THAT birds are the descendants of dinosaurs is now accepted by almost all evolutionary biologists. The clinching discovery was of animals that were clearly dinosaurs, and clearly could not fly, but which had feathers. That did raise the question, though, of why one twig of the great dinosaur tree had developed such strange outer vestments, even before it developed wings.

If a discovery announced in this week's Nature has been interpreted correctly, that question is about to get even stranger. For Zheng Xiaoting of the Shandong Tianyu Museum of Nature in Pingyi, China, and her colleagues, are suggesting that it was not just the part of the clan that led to birds that was feathered. What they have found makes it likely that many dinosaurs had something rather like feathers, and that those which did not had lost a primitive characteristic of the group in the way that elephants and hippos have lost most of the hair that is characteristic of mammals.

The crucial discovery comes from a famous fossil bed in Liaoning province. Dr Zheng and her colleagues propose to call it Tianyulong confuciusi. The new species belongs to a group of dinosaurs called the Ornithischia. This group, which includes such famous names as Stegosaurus and Triceratops, is one of the two clades into which the dinosaurs are divided. The other is the Saurischia, whose famous members include Diplodocus and Tyrannosaurus. The Saurischia also include all the immediate ancestors of birds. Which is why Dr Zheng was stunned to find on Tianyulong confuciusi what are conservatively described in her paper as “long, singular and unbranched filamentous integumentary structures”. In other words, things than look suspiciously like the central shafts of feathers.

Li Da Xing

Beautiful plumage

Such “protofeathers” have been found on other dinosaurs, but until now those species have, like those that sport true feathers, all belonged to a part of the Saurischia that includes the birds. Yet the split between Ornithischia and Saurischia goes back to the very beginning of the dinosaurs, 80m years before the first birds and about 100m years before T. confuciusi. So if the feather-like structures of T. confuciusi really have the same origin as feathers, then such structures must have been there from the outset, and be characteristic of dinosaurs as a whole.

Jurassic lark

That does not explain what they were for. A few researchers argue that they were actually internal to the skin, and thus not related to feathers at all. If they really were external, the most likely answer is that they served the same function—insulation—as hair does on a mammal. But they may, like hair and modern feathers, have had a secondary, signalling function.

This discovery raises another question, too. Taxonomically, the very definition of a bird was until recently an animal that has feathers. Now, taxonomists argue that since birds are descended from dinosaurs they should be classified merely as a subgroup of the Dinosauria. But if feathers truly are the diagnostic criterion, then perhaps things should be the other way round, and Stegosaurus, Triceratops, Diplodocus, Tyrannosaurus and their kin should no longer be thought of as terrible lizards, but as overweight, flightless birds.