Terror Bird Rises Again

Fossil find may rewrite the book on ancient bird group

12-01-2006 // Roger Di Silvestro

High school student Guillermo Aguirre-Zabala was wandering among rocky outcrops near the railroad station in his village of Comallo, Argentina, two years ago when he found large limb bones and a skull that suggested some nightmare creature out of the film Lord of the Rings. The skull was more than 2 feet long—the size of a horse skull—and tipped with a sharp, hooked beak like that of a monstrous eagle.

The nearly complete skull was a fossil, and paleontologists soon stepped in to give the skull a close look and to excavate more bones at the site. What they had on their hands proved to be the remains of the largest known bird species ever found, a flightless predator that had stood about 10 feet tall and that would have tipped the scales at about 400 pounds. Its foot and leg bones indicate to paleontologists that it was a fast runner, speedy enough to run down and nab prey that included reptiles, rodents and other small mammals that scurried across the plains of Patagonia some 15 million years ago.

Not yet given a technical name, the bird is one of a group of closely related species that paleontologists jawbreakingly call “phorusrhacids” or, more mercifully, “terror birds.” All of these birds were flightless, and most ate only plants. Previously known species ranged in size from about 2 feet to 9 feet tall and lived in South America between 2 million and 60 million years ago, an era in which the continent was not yet joined by the Panamanian land bridge to North America. Apparently, when the land bridge appeared, the terror birds disappeared.

The new fossil is overturning some of the traditional wisdom concerning terror birds. Previously found skulls of large terror birds were fragmentary, forcing scientists to model them after the better-known skulls of smaller relatives. The new skull is 10 percent larger than the size scientists had postulated for large species, suggesting that the skulls of those birds need to be remodeled. Moreover, paleontologists had surmised that the larger species were slow runners, but the leg and foot bones of the new giant suggest otherwise. “We conclude that reconstructions of the skull of gigantic phorusrhacids on the basis of smaller relatives is unwarranted, and that the long-established correlation between their corpulence and reduced cursorial agility needs to be re-evaluated,” wrote Luis Chiappe and Sara Bertelli of the Natural History Museum of Los Angeles County in a recent Nature article about their studies of the new fossil.—Roger Di Silvestro

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