How Many Tyrannosaurus Rexes Ever Lived?

How Many Tyrannosaurus Rexes Ever Lived on Earth? Here’s a New Clue.

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A truly bold estimate. How many Tyrannosaurus rexes ever lived? When the asteroid struck Earth 66 million years ago, roughly 20,000 adult Tyrannosaurus rexes were living in North America. According to the simulation, there was a 97.5% probability that between 1,300 and 328,000 of them lived at the same time. This result was calculated on the assumption that, since the dinosaurs were mesotherms (between cold-blooded and warm-blooded), they would have had a population density somewhere between that of modern carnivorans and the Komodo dragon. If the figure of 20,000 living at the same time is correct, then since Tyrannosaurus rex survived for 2.4 million years, it can be estimated that a total of 2.5 billion of them lived. With 20,000 living simultaneously across North America, you can think of it as roughly two adults living in an area the size of Washington, D.C. On the other hand, a population density this low could be too small—it is a number small enough that a single epidemic could wipe them all out. Compared with modern predators, the estimated 20,000 amounts to slightly fewer than 4,000 tigers living and slightly more than 4,000 lions. It is a wonderful attempt to reconstruct an ancient ecosystem.