It was only discovered in the early 1990s, but China's Yixian formation has already transformed our knowledge of dinosaurs. Jeff Hecht uncovers the fossil bonanza

When dinosaurs roamed Yixian 125 million years ago, it probably looked similar to New Jersey's pine forests

MEIlong, the "soundly sleeping dragon", is small enough to hold in your two hands. Its head is tucked under its forelimb, like a sleeping bird with its head under a wing. When I saw it in Xu Xing's office at the American Museum of Natural History in New York, the long tail that was wrapped around its body when it was discovered had been removed for analysis. Otherwise the tiny bones of this little dinosaur were arranged just as they were when it went to sleep for the last time, some 125 million years ago in what is now China.

Such a complete, exquisitely preserved skeleton of a small dinosaur was something that palaeontologists could only have dreamed about a decade ago. In recent years, however, they have become rather accustomed to dream discoveries. Mei long is just the latest in a string of stunning finds to come out of a rock formation called Yixian in the Liaoning province of north-eastern China (see Map, page 45). The Yixian formation is stuffed full of dinosaurs, as well as birds, mammals, lizards, fish, turtles, insects and plants, all preserved in great detail. And over the past 10 years, this fossil bonanza has helped to transform our knowledge and understanding of dinosaurs, the world they inhabited, and the animals they shared it with.

Well before Mei long turned up, Yixian was well known for its beautifully preserved fossilised fish, insects and turtles, which were - and still are - collected and sold by peasant farmers. The dinosaur story, however, only began in earnest about a decade ago, when word began circulating that Chinese scientists had found an amazing little

predator named sinosauropteryx (New Scientist, 19 October 1996, page 7).

Sinosauropteryx- meaning " Chinese winged lizard" - made its informal world debut in October 1996 at the Society of Vertebrate Paleontology's annual meeting at the American Museum of Natural History. It wasn't on the programme, but Phil Currie of the Royal Tyrell Museum in Drumheller, Canada, brought snapshots from China. Circles of scientists gazed in amazement at the photos: the creature was clearly covered in what looked like downy feathers. Among the onlookers was Yale University's John Ostrom, the grand old man of dinosaur palaeontology. Three decades earlier, Ostrom had proposed that birds evolved from dinosaurs. By 1996 his walk was slow and his hair was white, but when he saw the pictures of sinosauropteryx his eyes lit up.

It was a marvellous specimen. Its body had fallen to the bottom of a lake, probably during a volcanic eruption, where sediments flattened it, preserving a perfect impression of the little dinosaur - including a fringe of feathery filaments around its body. Ostrom had not believed he would live to see a feathered dinosaur. Neither had the much younger Currie. Both were wrong.

More soon followed. In early 1997 The Academy of Natural Sciences in Philadelphia, Pennsylvania, arranged a whirlwind tour of China for Ostrom and three others: Larry Martin of the University of Kansas, pterosaur expert Peter Wellnhofer of Ludwig Maximilian University of Munich, Germany, and feather-development specialist Alan Brush of the University of Connecticut. After showing his guests the sinosauropteryx specimen, Ji Qiang of the National Geological Museum in Beijing stunned them with two additional feathered dinosaurs that no one in the west knew existed. He called one protarchaeopteryx because it looked like a primitive version of the oldest-known bird, archaeopteryx. Another he called caudipteryx, meaning "winged tail".

Up to that point, feathers were thought to be unique to birds. Dinosaurs were believed to have been scaly: the few impressions of dinosaur skin that had been found all showed reptilian scales. And while microscopic examination showed that sinosauropteryx's frill was made of filaments rather than real feathers, both protarchaeopteryx and caudipteryx had the real thing. Suddenly fully fledged feathers were no longer strictly for the birds; dinosaurs had feathers, too.

Brains buzzing, the four headed home. Ostrom declared the Yixian fossil beds held a century's worth of work. Soon palaeontologists from all over the world were packing their bags for China, hoping to find fossils that would solve old mysteries and present new ones. A few years on, they are ready to start saying what it all means.

Fossils are failures of nature's recycling ►

The fossilised soft tissue seen in this sinornithosaurus is very rare

"Some were buried alive in fine ash, a dinosaurs' Pompeii that produced amazing 3D fossils"

system. Big dinosaur bones survive because of their sheer size, but the smaller bones are likely to be lost, and teeth are often the only traces of small animals. Soft parts such as skin and feathers almost never fossilise. Yet once in a very great while, nature slips up and leaves a treasure trove of fossils preserved in exquisite detail. The Burgess Shale in British Columbia, Canada, is arguably the most famous of these, preserving strange soft-bodied creatures from the Cambrian explosion, the first great blossoming of animals more than 500 million years ago. The limestone of Solnhofen in Bavaria, Germany, where archaeopteryx was discovered in i860, is another.

Now the Yixian formation has joined the pantheon - and arguably has topped the lot. "It's the biggest fossil bonanza ever," says Martin. A handful of other sites preserve some exceptional fossils from the Mesozoic era, the age of the dinosaurs (see Timeline, page 44), but none has the diversity of the Yixian formation. It reveals "almost every group of animals we would expect to see", says Paul Barrett of the Natural History Museum in London. Only crocodiles are missing. No one knows why, since they are abundant in Solnhofen and other Mesozoic sites.

Spread across a broad area of north-eastern China, the Yixian formation is in an area of semi-arid hills criss-crossed by streams that cut through the sedimentary rocks and expose the deposits. In many respects it resembles the fossil-rich badlands of western North America. When sinosauropteryx and its kin roamed the region 125 million years ago, it was covered by forest and quiet freshwater lakes. Nearby volcanoes erupted from time to time, spreading thick layers of ash that killed many animals. Birds may have succumbed to fumes from the eruptions and fallen into the lake; suffocated dinosaurs may have sunk to the bottom when swimming or after being washed into the water. Some palaeontologists think that volcanism may have occasionally released lethal clouds of carbon dioxide above the lake, like those of Lake Nyos in Cameroon, downing flocks of birds. Today, the fossil beds consist of alternating layers of volcanic ash and lake-bed sediments that are well over a kilometre thick in places. So far they have yielded more than 20 new dinosaurs, and it is anyone's guess how many more remain to be discovered.

The spectacular fossils have kept scientists so busy that they have yet to figure out the details of the fossilisation process. But it is clear there are two different processes at work, both linked with eruptions - and both remarkable. Some dead animals and plants found their way into a deep lake and sank to the bottom where volcanic ash mixed with lake sediments to preserve flattened imprints like that of sinosauropteryx. Others were buried alive in fine ash, in a dinosaurs' Pompeii, producing 3D fossils such as Mei long. The two types are complementary. Ash fossils preserve the 3D shape but not the soft parts; lake-bed fossils flatten skeletons, but preserve details such as soft tissue and body coverings.

Initially, Chinese geologists believed the Yixian rocks formed in the late Jurassic period, which ended about 145 million years ago. But those dates were only estimates based on comparing the fossils with those found elsewhere. When geologists dated the rocks radiometrically- the gold standard for geologic age- they found that the oldest rocks were just 127 to 128 million years old, with some as young as 121 million years. That placed them in the early Cretaceous period, the final and most spectacular chapter in the age of the dinosaurs. The bulk of the fossils date from 127 to 125 million years ago, says Barrett.

The fossil beds contain an astounding diversity of life. They preserve the oldest known flowers, illuminating the birth of flowering plants. They also include mammals-some with fur - at a key time in the evolution of modem groups. Among these are stunning surprises like Repenomamus giganticus, a badger-sized mammal that dined on baby dinosaurs at a time when mammals were supposed to be tiny, timid and nocturnal. But amid all this diversity it is the little dinosaurs that have attracted the most attention - and not just because they are feathered.

One of their principal attractions is their size. Big dinosaurs might get the public's pulse racing, but as far as evolutionary biologists are concerned the little guys are where the action is. Small animals tend to be highly adaptable generalists that go on to evolve specialist traits and then expand in size.

Before Yixian, no one doubted that small dinosaurs had existed. Few had been found, though. There were teeth, some scattered bones, and a handful of partial skeletons. But it wasn't clear how many of the little fossils were youngsters of species that we already knew about. The Yixian fossils have changed all that, and the evolutionary biologists have not been disappointed. Xu says most of the evolutionary innovation at Yixian seems concentrated in the smaller dinosaurs. Many are the earliest known members of groups that went on to dominate the Cretaceous, from triceratops to Tyrannosaurus rex.

That's not to say that there were no big dinosaurs in Yixian 125 million years ago, but their remains are few and far between Perhaps they didn't float far enough from shore to be dropped deep in the ancient lakes, and so never reached the fossil-forming sediments. Another possibility is that the local fossil-hunting farmers are not looking for big animals. Palaeontologists have found the remains of an elephant-sized plant-eater that appears to be related to diplodocus and other long-necked browsers, though it has yet to be properly described. "The farmers chucked it over the hillside," Currie says.

The single most common small dinosaur in the Yixian formation is psittacosaurus, a parrot-beaked plant-eater that walked on two legs. Psittacosaurus belongs to the large group of plant-eating dinosaurs called ornithischians, which included the later duck-billed dinosaurs. Adults grew to be a couple of metres long and had large numbers of young; one was found incubating a nest of over 30 juveniles. Thousands of psittacosauruses have been found in both sediments and ash, and the little ones were the favourite meals of local predators, including the badger-sized mammal repenomamus. But the psittacosauruses were doing something right; their descendants include the horned dinosaurs like triceratops and the dome-headed pachycephalosaurs.

Most of the more than 20 species of Yixian dinosaurs are theropods, a big class of bipedal dinosaurs typified by velociraptor. The Yixian theropods are strikingly diverse, ranging from the clearly dinosaurian, such as sinosauropteryx, to the very bird-like, such as caudipteryx. Most were predators but at least two- beipiaosaurus, a shaggy 3-metre beast that was the largest of Yixian's feathered dinosaurs, and the curiously buck-toothed incisivosaurus - were plant-eaters. Most ran on two legs, with two grasping arms in front and a long tail behind. They all had feathery coats. True birds are theropods too, and some 20 of them have been discovered at Yixian (see "Dinosaurs among us?", page 46).

The ubiquity of feathers among the Yixian theropods was a huge surprise. Palaeontologists had long suspected that some theropods would bear feathers-after all, the consensus has long been that birds evolved from small, predatory theropods. But they never thought they would see feathers on almost everything.

One such surprise was a creature called dilong, a 1.5-metre theropod discovered by Xu

"Big dinosaurs might get pulses racing, but when it comes to evolution the little guys are where it's at"

and Mark Norell, who is also at the American Museum of Natural History. Dilong was covered in 2-centimetre-long filamentary feathers. It came as a surprise, then, when it turned out to be a forerunner of the tyrannosaurs. "I never expected tyrannosaurs would have feathers," says Norell. Adult T. rexes apparently did not have feathers; the handful of skin impressions discovered so far show scales. However, juveniles may have retained a downy coat of feathers to keep them warm.

Feathers are now so established on the dinosaur scene that when palaeontologists discovered the bones of a small theropod called bambiraptor in Cretaceous rocks in Montana, they simply assumed it was covered in feathers. Likewise Mei long: the ash that buried the sleeping dragon did not preserve its skin, but nobody doubts it was feathered.

As yet, only theropods are known to have ►

Microraptor (left) and beipiaosaurus (right), under the watchful eye of Xu Xing

Farmers are digging up so many fossils that scientists are struggling to keep pace

"Without a time machine, these fossil deposits are our best window onto a vanished world"

Farmers are digging up so many fossils that scientists are struggling to keep pace had feathers. But there are hints that skin coverings might be more common than previously thought. Some fossils of the parrot-beaked psittacosaurus have frills of filaments along the tops of their tails.

And with all these feathers flying around, one obvious question is what the Yixian fossils tell us about the evolution of birds and the origin of flight.

In the 19th century Thomas Huxley pointed out that the skeleton of archaeopteryx resembled the only dinosaur known from Solnhofen, a small theropod named compsognathus. But at the time it was hard to envisage how supposedly sluggish, cold blooded dinosaurs could have evolved into active, warm-blooded birds. A century later Ostrom revived the question by showing striking similarities between archaeopteryx and deinonychus, one of the dromaeosaurs -a family of lightly built, agile carnivores that includes velociraptor (see Diagram, page 46). He suggested that birds had evolved from fast-running feathered dinosaurs that eventually became airborne.

More evidence that birds had evolved from dinosaurs emerged from a technique called cladistics, which builds evolutionary trees based on shared traits: the more traits two species share, the closer their relationship.

This kind of analysis usually places birds within the theropods, close to two groups of dinosaurs: dromaeosaurs and troodonts, the group to which Mei long belongs.

By the early 1990s, most specialists were convinced and began calling birds "avian dinosaurs", which meant traditional dinosaurs had to be rebranded as "non-avian dinosaurs". However, a small but vocal group, including Martin, disagreed. They believed that birds evolved from tree-climbing non-dinosaur reptiles that evolved flight by gliding down to the ground. One of their best arguments was that it wasn't obvious how "ground-up" flight could have evolved, as runners would not

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