23 June 2000
Feathers On Reptile Older Than Dinosaurs
by Kate Melville
A small, flying reptile older than most dinosaurs may have sported a set of feathers, suggests a study in the 23 June issue of Science. The evidence comes from a fossil approximately 220 million years old, about 75 million years older than the oldest known bird, Archeopteryx.
According to the interpretation by Terry Jones, of Oregon State University, and coauthors, the strong similarities between the fossil feathers and modern day bird feathers suggest an evolutionary link between the two.
Although the exact relationship between the reptile, named Longisquama insignis, and modern birds is uncertain, these findings may pose a challenge to the prevailing theory that birds evolved from dinosaurs. (See the related News article by Erik Stokstad, to be available on 21 June.)
Longisquama was an archosaur, a member of the group of reptiles that gave rise to dinosaurs, crocodiles, and birds. The lizard-like creature was "a little tiny guy, about ten inches long from head to tail," said coauthor John Ruben, also of Oregon State University. Elongated scales adorned Longisquama's long arms, perhaps for use in gliding, and its back.
According to coauthor Larry Martin, of the University of Kansas, the fossil was discovered more than three decades ago in central Asia by a Russian paleontologist who specialized in insects. When the scientist published the first report of the fossil in 1970, he described a row of long narrow appendages down the animal's back, interpreting them as a frill of extremely long scales.
Martin contends, however, that "the slightest breeze would have toppled the animal over" if the structure had been a frill.
A later Russian study revised the picture, describing two parallel rows of supposed scales and proposing that Longisquama may have been able to pivot to form a pair of wing-like structures. Still, the appendages continued to be described as scales.
"The scientists assumed that, because of the fossil's age, they just couldn't be feathers," Ruben said.
The specimen remained at a Russian museum for years until, approximately one and a half years ago, the museum organized a traveling exhibit that came to the University of Kansas Natural History Museum. Martin specifically requested that they bring along Longisquama.
Martin and his coauthors pored over every detail of the fossils, which included most of the skeleton except for the hind end, and impressions made by the skin. The scientists found three key clues that convinced them that the structures along the animal's back were feathers instead of scales.
First, the appendages showed some of the most recognizable features of a modern-day feather. The scientists identified a long, thin tube called a "shaft" running down the center of each appendage. A short distance from the base, a dense row of fine strands called "pinnae" project from either side. Neither the shaft nor the pinnae are typically thought to be features of reptilian scales. The shaft also comes to a point at the base and appears to poke into a follicle in the skin. In contrast, modern scales form in continuous sheets.
The Science authors also discovered that the base of the shaft was divided into several subsections, which is thought to be a signature of the complex developmental patterns unique to feathers.
The third clue also indicated that the appendages followed the same growth pattern of modern feathers. The pinnae of modern feathers first develop inside a tube called a feather sheath and then unfurl as the feather grows. The Longisquama fossil shows a new feather that seems to be developing in the same manner. At the base end, the entire structure is preserved and only the surface of the outer wall is visible. Nearer to the center, the wall is broken off, exposing the pinnae inside. Further toward the tip, the pinnae seem to have flaked off, and the wall of the opposite side of the structure appears.
For Martin, this evidence clinched it. "I'd been holding back until that point, thinking that these were unusual and feather-like scales, but scales nonetheless. The results were startling," he said.
Longisquama's precise classification within the archosaur group and its evolutionary relationship to birds is still unclear. The authors believe, however, that it's unlikely that features as complex and specialized as feathers evolved more than once. More fossil evidence will be necessary to explain the evolutionary origins of modern-day feathers. Currently, the Longisquama skeleton and several additional pieces described in the Science paper are the only specimens of their kind.