Paleontologists have discovered a fossil partially covered with broad, unbranched filaments — a type of structure previously theorized to exist on primitive feathered dinosaurs but not found until now.
Flight feathers on modern birds have a central shaft and stiff fibers, called barbs, that branch from that shaft. Barbules, smaller fibers that branch from the barbs, are tipped with small hooks that latch on to adjacent barbs or barbules, stiffening the feather into a single vane.
This arrangement is so complicated that many scientists theorize it could have evolved only once (SN: 8/18/01, p. 106). But paleontologists have proposed that a variety of simpler structures — including peculiar, branched structures colloquially called “dinofuzz” — evolved before feathers. Now, researchers have finally found an important yet long-missing piece of the feather lineage: single, unbranched filaments.
The unbranched structures appear on a newly discovered specimen of Beipiaosaurus, a feathered dinosaur that lived in what is now China about 120 million years ago, says Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. The filaments, which measure between 10 and 15 centimeters long, are broad — about 2 millimeters wide for most of their length, he and his colleagues report online January 12 in the Proceedings of the National Academy of Sciences. Apparently the structures were stiff, because none of those preserved in the fossil are bent into a curved position.
Finally, Xu notes, the structures don’t cover the creature’s body: They’re found only on the creature’s head, neck and tail. The filaments couldn’t have generated lift, so they’re not flight worthy, and they’re too sparse to have retained the creature’s body heat. Xu and his colleagues therefore speculate that the filaments served as display structures, just as many similarly placed feathers do on modern birds.
Initial analyses of the fossils of Beipiaosaurus, a creature first described about 10 years ago, found only modern-style feathers with barbs and barbules, Xu says. When he and his colleagues removed more rock from some of those specimens, however, the team discovered the same sort of stiff, unbranched filaments seen on the newer fossils.
The appearance of the newly described filaments “is not surprising,” says Richard Prum, an evolutionary biologist at Yale University who has long speculated about the development and evolution of feathers. Xu’s team “makes a good argument that [the filaments] were used for display,” he notes. Despite the presumed stiffness of the filaments, the structures probably didn’t serve a defensive function, like a porcupine’s quills do, Prum adds.
Having different types of feathers on the same creature isn’t surprising either, Prum continues. Today’s birds often sport many types of feathers: feathers for flight, feathers on their bodies for thermoregulation and downy feathers as an undercoat. Some modern species, such as vultures and cassowaries, even sport bristly structures similar to those seen in Beipiaosaurus, Prum notes. “We know that feather follicles can do this.”
Beipiaosaurus is just one of a host of feathered creatures that have been unearthed in China in recent years (SN: 1/25/03, p. 51; SN: 8/19/00, p. 119). While many paleontologists contend these fossils strengthen the idea that birds are, in fact, the modern descendants of dinosaurs, others aren’t so sure. “Many of us believe that these ‘feathered dinosaurs’ are actually flightless birds,” says Alan Feduccia, a paleontologist at the University of North Carolina at Chapel Hill. For one thing, the intricate arrangement of bones in those ancient creatures’ arms and hands matches that seen in the wings of modern birds, he notes.
“Are we sweeping important questions under the rug by saying that these [creatures] are feathered dinosaurs?” Feduccia asks.