2.5 billion T. rex roamed Earth, study finds

Imagine 2.5 billion Tyrannosaurus rexes

That's how many scientists think roamed the earth

Researchers factored in size, life expectancy, and geographic range

T. rexes inhabited North America for 2.4 million years

Their skulls alone were about 5 feet long

(CHARLES MARSHALL, UNIVERSITY OF CALIFORNIA MUSEUM OF PALEONTOLOGY DIRECTOR): "The key step is using data from living animals, where there's a clear relationship between population density - the number of individuals per unit area and body mass. There are relatively few elephants. There are a lot more zebra. There are bucket loads of of rabbits and mice."

"We computed the generation time for the first time at about 19 years. The geologic longevity is roughly 2.5 million years. That means it lasted for about 125,000 generations. Our species has been around for about a tenth that. And multiplying the number of generations by the standing population size gave us the 2.5 billion."

T. rexes have been extinct for 66 million years