Megaraptor

Jeff Poling

Scientists have discovered the largest 'raptor-like dinosaur found to date, in the barren hills of northwestern Patagonia in Argentina.

The discovery was announced December 2, 1997, at Houston's Museum of Natural History. Argentine paleontologist Fernando Novas unveiled a cast of the animal's 13-inch slashing toe-claw. The total find consists of the claw, a hind-leg bone and a fore-leg bone. The fossil was named Megaraptor namunhuaiquii, "large thief with lance feet," by Dr. Novas.

Most dromaeosaurids, often erroneously called "raptors," typically stood about the height of a human, according to Peter Dodson, professor of veterinary anatomy and geology at the University of Pennsylvania in Philadelphia. They averaged 8 to 9 feet long and may have weighed 150 to 185 pounds. The largest confirmed dromaeosaurid, Utahraptor, may have reached lengths of 16 feet. Megaraptor, if a true dromaeosaurid, would be the largest by far at 25 to 30 feet long and 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."

Most known dromaeosaurids are north american or asian. The discovery of Megaraptor's therefore has come as a bit of 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."

Sues explains that smaller dromaeosaurids 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 Megaraptor's appearance in South America, he says, is that a land bridge or archipelago could have existed at the time, allowing dromaeosaurids to migrate south, much as placental mammals did in the Cenozoic. Evidence of such a bridge, if it existed, would likely have been destroyed due to the extreme geologic activity of the Caribbean.

The more likely explanation, Sues says, is 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, populations continued to evolve separately and ended up with similar forms. Megaraptor would not, then, be a dromaeosaurid.

The extreme size of Megaraptor, dromaeosaurid or not, is not much of a surprise. "Most South American dinosaur fauna are oversized forms," says Rodolfo Coria, another Argentine paleontologist who uncovered Giganotosaurus, possibly the largest theropod to have ever existed. "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."

From: "Thomas R. Holtz, Jr." <th81@umail.umd.edu>

Unfortunately, the technical paper describing Megaraptor is not yet out. Since they've held a press conference on the critter, I don't think I'd be leaking any confidences here, so:

Megaraptor is known mostly from some forelimb and hindlimb elements, but neither are complete. The sickle claw is quite large, and has dromaeosaurid curvature and cross-section. Its metatarsus is not like a (typical) dromaeosaurid, in that it is fairly gracile (dromaeosaurids have shorter, stockier metatarsi than almost any other group of theropod). However, it does not seem to have a pinched third metatarsal, so it's not a troodontid.

Be careful of most of the data in these press releases: without any skull or vertebral elements to go by, estimates on length are "iffy" (to use the technical jargon... :-).

A visitor to my site just forwarded me an article saying that Novas thought Megaraptor evolved convergently with dromaeosaurs. Is this true? If so, what does he think Megaraptor is? A huge troodontid? What?

Novas has not committed on the subject. The foot is more gracile than any typical dromaeosaurid, but it is not arctometatarsalian.