One Tyrannosaurus rex seems scary enough. Now picture 2.5 billion of them. That’s how many of the fierce dinosaur kings probably roamed Earth over the course of a couple of million years, a new study finds. Using calculations based on body size, sexual maturity, and the creatures' energy needs, a University of California-Berkeley team figured out just how many T. rex—the biggest land-living carnivore of all time—lived over 127,000 generations, according to a study in the journal Science. It's a first-of-its-kind number, the AP reports, but just an estimate with a margin of error that is the size of a T. rex. "That's a lot of jaws," said study lead author Charles Marshall, director of the University of California Museum of Paleontology. "That's a lot of teeth. That's a lot of claws."
The species roamed North America for about 1.2 million to 3.6 million years, meaning the T. rex population density was small at any one moment. There would be about two in a place the size of the Washington, DC, or 3,800 in California, the study said. Marshall said the estimate helps scientists figure the preservation rate of T. rex fossils and underscores how lucky the world is to know about them at all. About 100 or so T. rex fossils have been found—32 of them with enough material to show they were adults. If there were 2.5 million T. rex instead of 2.5 billion, we would probably not know they existed, he said. Given the uncertainties, the Berkeley team said the total population could be as low as 140 million or as high as 42 billion. The science is important, "but the truth, as I see it, is that this kind of thing is just very cool," said Purdue geology professor James Farlow.
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