Giant Dinosaur Fossil Forces Scientists to Question Theories

The barren hills of northwestern Patagonia in Argentina have yielded fossil remains of the largest raptor-like dinosaur ever discovered.

Nearly four times longer and eight times heavier than the voracious velociraptors of "Jurassic Park," the 90-million-year-old creature is challenging notions of how raptors may have behaved and how widely they were distributed. It also represents the latest in a string of South American fossil finds that are yielding clues to issues ranging from the evolution of birds to the splitting of continents.

The discovery was announced Dec. 2 at Houston's Museum of Natural History, where Argentine paleontologist Fernando Novas unveiled a cast of the raptor's 13-inch toe claw. So far, the claw, a leg bone, and two arm bones have been unearthed. Dr. Novas named the creature Megaraptor namunhuaiquii, loosely translated as "large thief with lance feet."

The moniker "thief" may be a bit tame. Megaraptor, Novas says, is a very distant relative of a group of meat-eating predators, including Tyrannosaurus rex.

Most raptors typically stood about as high as a human, says Peter Dodson, a professor of veterinary anatomy and geology at the University of Pennsylvania in Philadelphia. From tail to nose, they averaged 8 to 9 feet long and tipped the scales at between 150 and 185 pounds, while at least one species, Utahraptor, reached lengths of 16 feet.

Megaraptor, by contrast, is 25 to 30 feet long and probably stood 13 feet tall.

"We've tended to view raptors as small, swift, vicious, and cunning hunters rather than brutal and strong," Dr. Dodson says, "Now, our confidence in that picture is being shattered a bit."

Megaraptor's home turf also comes as a surprise.

"This is the first record from South America of this group of dinosaurs," says Hans-Dieter Sues, a paleontologist at the Royal Ontario Museum in Toronto. "Previously, we thought they only lived in the Northern Hemisphere."

He explains that smaller raptors were widely distributed in Laurasia - what was to become North America and Asia. But Gondwanaland, which would later split into Africa and South America, was isolated from Laurasia.

One possible explanation for raptors' appearance in South America, he says, is that a land bridge or archipelago could have existed at the time, allowing raptor-like creatures to migrate south. The Caribbean is so geologically active that any evidence of such a bridge is likely to have been destroyed.

The more likely explanation, he says, holds that Megaraptors and their northern counterparts evolved separately from common ancestors that had a worldwide distribution. Once the earth's land mass began to break apart, dinosaurs evolved into what Dr. Sues terms "more provincial" species.

Whatever the explanation, is it clear that South America represents a unique natural museum, and did even in the days of the dinosaurs themselves.

"Most South American dinosaur fauna are oversized forms," says Rodolfo Coria, another Argentine paleontologist who uncovered Giganotosaurus, the largest-ever flesh eater. "They represent primitive assemblages of dinosaurs that were widely distributed around the world during the Jurassic period, but survived another 50 million years into the Cretaceous period in South America. This was their last bastion before they became extinct."

Dinosaurs such as Megaraptor and Giganotosaurus, which stood high above their northern counterparts, also may represent the limit on size for predators that walk on their hind legs. "Physiology is the limiting factor," says Dodson. "There may be other discoveries in their size range, but not 50 percent larger."