One Tyrannosaurus rex seems scary enough. Now picture 2.5 billion of them. Thats 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 Thursdays journal Science. Its a first-of-its-kind number, but just an estimate with a margin of error that is the size of a T. rex.

Thats a lot of jaws, said study lead author Charles Marshall, director of the University of California Museum of Paleontology . Thats a lot of teeth. Thats a lot of claws.