In All of Time, 2.5 Billion Tyrannosaurus Rexes Have Roamed Earth

The study used calculations based on body size and metabolism rate to estimate out how many dinosaurs lived throughout the species existence

A photo of the Tyrannosaurus Rex specimen at the Field Museum of Natural History in Chicago, IL
Researchers calculated that the T-Rex population, at any given time was 20,000 adult individuals, continued for 127,000 generations, and each generation lasted for 19 years. Evolutionnumber9 via Wikicommons under CC BY-SA 4.0

The iconic Tyrannosaurus rex is a ferocious predator with razor-sharp teeth that lived 68 million years ago during the Late Cretaceous period. Despite the dinosaur's gargantuan popularity in modern media, researchers actually haven't estimated how many individuals in the species existed during their reign on Earth in total—until now, reports Kenneth Chang for the New York Times.

By using approximations of the fierce dinosaur’s body mass, sexual maturity, and metabolism, scientists now estimate that 2.5 billion T. rexes walked the Earth during its existence, reports Karina Shah for New Scientist. The new study was published this week in the journal Science.

Charles R. Marshall, a University of California, Berkeley paleontologist, was fascinated with the idea of how many of the mega-predators walked the Earth whenever he held a T. rex fossil, reports the New York Times.

“Were there are a million, a billion, a trillion T. rexes? Is this one in a million, one in a billion, one in a trillion? How on earth could we know that number? We all know fossils are rare, but how rare are they? And so it really started with that question,” Marshall tells the New York Times.

To find out, Marshall and his team used Damuth’s law to estimate the T. rex population. Under Damuth’s law, researchers calculate average population density using the animal’s body mass. Damuth’s law states the larger the animal, the less dense its population, reports Seth Borenstein for the Associated Press. While not universal, it is accurate in larger animals, like mammals and lizards, the New York Times reports.

Using previous studies, climate models, and locations of T. rex fossils, the research team found the average body mass of a T. rex was about 11,464 pounds, and the geographic range of the species was about 888,000 square miles across North America, reports New Scientist. Marshall and his team also calculated the average T. rex lifespan and when it reaches sexual maturity. The T. rex can live up to its late twenties and reaches sexual maturity at 15.5 years, reports Amy Woodyatt for CNN. The amount of energy a T. rex needed to survive was also calculated to be around the same amount as a Komodo dragon or a lion, AP reports.

The team then used these estimations and data from other living species to calculate T. Rex abundance, at any given time, was about 20,000 adult individuals, continued for 127,000 generations, with each generation lasted for 19 years, CNN reports. The species was on the planet for 1.2 million to 3.6 million years, so the population density was really small at any given time.

To put this estimation in perspective, only two T. rexes would occupy a place the size of Washington, D.C., or 3,800 would roam an area the size of California, the AP reports. In total, the overall T. rex population to ever exist was estimated at 2.5 billion.

The study also helped researchers understand the preservation rate of T. rex fossils. Based on this population density, only one in 80 million T. rexes were preserved as fossils, New Scientist reports. If the T. rex population were 2.5 million instead of 2.5 billion, then we would probably never have known the T. rex existed at all, Marshall explains to AP.

“Studies like this are the first step in recreating ancient ecosystems. We need to move beyond what fossils were found and where to the larger picture: how the ecosystem functioned, ” says Felisa A. Smith, a biology professor at the University of New Mexico, to the New York Times.

Marshall and his team are planning on applying the method to calculate other population densities of dinosaurs that also lived during the Cretaceous period to model how ecosystems looked at that time, reports New Scientist.