Rewriting the history of feathered dinosaurs

Since 1996, nearly 50 new feathered dinosaur species have emerged from the fossil fields of China. Palaeontologists thought they had a good idea of the diversity of these fluffy carnivores. But a recent finding left them scratching their heads.

The Shandong Tianyu Museum of Nature in Pingyi is the place to go if you want to experience China’s dinosaurs in all their weird and wonderful glory. Five hundred kilometres south of Beijing, it’s the largest museum of its kind in the world, with fossils of more than 1,000 complete dinosaurs, 2,300 early birds and plenty of creatures that bridge the boundary. Arranged in 28 halls in three nondescript-looking buildings, these spectacular displays merely hint at the treasures behind closed doors, where a backlog of new feathered dinosaur finds is stacked up waiting to be studied and named.

Species described from the collection here include Tianyuraptor, which resembles Velociraptor; Tianyulong, one of the first known feathered herbivorous dinosaurs; and the four-winged flying dinosaurs Anchiornis and Xiaotingia. Many museums have single specimens of dinosaur species in their collections; Shandong often has several hundred. “You don’t need to do too much to dig up fossils in China,” local palaeontologist Wang Xiaoli told a reporter from The New York Times. “When the wind blows, they reveal themselves.”

Not much surprises the scientists and technicians who work on the geological riches that pass through these doors, but one fossil sold to the museum in 2007 had them stumped. “Over the past 20 years I have discovered many dinosaurs, so to a degree I am used to finding strange species,” says Xu Xing, the world’s most prolific living dinosaur hunter. Based at the Institute of Vertebrate Palaeontology and Palaeoanthropology in Beijing, Xu has been involved in the discovery and naming of approximately 60 dinosaur species, from the magpie-sized, four-winged flyer Microraptor, found in 2000, to the giant 9-metre-long fluffy tyrannosaur Yutyrannus, described in 2012. “But still,” he says, “when I saw this particular discovery I was shocked.”

Just when palaeontologists though they’d seen it all, this weird little fossil would reveal a bizarre experiment in the evolution of flight in dinosaurs – a process that also gave rise to modern birds.

To understand why it was so unusual, we must look back to 1996, to the very first discovery of a feathered dinosaur.

In the early 1990s, nobody could have predicted the incredible flurry of dinosaur discovery that was about to begin in China. With the rebuilding of academic institutions after the cultural revolution, and foreign-trained Chinese palaeontologists returning home to prospect, China’s Cretaceous- and Jurassic-era rocks began to give up their riches. Something like 160 species of dinosaur have been described across China since 1990, and the rate of new discoveries may not have peaked.

It wasn’t just a matter of numbers; the remarkable preservation of fossils from north-eastern China (particularly the provinces of Liaoning and Hebei) was unique too. One of the things recorded in exquisite detail was feathers – the defining characteristic of birds. Most experts had come to accept that birds were the descendants of so-called theropod carnivorous dinosaurs (such as Velociraptor) based on the similarity of their skeletons, bones filled with air pockets, wishbones and many other features besides.

But while the fossils of archaic birds such as Archaeopteryx (which flew with feathered wings but also had teeth and a long bony tail) had been found as early as 1861, few thought we would ever find actual dinosaur fossils with feathers. That’s why palaeontologists were astounded when, in 1996, a fluffy 1.5-metre-long dinosaur called Sinosauropteryx was found in Liaoning, near the border with North Korea. The region preserves a Pompeii-like world of dinosaurs in fine-grained sediments, including volcanic ash. Sinosauropteryx’s feathers were not flight-worthy, however. They were a short, downy fuzz visible on the head, back and tail of this vividly preserved animal.

Since that finding, we now know of about 50 species of dinosaur for which there is direct evidence of feathers. Some have halos of fluff or beautiful fans of flight feathers delicately traced into their remarkable fossils; others have a distinctive pygostyle tailbone that would have been an attachment point for feathers; still others have bumps along their forearms – “quill knobs”, where feathers attach to ligaments in the wings of modern birds. Most of these species come from the Chinese provinces of Liaoning, Inner Mongolia and Hebei, but a handful are from the nations of Mongolia, Burma, Madagascar, Germany, Canada and Russia.

Together these fossils show that most theropod dinosaurs all over the world had feathers.

While more than 90% of fossil sites worldwide preserve only bones, Liaoning in particular preserves feathers and soft tissues. These unlikely fossils have given us a tiny window into a world we had no idea existed. We now have a remarkable evolutionary sequence of fossils from dinosaurs to birds. It shows how feathers evolved from very simple filamentous structures through to complex, branched flight feathers, and also how flight itself might have developed.

Sinosauropteryx’s downy fuzz was probably used for insulation. In later models of dinosaurs, feathers also began to be used for display. The massive Gigantoraptor, an 8-metre-long, parrot-beaked omnivore found in the Gobi Desert of Inner Mongolia, for example, likely used great fans of tail feathers for mating displays. Only much later in the evolutionary process did feathers begin to be used for flight, such as in the four-winged, pigeon-sized dinosaur, Microraptor, found in 2000.

Another four-winged flyer, the eagle-sized Changyuraptor was discovered in 2014, and there are likely others yet to be found. The wings on the hind limbs and forelimbs of these animals helped them glide between the trees of China’s Jurassic and Cretaceous forests. It is unlikely they were capable of full powered-flight, as they lacked the keeled sternum to which large flight muscles are anchored in today’s birds.

The four-winged model was one early experiment in flight within the group of animals from which birds evolved. Many early birds would have had large feathers on their hind limbs and feet too, as some chickens do today. 

Apart from these small dinosaurs, palaeontologists were also surprised to discover that much larger carnivores also had feathers. A number of feathered tyrannosaur species were found in China including Dilong in 2004, and the much larger Yutyrannus in 2012. This shaggy 9-metre-long creature was much nearer in size to T. rex, making it plausible that the tyrant king itself had feathers.

Then there’s Zhenyuanlong, a 2-metre-long dromaeosaur related to Velociraptor, described as a “fluffy, feathered poodle from Hell”. Found in 2015 in the 125-million-year-old deposits of the Yixian Formation in Liaoning, it has the largest feathered wings yet found on a dinosaur.

Weird dinosaurs indeed, yet none of them were as perplexing as the specimen that Xu first examined in detail in 2009. At first it was just an overall impression. But it would soon become clear this dinosaur was sporting features that surely didn’t belong in the same animal.

Many of the treasures that arrive at the Shandong museum have been dug up by farmers who supplement their meagre income by excavating dinosaur remains and selling them on to dealers and institutions. In theory, it has been illegal to buy fossils collected by such “amateurs” since 2008. But better that these priceless fossils are available for science, the palaeontologists argue, than lost to the living rooms of wealthy private collectors.

The majority of feathered dinosaur fossils have come from Liaoning, but some – such as the enigmatic specimen – are from the neighbouring province of Hebei. This is a relatively new locality for fossil hunters, dated to the mid-to-late Jurassic, 160–165 million years ago. “Fossils in northern Hebei are not as rich as Liaoning, but often they are well preserved; in some cases, feathers and soft tissues are even better preserved,” says Xu.

The new specimen was found by farmer Wang Jianrong, who dug it from a fossil quarry near the village of Mutoudeng in Qinglong County. Here Hebei’s small mountains, deep valleys and rolling foothills are pocked with pits dug by fossicking farmers. Wang may not have realised he had found something important, as by the time the specimen arrived at the Shandong museum it was broken into several pieces and parts were missing.

Xu first saw the strange new specimen in 2009, shortly after it arrived at the museum. It was broken and partially covered with rock. He could tell it was a feathered dinosaur but, running his hands over it, he decided there was definitely something strange about it. In 2013 he sent his technician, Ding Xiaoqing, to the Shandong museum and asked her to clean the overlying rock away.

Several months of work later, Xu could see from the distinctive skull shape and the hand that it was a scansoriopterygid (“climbing wings”) dinosaur. This feathered group includes the species Epidexipteryx and Epidendrosaurus (previously called Scansoriopteryx), which have forward-slanting teeth and weird fingers. Discovered in 2002 in Liaoning, Epidendrosaurus was thought to have been a sparrow-sized climber and tree-dweller, with an elongated third finger. Its purpose? Possibly to skewer insects hidden inside tree hollows, much as the aye-aye lemur of Madagascar does today.

The slightly larger, pigeon-sized Epidexipteryx, found in Inner Mongolia in 2008, was weirder still. Like Epidendrosaurus, it was covered in downy fuzz but had four long, ribbon-like feathers emerging from its tail. Its third finger and putative skewer may have been half as long again as its entire body.

As with other scansoriopterygids, the feathers of the new creature were simple, short and brush-like, and would have covered its whole body; scansoriopterygids lacked the large vaned feathers with complex structure that are found on modern birds and many theropod dinosaurs. Their feathers, known as “dino fuzz”, are more like the down on a chick – made of fluffy filaments with no central vane or complex branching structure found in what we typically think of as a feather.

More of these fuzzy feathers were preserved on another piece of the fossil, surrounding part of a hind limb and foot: an analysis with an electron microscope revealed the presence here of melanosomes – tiny packages of pigment responsible for feather colour in living birds.

Furthermore, as Ding slowly removed the matrix of rock encasing the fossil, the experts noticed two rod-like structures near the wrist that were like nothing they had ever seen before. “This confused us for quite a long time,” Xu says. “It was apparently part of the skeletal system, but something you never see in other dinosaurs.”

Xu’s colleague Corwin Sullivan first saw the fossil in early 2014 when he, Xu and American palaeontologist Jingmai O’Connor were visiting the Shandong museum. “What really puzzled us were these structures extending from both wrists that looked very much like bones on the slab, but of course theropods aren’t supposed to have bones in this position,” he recalls. “I remember standing around with Xu, Jingmai and Zheng trying to work out whether these were ligaments or some other kind of soft tissue, but we couldn’t come up with anything that really made sense.” (Later humorous suggestions included that they might have been used as giant chopsticks.)

Sullivan happened to be working on a book with a section on gliding and flying animals; a photo of a flying squirrel grabbed his attention. It showed the animal’s “styliform elements” – rods of cartilage that protrude from the wrists of some gliding mammals. These rods allow them to hold out their patagia, membranes of skin that extend from their wrists to their ankles, allowing them to fly.

“When I read that thing about flying squirrels, the fossil just flashed into my mind,” Sullivan says. “The idea was hugely exciting … but I reined myself in, thinking there were probably a dozen reasons this couldn’t be the same structure.” As he began to research it in more detail, however, he realised he was on to something.

Modern birds do in fact have a small skin flap in front of the elbow called a “propatagium”, which forms part of the wing, but is covered by the much larger flight feathers. “You can imagine that if this propatagium was also present in theropod dinosaurs that are the ancestors of birds, then what’s going on in the scansoriopterygids is that it has expanded to form a wing membrane,” Xu says.

“Finally, we noticed there were patches of membrane preserved near the styliform elements… We suddenly realised this was a really bizarre dinosaur with kind of pterosaur- or bat-like wings. We all got so excited, and we just couldn’t believe that this had happened… Wings in my mind always had beautiful, big flight feathers. These are the major components of dinosaur wings. Here we had something totally different.”

Xu realised that what he had on his hands was the Jurassic equivalent of a platypus, the Australian oddity that stumped 18th century British naturalists, who at first considered the stuffed specimen they received a fake. This new dinosaur had a feathered body but wings made of skin flaps, resembling those of pterosaurs – distant cousins of dinosaurs that evolved independently.

Working with colleagues including Sullivan, O’Connor, Zheng and Xing Lida, based at the China University of Geosciences, Xu began to amass the evidence needed to back up their extraordinary find. Just as for 18th century naturalists, fake specimens have been a big problem in China. In 1999, for instance National Geographic heralded the discovery of Archaeoraptor, a “missing link” between dinosaurs and birds; it turned out to be a skillfully glued-together composite of bird and dinosaur fossils. Xu’s expert eye is rarely fooled, but he realised a bat-winged dinosaur was so improbable other palaeontologists might have doubts.

Before publishing in Nature in May 2015, the team did a variety of analyses to prove, for example, that the fossilised styliform element had a chemical composition suggesting it had originally been made of bone or cartilage. Crucially, the experts knew it had not been glued together by a hoaxer – Xu’s own technician had prepared the fossil out of the rock at the Shandong Museum. “I am 100% sure this is not a faked fossil,” Xu says. “Sometimes I am not so certain if a fossil is real, or I think something may be wrong with it, but in this case I am quite certain.”

Furthermore, the researchers were able to pin down Wang, the original discoverer and visit the specific region within the quarry from which he’d extracted the specimen, to determine the geological layer and age. The team called the species Yi qi (pronounced “ee-chee”) which means “strange wing” in Mandarin.

The announcement of the species was greeted with a mixture of bemusement and palpable excitement by a palaeontological community that had become somewhat desensitised to the feathered dinosaur riches pouring from China.

In an accompanying commentary, also published in Nature, palaeontologist Kevin Padian noted that in the two decades since Chinese finds had cemented notions about the dinosaurian origin of birds, “the picture of the evolution of feathers and flight has become richer and more complicated” with the discovery, “seemingly on a monthly basis”, of other feathered dinosaurs: “But things have just gone from the strange to the bizarre.”

Daniel Ksepka, from the Bruce Museum in Connecticut, similarly described the discovery as “refreshingly weird”, adding that “palaeontologists will be thinking about Yi qi for a long time”.

It’s probable that gliding membranes were present in all the scansoriopterygid dinosaurs. This explains their puzzlingly long fingers: they were not skewers for picking insects out of trees but struts for membranous wings. “I tend to believe that Epidexipteryx and Epidendrosaurus both have wings like Yi,” says Xu. “All the members of this group probably have styliform elements and membranes attached. You need fossils to support this, but if you ask me I believe it’s possible.”

It’s not clear what kind of flight mechanism these “bat” dinosaurs employed, but they may have used a mixture of gliding and flapping. Xu’s team attempted, without much luck, to make structural models based on the fossil, to test them aerodynamically. Now they are creating three-dimensional computer models instead. The experts also reappraised the handful of other scansoriopterygid fossils in light of what they know about Yi, but haven’t yet found direct evidence of the styliform element or membranous wings.

“Given that the scansoriopterygids are so interesting and so bizarre, we want to do more work on this group,” Xu says. He plans to search for fossils in several localities in Inner Mongolia and Hebei.

In 2016, Xu told me, his team carried out a study of the melanosomes on a few parts of the fossil. “I will say Yi qi probably had a green or brown colour. It’s a guess, but the probability is high because the melanosome [structures] appear similar to modern birds with green or brown feathers.” If proven correct, this is the first time that green has been detected in a feathered dinosaur fossil.

The really big question is why this group evolved a second method of dinosaur flight when many closely related theropod lineages had species with very large flight feathers.

“Why evolve a completely different flight mechanism and body plan?’ asks Xu. “This is really bizarre and, so far, I don’t have a good answer.” He believes that whenever big evolutionary transitions take place – such as that from terrestrial dinosaurs to flying birds –strange experiments take place to fill the new niche.

Some of those experiments may well be recorded in the rocks of Hebei and Liaoning. “Yi qi was totally unexpected. We couldn’t believe it. If you know dinosaurs very well and the transition well, then you’d never expect there would be a dinosaur with bat-like or pterosaur-like wings instead of feathered wings. Discoveries like this will continue to emerge and demonstrate how complex the transition to birds was’, Xu says. “I would not be surprised if we find even more bizarre species in the future.”

Credit: Edited excerpt from Weird Dinosaurs: The Strange new fossils challenging everything we thought we knew by John Pickrell, New South books.