When dinosaurs took to the skies

Newly discovered flying-dinosaur fossils with exceptionally well-preserved feathers are revealing more about how dinosaurs and early birds took flight. Daniel Cossins reports.

An illustration of Changyuraptor yangi, the largest four-winged dinosaur. – Stephanie Abramowicz, Dinosaur Institute, NHM

The story of how feathered flight evolved and birds arose from dinosaurs is written in stone – most of it in stone unearthed in north-eastern China. Over the past 20 years the region has yielded dozens of fossils with exceptionally well-preserved feathers. Among the most intriguing are the four-winged theropods – pigeon-sized predatory dinosaurs with long feathers on all four limbs. These animals were almost certainly capable of flight, though it’s not clear whether they flew by gliding or flapping.

Now, thanks to another exquisite fossil recently excavated in the area, palaeontologists have described the largest-ever four-winged dinosaur. This species, named Changyuraptor yangi, was the size of a large eagle – 4 kilograms and 1.3 metres from snout to tail. In comparison, Microraptor gui, the next largest four-winged dinosaur known, weighed just one kilogram.

“The most interesting thing is how big it is,” says Nick Longrich, a palaeontologist at Bath University in the United Kingdom, who was not involved in the work. “Four kilos? That’s impressive. This is a sizeable beast even by the standards of modern birds, and it shows that these four-winged dinosaurs could scale up the design.”

Changyuraptor is not just distinguished by its size. Its 30 cm-long tail feathers are the longest recorded for any dinosaur. “It has the most incredible plumage I’ve seen in all my years looking at fossils,” says Luis Chiappe of the Museum of Natural History of Los Angeles. Chiappe led the American and Chinese palaeontologists who found the specimen and described it in Nature Communications in July.

Like other four-winged dinosaurs, Changyuraptor’s wings were covered in quill-like feathers similar to those on modern birds, rather than the primitive, downy feathers found on some flightless dinosaurs. Because the animal’s tail feathers were so well preserved the researchers could analyse how they might have worked aerodynamically in flight. “The length and shape of the tail would have allowed them to adjust their pitch – that is, whether they were flying nose up or nose down – as well as to slow their speed during descents and help them land safely,” says Chiappe. “That’s important because landing is pretty dangerous for heavy animals coming down fast.” In short, the long tail feathers helped Changyuraptor to avoid crash landings.

Dave Hone, a palaeontologist at Bristol University in the United Kingdom, says that the case the authors make for the tail’s function is convincing. From looking at smaller Microraptor species a lot of people had assumed that the tail was used, at least in part, for steering and control in the air, he explains, “but this paper analyses how the tail feathers would actually operate in flight.”

As palaeontologists we don’t normally get datasets like this to play with.

The information on Changyuraptor adds to the growing knowledge that the period of the late Jurassic and early Cretaceous was remarkably rich for the evolution of feathered flight. Within this, Changyuraptor represents a side-branch in avian evolution rather than a transitional step to modern birds. “There were lots of different lineages experimenting with modes of flight at this time,” says Longrich, “the lineage that became modern birds was just one of them.”

Archaeopteryx is the iconic member of that lineage, considered by many to be a freeze-frame of the transition from dinosaur to modern bird. Earlier in July palaeontologists in Germany published in Nature details of a newly discovered Archaeopteryx specimen. This one – the 11th to date – is a beautifully preserved imprint. It lived some 25 million years before Changyuraptor.*

Excavated in Bavaria, the new Archaeopteryx specimen is so well preserved it clearly shows the species didn’t just have feathered wings and tail. Its body was covered in quill-like feathers – long on the wings and tail and shorter on the hindlimbs. After comparing this coverage to other feathery dinosaurs and early birds the authors suggest that its feathers evolved initially for insulation or display before being co-opted for flight.

Chiappe agrees that Archaeopteryx flew without the use of hindwings. Longrich, on the other hand, thinks the authors have underestimated the size and importance of the specimen’s hindlimb feathers. “I think they were probably used for flight,” he says. “To me, it appears to have the four-winged design, and is therefore more primitive than previously thought.”

For Chiappe, the question now is this: did the common ancestor of Archaeopteryx and Changyuraptor have long-feathered hindwings for flight that were later phased out by early birds, or did the four-winged dinosaurs independently evolve the trait in what turned out to be an evolutionary dead end?

However the story plays out, Hone says we should never get blasé about new fossils with beautifully preserved feathers. “This is an extraordinarily rare resource,” he says. “Every new specimen is of extreme interest because as palaeontologists we don’t normally get datasets like this to play with.” Longrich agrees, “After years of focusing on the skeleton, because that’s all we had, we’re now able to really starting to look at the feathers in more detail.”

*This article has been altered to correct the age of Archaeopteryx.

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