The tough thing about being on top is that someone's always trying to dethrone you, and last week two paleontologists announced that they had toppled the biggest king of all. In a remote corner of Argentina, Rodolfo Coria and Leonardo Salgado excavated the remains of a carnivorous dinosaur larger than Tyrannosaurus rex. Named Giganotosaurus, the two-legged meat-eater was "probably the world's biggest predatory dinosaur," Coria and Salgado write in the journal Nature. Based on the 70 percent of the skeleton unearthed since the summer of '93, the creature was 12.5 meters long, one meter longer than the largest T rex (nicknamed Sue after its discoverer). And Giganotosaurus has such massive bones that it likely outweighed five-ton Sue by at least 4,000 pounds. It lived 100 million years ago, 30 million years before T rex evolved in North America. Both arose out of similar environments: a savannah-like landscape with lots of big sauropods to fill the belly of the beast.