How many T-Rexes roamed planet earth?

Computer simulations indicated, with 97.5 percent probability, that there were at least 1,300 adult tyrannosauruses at any given time but not more than 328,000. Credit...Science Photo Library/Science Source

T-Rex, poster child of dinos, roamed North America in pretty sizable numbers though the exact number remains elusive to say the least.

Before they were killed off by a meteor that hit Earth 66 million years ago, some 20,000 adults of the iconic ferocious dinosaur predator — Tyrannosaur rex — roamed North America at any given time, researchers have calculated.

Why it's hard.

Skeletal features can tell a lot about an animal. For example, someone looking at a human tooth could infer that it is suited for chewing both plants and meat, and the shape of the skeleton could yield an estimate of how fast a person can run. But the physical attributes cannot tell you how many people live in New York City.

For living species, John Damuth, a biologist at the University of California, Santa Barbara, came up with a mathematical relationship, now known as Damuth’s law, between the average body mass of an animal and its expected population density.

The relationship is not universal but generally holds for large classes of animals like lizards or meat-eating mammals. So, for Tyrannosaurus rex, they had to not only plug in the weight of the dinosaur — about six tons, give or take a few — but also derive other numbers in the law.

Best guess ...

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.

It least it's a smart guess, right? :) Awesome illustration of T -Rex without a doubt.

As a walker, T-Rex took all the time in the world. :) About 2.9 miles/hr.