In the age of dinosaurs, a succession of mega carnivores lorded over the landmass that is now North America. First came the 30-foot-long Allosaurus, 145 million years ago. Sixty million years later, the 40-foot-long T. rex reigned supreme. But scientists didn't know what, or who, prevented T. rex from taking the throne during the intervening period. That is, until paleontologists from the Field Museum and North Carolina State University, on a dig in Utah, unearthed bones from the leg, tail, and spine of an all-new species. The researchers dubbed the dino Siats meekerorum after a man-eating monster in local Ute legend. The beast would have rivaled T. rex in size and power had the two coexisted. But only after Siats went extinct could tyrannosaurs, then the size of white-tailed deer, evolve into fearsome T. rex and assume the role of apex predator.