Study: 2.5 billion T. rex used to roam the Earth, but not all at once By Seth Borenstein Paleontologists uncover large dinosaur bone in southwestern France Paleontologists said they discovered the bone of a large dinosaur at an excavation site in southwestern France in July. 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. 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 Thursday’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.