Size and Weight:
One of the most frequently misspelled of all dinosaurs, Giganotosaurus is often called "Gigantosaurus"--which may be appropriate, but still isn't quite kosher. However you spell it, though, this South American theropod was truly gigantic, at 40 feet long and 8 tons slightly outweighing even Tyrannosaurus Rex (but still not approaching the size of the biggest theropod of them all, Spinosaurus). Giganotosaurus also appears to have had a more formidable arsenal at its disposal, including bigger arms with three clawed, grasping fingers on each hand. (See 10 Facts About Giganotosaurus and a gallery of Giganotosaurus pictures.)
As large as it is, Giganotosaurus is a relatively recent find, the "type specimen" only being identified in 1993 by an amateur fossil hunter in Argentina. Based on an analysis of this skeleton, which is 70 percent complete, paleontologists have concluded that Giganotosaurus was closely related to the well-known Carcharodontosaurus ("great white shark lizard"), as well as the slightly lesser known Mapusaurus and Tyrannotitan. While Giganotosaurus may have rivaled T. Rex in size, it didn't have close to its chomping power: an analysis has shown that this Giant Southern Lizard's bite was three times less powerful.
A big carnivore requires big meals--which is why it's intriguing that paleontologists have unearthed the remains of a massive Argentinosaurus (a slow-witted sauropod, and possibly the biggest dinosaur that ever lived) near those of Giganotosaurus. While it's unlikely that a lone Giganotosaurus could have taken down a full-grown giant like Argentinosaurus, this discovery could be a hint that--just maybe--Giganotosaurus hunted its large, slow-witted prey in nimble packs. (For an analysis of this encounter, see Giganotosaurus vs. Argentinosaurus - Who Wins?)