretaceous period ever found at that time.

One of many finds

The significance of Zhou's discovery might well be understood by any palaeontologists studying the origin of birds. Bird fossils of the early Cretaceous period, some 120 million to 125 million years ago, have been very rare, especially those with crania.

Yet they represent a critical stage in the evolution of birds from the emergence of archaeopteryx 150 million years ago to that of modern birds less than 100 million years ago.

Zhou's discovery was followed by a series of large-scale explorations by institute researchers around the site in the following two years. They found 20 some bird fossils, including the 120-million-year-old Cathayornis.

These discoveries were followed in the subsequent decade by the finding of an even larger collection of bird and dinosaur fossils in the region, shaping some of the most contentious debates in palaeontology.

Besides early birds, feathered theropod dinosaurs were discovered on a continuous basis and these provide ever stronger support for the hypothesis that birds evolved from the bipedal theropod dinosaurs.

New evidence was also found on the origin of feathers and flight and this refreshed human understanding of the theropods' relationship with birds.

"New discoveries produced interesting results that suggest feathers may not be unique to birds and that flight might have evolved from gliding by tree-dwelling creatures," said Zhou, now an internationally established avian palaeontologist.

He said the discoveries, which belong to the world-famous Jehol biota, have influenced his academic interest and directed him from fish fossils to birds, which have been the focal point of the world-stunning palaeontological finds in China over the past decade.

Discoveries are still being made today, at a rate that "outstrips the rate of description," he said in an article reviewing the Jehol biota that was published in the scientific journal Nature late last month.

Overwhelming if not yet conclusive evidence has emerged in the biota - the region's animal and plant life - to shed light on some long-standing enigmas, such as the origin of birds. New queries have also been raised, along with new discoveries, that either challenge established viewpoints or raise new ideas in palaeontology.

Blurring line

Although the Jehol biota has been known among the palaeontological community for decades for its bountiful fossils, it was not until the mid-1990s that it drew worldwide attention after feathered dinosaurs and early birds were discovered.

The discoveries included the feathered dinosaurs Sinosauropteryx, Protarchaeopteryx and Caudipteryx, the primitive birds Confuciusornis and Liaoningornis, in addition to a wide range of fossil mammals, plants and insects.

Zhou said the dinosaurs unearthed since the mid-1990s are the first to be found covered with feathers.

The bird-like dinosaurs and dinosaur-like birds were found near the villages of Jianshangou and Sihetun, in rocks aged between 120 million and 130 million years old.

The first dinosaur found with feathers was named Sinosauropteryx prima. Its skeleton was surrounded by a halo of feathers.

Sinosauropteryx's very simple feathers were almost hair-like and could not allow it to fly.

Caudipteryx and Protarchaeopteryx were small dinosaurs with a downy covering of feathers over most of their bodies but they also had longer, more complex feathers on their arms and tails, arranged like those of modern birds. These long feathers had a central shaft with barbs of equal length on either side.

Like modern birds, these dinosaurs may have used long, patterned feathers to recognize one another, to attract mates and warn off rivals. They may also have used the long feathers to help cover and insulate their eggs and chicks in nests.

These dinosaurs had also developed a special bone in the wrist that enabled them to fold their long arms against their body, just as birds do today. It also allowed them to move their hands in a broad fan-shaped motion, which has today become an important part of the flight stroke in modern birds.

Some of the oldest known birds have been found in Liaoning Province and most have been identified as living between 120 million and 145 million years ago.

These birds lacked the long bony tail of their ancestors and had larger, keeled breast bones and true flight feathers, suggesting that they could fly for some distance.

With new discoveries being found regularly, Zhou admitted that the line between dinosaurs and early birds has been blurred in some ways, as the bodily features previously considered as being unique to one class, such as feathers, were later found to be shared by the other.

While debate has been aroused over whether the hair-like integumentary structures covering the newfound dinosaurs were true feathers or just filaments, a theropod called Microraptor discovered by Zhou's colleague Xu Xing further tightens the link between birds and dinosaurs.

The crow-sized Microraptor represents the most bird-like dinosaur found so far. Its integument, characterized by a rachis, appears to be exactly like feathers preserved on the bodies of birds in the same rock. It also has other features that further ally it with birds, such as the clawed feet that may have been adapted for tree-dwelling.

In the middle of last year, Zhou and his colleagues published information on their discovery of Jeholornis, which he described as the most dinosaur-like bird ever found. Jeholornis combines features typical of dinosaurs with features typical of more advanced birds.

Unlike other early Cretaceous birds, the turkey-sized Jeholornis had a 42-centimetre tail consisting of at least 22 individual bones, just like a dinosaur's tail. Yet it also sported an advanced shoulder girdle capable of powering flight.

In an article published in January this year in Nature, Xu, Zhou and his colleagues proposed that the most recent ancestor of birds and dromaeosaurs, the closest relative of early birds, was a four-winged creature that lived in and glided among trees.

Zhou said the similarity in anatomic features has increased the difficulty in distinguishing early birds from dinosaurs in some ways. "One major difference left now is that, I would say, birds fly by flapping, while the tree-dwelling, feathered dinosaurs could only fly for any distance by gliding," he said.

With new discoveries expected from the biota, it will be no surprise if the fossil of a dinosaur that flew by flapping is found one day, Zhou said. That might lead palaeontologists to rethink the notion of the bird-dinosaur divide.

"Some palaeontologists have already said that dinosaurs did not become extinct - they are just flying in the air," he said.

Refugium or cradle or both?

New discoveries from the Jehol biota have also raised the question of whether it was a refugium of late Jurrasic fauna or a cradle of new life.

Although radiometric dating provides overwhelming evidence of Early Cretaceous life there, the closest relatives of several Jehol animals and plants are from Late Jurassic or even older deposits.

Some of the relicts - animals or plants living in an environment that has changed from that which is typical for them - have never been found in any other post-Jurassic deposits.

This pattern has prompted the hypothesis that East Asia acted as a refugium in the early Cretaceous period for some of these more typically Jurassic species.

This hypothesis is supported by geological theories that East Asia seems to have been isolated from the rest of the Laurasia supercontinent from the Middle Jurassic to the Early Cretaceous period.

Yet the Jehol biota is also characterized by its having a great deal more fauna and flora that are typical of the Cretaceous period.

Zhou proposed that the flourishing of the Jehol biota during the time when East Asia's isolation ended may offer a partial explanation for the mixed composition of the region's animal and plant life.

The presence of the mixed groups of fossilized birds, dinosaurs and plants "may simply reflect the long period of isolation through the Middle Jurassic to the earliest Cretaceous, whereas the arrival of more cosmopolitan forms probably signalled the breaching of various palaeogeographical barriers such as regression of the Turgai Sea," he wrote.

The Jehol biota can be viewed as a window on succession in an Early Cretaceous terrestrial biome - an ecological community of plants and animals adapted to a region's particular conditions - in which an established biota merged with and was partially replaced by a new biota composed of both immigrants and relicts.

"In that sense, the Jehol biota is not just a biological refugium as proposed by some palaeontologists but also a cradle for new life," Zhou noted.

Problems remain

Despite the many exciting discoveries, Zhou noted that advances in the understanding of the Jehol biota have been hampered by illegal collecting, the manufacture of fake and composite specimens, and the illegal sale and export of fossils.

Currently, the main problem lies with private collectors who do not record the contextual information about the locality and stratigraphy that are essential for placing the organisms within their palaeo-environmental settings.

To bypass these problems, excavations sponsored by research institutes - under the supervision and guidance of local authorities - must become the norm, he suggested.

Systematic, responsible collecting and vigilant site management are needed to stop the loss of important and unique specimens, he added.

"The Jehol biota currently represents our best chance of viewing the composition and dynamics of an intact Early Cretaceous terrestrial ecosystem," he wrote. "Continuing study of the fauna, flora and palaeo-environment is likely to yield exciting new results for years to come."