Imagine two-point-five billion school bus-sized predators roaming the Earth. Researchers at the University of California - Berkeley think that's how many Tyrannosaurus rexes lived and died during the creature's estimated two-and-a-half million years in North America. The team estimated that around 20-thousand adult T-rex existed at any one time, with one meat-eating dinosaur for every 38 square miles. They also believe there were about 127-thousand generations of the species, bringing the team to their total population estimate