Downy. The filaments covering Sinornithosaurus appear to have features of feathers.

A Branch in Feather Evolution

The origin of feathers has always been mysterious. One reason is that the oldest known bird, Archaeopteryx, sported essentially modern plumage. Now three paleontologists have found primitive branching structures--reminiscent of feathers--on filaments that once covered a dinosaur. The team argues that this kind of dinosaurian fuzz evolved into full-fledged feathers.

A modern feather has a central shaft from which smaller strands branch and branch again. In flight feathers, the branches hook together to make a stiff, aerodynamic vane. Some researchers have proposed that these feathers evolved from long scales, such as those found on gliding reptiles (ScienceNOW, 22 June 2000). Four recently discovered dinosaurs from China, however, had body parts covered with filaments. They looked like primitive feathers to many paleontologists. Others disagreed, saying the filaments were fibers from decaying skin and muscles.

Now a trio of scientists--Xu Xing and Zhonghe Zhou of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, and Rick Prum of the University of Kansas, Lawrence--have taken a closer look at one of these known dinosaurs. They picked the 125-million-year-old Sinornithosaurus, which had the best preserved filaments and belongs to a group of dinosaurs widely believed to be the closest relatives to birds. Once preparators had cleaned and chipped away more of the rock, they found that the 30- to 45-millimeter-long filaments that covered many parts of the body were "clearly preserved" as external structures.

The structures resemble feathers, the researchers say. Many filaments are bundled together, and in some of these the filaments join at the base, making a downy tuft. Other structures show tips of filaments curving away from the bundle at various points--like filaments branching from the shaft in a feather vane, Zhou says, although he admits that this could be an artifact of preservation. All told, the filaments "might provide a window for us to view the early stages of branching of feathers," Zhou says.

"This is some of the best evidence that we've got that filaments might be part of feather evolution," says Larry Witmer, a paleontologist at the Ohio University College of Osteopathic Medicine in Athens. But others are unconvinced. "They certainly do not tangle like wet down feathers," says paleontologist Larry Martin of the University of Kansas, Lawrence.

Related sites
Technical information on Sinornithosaurus, including images
Online exhibit of "feathered dinosaurs" by Yale's Peabody Museum