While slightly crushed, the skull is largely intact.
"This is by far the best skull preserved" for a large terror bird, Chiappe said.
"We also have some of the foot bones [from] the same animal, which is great, because it allows us to make inferences about the speed of this animal."
A patchy fossil record has raised questions about the speed and agility of different species of terror birds.
Scientists have unearthed many examples of smaller phorusrhacid species that grew between 2 to 3 feet (60 to 90 centimeters) tall.
But examples of the largest species, which like many ancient top predators lived in relatively small numbers, have been scarce.
"Over the decades [scientists] have reconstructed the gigantic members of the terrors birds as a scaled up version of the small ones," Chiappe said.
Such portraits invariably painted the larger species as slower and lumbering.
Writing in tomorrow's issue of the journal Nature, Chiappe and colleague Sara Bertelli say analysis of the new skull and foot bones indicate that large terror birds were both nimble and swift.
The notion that larger species are slower is "largely incorrect," Chiappe said. "We don't really see that inverse correlation between large size and less agility."
Chiappe estimates that the new species ran as fast as modern day emus or rheas but were not as speedy as ostriches, the world's largest and fastest living birds.
Exploring the Unknown
Chiappe says future biomechanical studies of fossils like the recently found foot bones could determine just how fleet-footed large terror birds were.
But overall, much about terror bird behavior remains unknown.
Scientists still aren't clear, for example, whether the birds hunted in packs like velociraptors or individually like large predatory cats.
A 2005 study suggested that the giant birds may have used kung fu-style kicks to break the bones of their prey and extract marrow.
Chiappe says he's particularly keen to run CT scans on the brain case of the new specimen and smaller phorusrhacids, to learn more about the species.
Editor's Note: Luis Chiappe has received funding for his research from the National Geographic Society's Committee for Research and Exploration.
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