• Source: Associated Press One Tyrannosaurus rex seems scary enough. Now picture 2.5 billion of them. That’s how many of the fierce dinosaur king probably roamed Earth over the course of a couple million years, a new study finds. Life-sized Tyrannosaurus rex models are unloaded for a dinosaur exhibition in Potsdam, Germany, in 2006. Source: Associated Press Using calculations based on body size, sexual maturity and the creatures' energy needs, a team at the University of California, Berkeley figured out just how many T. rex lived over 127,000 generations, according to a study in today's journal Science. It’s a first-of-its-kind number, but just an estimate with a margin of error that is the size of a T. rex.