The researchers estimate that the spacing would allow for a total of 14 quill knobs and feathers on the velociraptor's arms.
The study will appear in tomorrow's issue of the journal Science.
Researchers had previously found evidence of feathers in many theropods, the group of two-legged dinosaurs that includes velociraptors. Theropods encompass a nonflying family of dinosaurs called dromaeosaurs, or raptors.
The latest discovery adds to a wealth of evidence that theropods gave rise to modern birds, scientists say. (Related: "New 'Mini' Dinosaur a Step in Bird Evolution Path" [September 6, 2007].
Kevin Padian is curator of the University of California Museum of Paleontology and was not involved in the study.
"This is both expected and welcome—expected because we know feathers were present in other members of this group and welcome because it's nice to have the confirmation," Padian said.
"Probably the few scientists who still don't accept that birds evolved from these dinosaurs will try to explain away the evidence," he said.
"But this finding certainly passes the walks-like-a-duck, talks-like-a-duck test."
Determining which nonflying dinosaurs might have had feathers is difficult, however, because feathers are generally not preserved in the fossil record.
"The [fossil] deposits that have typically preserved these nonflying dinosaurs with feathers have a preservational bias toward very small animals," lead author Turner said.
"We don't know much about what the large members of the [dromaeosaurs] group looked like externally," he added.
"What this discovery helps to establish is that even as these groups go through these evolutionary trends toward increased size, it appears that they are retaining their feathers.
"That's an important piece of information, since many of these dinosaur groups start off small, have feathers, and then get big."
Researchers are not sure what the velociraptor's feathers were used for, however.
"It's often difficult to determine function from specific structures in the fossil record, because most features in most living animals don't have one specific function," Turner said.
Luis Chiappe, director of the Dinosaur Institute at the Natural History Museum of Los Angeles County, was not involved in the study.
"The presence of quill knobs in an animal like this one can be explained by the fact that these dinosaurs used their feathered limbs to generate additional thrust during running," Chiappe said.
Chiappe speculates feathered dinosaurs ran while flapping their feathered forelimbs, and that flight may have originated as a byproduct of this behavior.
Turner said the raptors' feathers would certainly not have been used for flying.
Velociraptors are too large and their forelimbs are much too short in relation to their body size to support flying, or even gliding.
The feathers may, however, be a leftover feature of their ancestors.
"Potentially the [feathers] could have been used in the ancestor of birds and velociraptors for flight, or more likely as a gliding function," Turner said.
The animals may also have used the feathers for insulation or as a display to attract mates.
"Whittling it down to the exact function is probably something we won't be able to do," Turner said. "But we can continue to narrow it down."
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