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

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That includes the metabolism. Dinosaurs are no longer thought to be coldblooded like modern-day lizards, but they were probably not as warm-blooded as mammals. So Dr. Marshall’s team assumed a physiology in between that of meat-eating mammals and Komodo dragons. They also had to account for some uncertainty about where the dinosaurs lived in North America. Paleontologists don’t know whether the range of T. rex was limited to where fossils have been found in the western United States and Canada, or if it stretched to other places with similar climates back then, from Alaska to the East Coast.

Because so much is unknown about dinosaurs, the scientists were not looking to come up with a single definitive answer, but to provide limits on what they thought a plausible number might have been. “For most of the paleontological data, I don’t know how to guess a number,” Dr. Marshall said. “But I can tell you what a good minimum is, and what a good maximum is.”

The calculations yielded a most likely standing population of 20,000 Tyrannosaurus rex adults. That would indicate a sparse distribution equivalent to two adults in an area the size of Washington, D.C.

But the uncertainty around that estimate was very broad. The same computer simulations indicated, with 97.5 percent probability, that there were at least 1,300 adults but not more than 328,000.

If the 20,000 number is correct, over the 2.4 million years that T. rex walked the Earth, there would have been a total of some 2.5 billion adults that ever lived.

But even Dr. Marshall thinks the 20,000 number is likely low. “It just seems inconceivable you can last a couple of million years with those few individuals,” he said. “You just need some horrible plague or something and you’re gone.”

He said he thought the population could have been tens of thousands or maybe 100,000 or 200,000. A large part of the uncertainty is that Damuth’s law is not absolute. Jaguars and spotted hyenas are both meat-eating mammals of similar size, but the population density of hyenas is some 50 times higher.

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