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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

More than 2 billion Tyrannosaurus rexes walked the Earth. That’s according to a new study published in the journal Science.

The reptiles were not all alive at the same time. They were around for about 1 million to 4 million years, about 70 million years ago. There may have been about 20,000 adult T. rexes at any one time.

Only about a hundred T. rex fossils have been found. Researchers arrived at the number 2 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. He’s also 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.”