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Counting T. Rexes

OPEN WIDE This life-size model of a tyrannosaur can be found in a park in Germany. CHRISTOPHE GATEAU—PICTURE ALLIANCE/GETTY IMAGES

Some 2.5 billion Tyrannosaurus rexes walked the Earth. That’s according to a new study published in the journal Science.

The reptiles were not alive all at the same time. They roamed for about 1.2 million to 3.6 million years, nearly 70 million years ago. At any one time, there may have been about 20,000 adult T. rexes.

Only about 100 T. rex fossils have been found. Researchers arrived at 2.5 billion by looking at key facts about T. rex, including its average body size and energy needs.

Experts were surprised by the estimate. Charles Marshall is a biology professor at the University of California, Berkeley, and the study’s lead author. “That’s a lot of jaws,” he says. “That’s a lot of teeth. That’s a lot of claws.”