T. rex were once so common on Earth you could probably find 2 roaming a Washington DC-sized area, study suggests

Story Highlights

• “That’s a lot of jaws,” said study lead author Charles Marshall. “That’s a lot of teeth. That’s a lot of claws.”
• The species roamed North America for a few million years.
• The authors estimated that the species' population density was about 3,800 T. rex in an area the size of California.

We all know that the most terrifying dinosaur of them all – Tyrannosaurus rex – once roamed the Earth. But just how many of these fearsome beasts were there?

According to a new study that estimated the population traits of the iconic, long-extinct species, over their entire late-Cretaceous-Era reign, the total number of Tyrannosaurus rex that ever lived on Earth was roughly 2.5 billion.

“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 a few million years, meaning the T. rex population density was small at any one specific moment. In fact, at any one time, around 20,000 T. rex would have been alive on the planet. 

The authors estimated that the species’ population density was about 3,800 T. rex in an area the size of California – or two in an area the size of Washington D.C.

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Using calculations based on body size, sexual maturity and the creatures’ energy needs, researchers figured out just how many T. rex lived over 127,000 generations.

Marshall’s team calculated the population by using a general biology rule of thumb that says the bigger the animal, the less dense its population.

In this Tuesday, March 7, 2006 file photo, life-sized Tyrannosaurus rex models are unloaded for a dinosaur exhibition in Potsdam, Germany. A study released on Thursday, April 15, 2021, calculates that 2.5 billion Tyrannosaurus rex prowled North America over a couple million years or so, with maybe 20,000 at any given time. (Photo: Sven Kaestner, AP)

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.

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Only about 100 or so T. rex fossils have been found – 32 of them with enough material to figure they are adults. If there were 2.5 million T. rex instead of 2.5 billion, we would probably have never known they existed, he said.

Thomas Holtz, a vertebrate paleontologist at the University of Maryland in College Park, calls the calculation an “interesting speculation,” adding that “we always knew that the chance of any individual becoming a fossil was exceedingly rare, but we lacked the calculation to figure out how rare,” according to the journal Nature.

The study appeared in the peer-reviewed journal Science, a publication of the American Association for the Advancement of Science.  

Contributing: The Associated Press

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