"It's the first documentation of giant raptors in Patagonia measuring 5 meters [16.5 feet] or more," Novas said. "No one expected this, it's a new lineage."
The researchers call Austroraptor "bizarre" because of its short arms, which, along with its large size, distinguishes the new raptor from its unenlagiine relatives.
Novas says the new raptor is the first ever found with short arms.
Its shorter arms and more robust thigh bones, which supported the heavy animal, rule out any possibility of flying, Novas said. Smaller, longer-armed crow- and turkey-size relatives in Patagoni probably did fly, he added.
The size of the Austroraptor probably made it a vicious predator, said Paul Sereno, a paleontologist at the University of Chicago. The new find places raptors in the "big league" of dinosaurs, he said.
"This was a monster raptor that makes the Velociraptor look like kid's play," said Sereno, also a National Geographic Explorer-in-Residence. "At five meters long with a sickle-shaped claw, it was an amazing predator."
"Normally you'd hold an ostrich-size raptor's skull in your hand. This one's skull is almost a meter long."
The new fossils, combined with new finds from China, show that dromaeosaurs "weren't trying to become birds—they were, like all animals, diversifying over time," said Thomas Holtz, a vertebrate paleontologist from the University of Maryland.
(Related: "New Birdlike Dino Adds to Debate on Origins of Flight" [October 18, 2005].)
The new dinosaur also has a "weird" long skull, Holtz said.
"On first glance it looks like a pterodactyl," he said. "In fact, there has been a small and closely related dinosaur found in South America which has a similar skull. This suggests the whole radiation of southern dromaeosaurs have these long tapering skulls.
"What it does show is that we think we know a lot about the raptors, but even a relatively familiar group of dinosaurs continues to surprise us with their diversity," Holtz added.
Suddenly, the world of southern raptors looks a lot bigger and more complex.
Not only was the Southern Hemisphere home to giant raptors as well as smaller, birdlike specimens, but those large southern raptors were still living at a time when their northern counterparts had died off, Novas said.
"It means that raptors here had very different evolutionary paths from those up north," he said.
"This Austroraptor shows us that here in South America giant raptors evolved and survived until the end of the dinosaur age.
"This new evidence clearly indicates that South America was the site of a very prolific lineage of carnivorous dromaeosaurs, whose evolutionary history now begins to reveal itself," Novas added.
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