[Photo] An artist's rendering of a new species of phorusrhacid, or terror bird, shows the massive creature's sharp, curving beak. The fossil skull of the new specimen???seen in the bottom photo alongside a modern California condor skull???is 2.3 feet (almost a meter) long and represents the largest bird skull ever found. Skulls photograph by S. Bertelli, illustration by S. Abramowicz

Just in time for Halloween, paleontologists have dug up a truly scary creature???Big Bird's bad, buff brother.

The real-life fossils belong to a new species of phorusrhacid, giant predators also known as terror birds that once dominated South America.

Terror birds were the biggest birds the world has ever seen, and the new species is by far the largest terror bird yet, says paleontologist Luis Chiappe, director of the Dinosaur Institute at the Natural History Museum of Los Angeles County, California.

"Some of these birds had skulls that were two and a half feet [almost a meter] in length. [They] were colossal animals," he said.

The new, currently unnamed species stood about ten feet (three meters) tall and had a head as big as that of a horse.

The largest terror birds could likely swallow dog-size prey in a single gulp, experts say.

The bird's most striking feature???literally???was its giant nose, a roughly 18-inch (46-centimeter) beak with a sharp, curving hook shaped like an eagle's beak.

Whether the flightless birds used their beaks to impale or bludgeon their prey is unknown, Chiappe says. But a single hit from their "massive skull[s] would have killed anything immediately."

Intact Skull

Terror birds were first discovered in the late 1800s and are believed to have become South America's top predators after the dinosaurs died off 65 million years ago.

A high school student unearthed the ancient remains of the new species three years ago in the Patagonia region of Argentina.

Chiappe and his colleagues examined the specimens and found that the fossils???a well-preserved skull and foot bones???are about 15 million years old.

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.