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On December 2, 1997 the discovery of Megaraptor namunhuaiquii ("large thief with lance feet") was announced at Houston's Museum of Natural History. It was discovered by paleontologist Fernando Novas in northwestern Patagonia in Argentina. Megaraptor was announced by the unveiling of it's giant 13-inch slashing toe-claw.
There has been quite a controversy over exactly what group Megaraptor belongs to. It's huge claw compares to that of the typical dromaeosaurids (Known as Velociraptor and it's larger cousin Utahraptor) in shape, but definitely not in size. A typical Velociraptor "averaged 8 to 9 feet long and may have weighed 150 to 185 pounds" (3). Utahraptor reached a length of 16 feet. Then the discovery of Megaraptor blew them away. "Megaraptor, by contrast [to other raptors], is 25 to 30 feet long and probably stood 13 feet tall" (5).
Very few bones of Megaraptor have been discovered. The explanation given by Tom Holtz of the University of Maryland, College Park is as follows; "Megaraptor is known mostly from some forelimb elements, but neither are complete. The sickle claw is quite large, and has dromaeosaurid curvature and cross-section. It's metatarsus is not like a (typical) dromaeosaurid, in fact 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" (1)
Another problem to consider about Megaraptor is that it is found in South America in the layers of the late-Cretaceous period (approx. 90 million years ago). At this time the Earth is split into two major continental masses: Laurasia and Gondwanaland. Laurasia contained what is today North America and Asia. Gondwanaland included South America and Africa. Velociraptor and Utahraptor were found exclusively on the Laurasian continent, while Megaraptor was found on Gondwanaland. The last "known" time that we are sure these masses were connected was during the Jurassic when the supercontinent of Pangea broke apart into the two continents which are being discussed. It is known that Velociraptor and other dromaeosaurids as well as Megaraptor were specifically Late Cretaceous animals. This means that their relation (if there is any) came from a common ancestor millions of years prior during the Jurassic, making them far distant "cousins." "'Most [members of the] 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 [Gondwanaland]. This was their last bastion before they became extinct" (3).
One theory as to how Megaraptor and the dromaeosaurids could be closer related would be if at one time there was a land bridge between Laurasia and Gondwanaland. The land bridge would have acted in the same way as the one which very recently in time connected Russia and Alaska by way of the Bering Strait, allowing humans and other Eurasian fauna to venture onto the North American continent. Such a land bridge would "allow 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" (5).
It is most likely possible that Megaraptor evolved from a more primitive dinosaur. To use another description from Tom Holtz, "Velociraptors had 'short, stocky feet' thought to have evolved to 'withstand the stresses of using the "sickle" claw,' but [Megaraptor had] the foot long, thin feet that nonetheless were capable of wielding the largest dino claw yet found" (2). The "short, stocky feet" mentioned were an adaptation made by "raptors" probably during the late-Cretaceous. "The foot anatomy is distinctive, suggesting a remarkable case of evolutionary convergence (the panda's 'thumb'-like appendage is the classic example) where an animal develops a feature much like another but from independent ancestry" (4). The slender feet of Megaraptor have a more primitive aspect. This leads to believe that Megaraptor evolved completely separately, from a creature of Gondwanan origin.
Novas, the man who discovered Megaraptor, believes that it probably is closely related to a group of meat-eating dinosaurs which include Tyrannosaurus rex. This means that Megaraptor descended from the Jurassic meat-eaters such as Allosaurus and it's predecessor Megalosaurus. Then as Pangea broke apart Allosaurus was split into two different taxa. In Laurasia evolution brought the former king of all dinosaurs Tyrannosaurus rex. However, in Gondwanaland the even greater giants (recently discovered throne takers) such as Giganotosaurus and Carcharodontosaurus took precedence. Even though Megaraptor indeed had a claw which was almost identical in design to that of Velociraptor, it is now thought that it was just an evolutionary feature which happened to evolve (in parallel convergence) on two separate continents.
One thing which needs to be made clear is that so little has been discovered of Megarapter that most of what has been discussed may not necessarily be true. This interpretation has been determined using the best available evidence known. However, since no part of the skull or vertebral elements have been found to date, one should approach such any information with caution, including this paper, before making any judgment. Especially be wary of press releases which sometimes overexcitedly report theories as evidence.