Giganotosaurus goes on the prowl. The theropod thrived 100 million years ago, some 25 million years before T. rex. Roughly six feet bigger than T. rex and a ton or two heavier, Giganotosaurus nonetheless was a relatively fast dinosaur whose pointed tail apparently provided balance and the ability to make quick turns as it ran. Fossil evidence suggests its prey included Argentinosaurus, a 125-foot-long herbivore.