The discovery of a number of fossils at a site in northwestern China was a closely guarded secret for nearly 10 years. Now the ancient relicts are set to go on display in the Xinjiang Uygur autonomous region

About 100 kilometers south of Hami, a prefecture in the Xinjiang Uygur autonomous region, is an area known as The Demon's Castle, named after the striking sand dunes and weird structures formed by the winds that have battered this barely populated, seemingly endless stretch of the Gobi Desert for millennia.

About 120 million years ago, however, the region in northwestern China was a vast lake, teeming with life and home to a large number of reptiles, mostly pterosaurs, the first vertebrates to master powered flight, which dominated the skies more than 70 million years before the first birds. Some scientists believe that if the mass extinction of the dinosaurs and related species about 65 million years ago had not occurred, pterosaurs may have evolved along the same lines as other flight-capable species, such as bats.

In Hami, their bones lay scattered across an area of 50 square kilometers, where they eventually fossilized and were then forgotten until their recent discovery by paleontologists.

"The discovery of Hamipterus is one of the most exciting finds in the 200-year history of the study of pterosaurs. It has helped to reveal their sexual characteristics and ontogeny (origins and development), and the structure of their eggs," says Wang Xiaolin, a paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology affiliated with the Chinese Academy of Sciences in Beijing, who leads an expedition team in Hami.

Pterosaur fossils have been unearthed at sites across the world since the 18th century, but early researchers were unsure about how to categorize these strange creatures with four fingers, the fourth of which was greatly elongated and served as a hook for membranes that stretched from the feet and formed the wings. Some researchers even suggested the fossils were those of the legendary phoenix.

In 1801, Georges Cuvier, a renowned comparative anatomist from France, gave the unique fossils an identity, calling them "pterodactyl", meaning a reptile with a long finger for flight. The name was later refined to "pterosaur".

Subsequent research revealed a great deal of basic information. For example, pterosaur fossils unearthed in Liaoning province and also in Kazakhstan were covered with filaments resembling hair, indicating that the animals were warm-blooded and needed the downy covering to regulate body temperature. Further discoveries allowed scientists to understand the workings of the reptile's circulatory, respiratory and flight systems, and that pterosaurs ate fish, insects and fruits.

In general, though, our knowledge of pterosaurs is limited because the fragility of the fossilized skeletons makes preservation difficult. Moreover, most known pterosaur species are represented by just one or two specimens, leaving population numbers and living habits a mystery.

Pterosaur eggs are even rarer than fossils, and only four isolated and flattened specimens had been discovered before research began at the Hami site.

In June last year, Wang and his team announced the discovery of five exceptionally well-preserved three-dimensional pterosaur eggs, exciting paleontologists across the globe. The team's research was published in Current Biology, a well-respected journal that specializes in research into molecular biology and evolution.

"The site is basically a treasure trove for pterosaur researchers. Even though we started excavating the site in 2005, we didn't make it public until last year, because we wanted to fully prepare protective measures for the area," Wang says.

Hundreds of 3-D, preserved male and female skulls and eggs were discovered during a decade of investigations and excavations.

On Nov 3, the Institute of Vertebrate Paleontology signed an agreement with the Hami government to develop a geopark and a pterosaur museum on the site.

"We hope more geologists and paleontologists and more tourists will visit the region in the years to come to experience the charm of this prehistoric master of the skies," Wang says.

The "treasure trove" has attracted the attention of researchers worldwide, including Alexander Kellner, a leading paleontologist at the National Museum of Brazil and the Federal University of Rio de Janeiro.

"The Hami site is really incredible. It has hundreds of pterosaur specimens that we can attribute to the same species. This is the closest we can get to one (or closely related) population of a flying reptile species," the Brazilian national wrote in an e-mail exchange with China Daily.

The only other sites comparable to Hami are a small deposit in Argentina and another site in Brazil, according to Kellner.

"We can expect more eggs, more specimens, particularly juveniles, and more material for histological sections that may help us to understand several biological aspects of this pterosaur species," he writes.

With one-fifteenth of the world's surface area, China has one of the most widely distributed collections of pterosaur and dinosaur fossils. For example, pterosaur fossils have been found in the western part of Liaoning province in the northeast and also in Zigong in Sichuan province, deep in the southwest, together with a number of dinosaurs, such as microraptors and Tuojiangosaurus, or Tuo River Lizard.

However, lax management in the early years of research resulted in a number of valuable fossils, even some from protected areas, being stolen, moved overseas and sold.

One of the best-known cases occurred in December 2006, when a nest of 22 fossilized dinosaur eggs that had been unearthed in Guangdong province in 1984 was sold to a bidder in the United States and resold at an auction, fetching $420,000.

When Chinese scientists saw photos of the nest on the Internet, they recognized it as coming from China, so they called on the government to intervene. In February 2012, after five years of negotiations, the nest became the first Chinese fossils to be recovered and returned via diplomatic channels.

"Even though we can recover stolen fossils, we will never know all the information about them, such as their exact location in the excavation site and the real story behind them," says Zhao Zikui, an expert on dinosaur eggs at the Institute of Vertebrate Paleontology and Paleoanthropology.

"As a result, when the fossils are stolen, they immediately lose essential information and their value for scientific research," he says.

The fossils at the Hami site will be fully protected. Scientists from Chinese Academy of Scienese will draw up specific plans for investigations, excavations, scientific research, site protection and the development of tourism. The plans, based on the experience gained from a decade of working at the site, will be implemented by the academy and the local government.

"The Gobi Desert and pterosaur fossils, together with some remnants of early man, make the site unique in terms of heritage and natural and cultural value. In the future, we will work with the Hami government to apply to the Chinese government for National Geopark status, and even World Geopark and Natural Heritage status awarded by the United Nations," Wang says.