According to experts, 2.5 billion Tyrannosaurus rex dinosaurs roamed North America during the late Cretaceous period.
The researchers estimate that about 20,000 adult T. rexes were alive at any given time during the species' lifetime - give or take a factor of ten.
Researchers published their report, titled "Absolute Abundance and Preservation Rate of Tyrannosaurus Rex," in the journal Science.
2.5 billion Tyrannosaurus rex Dinosaurs: That's a Whole Lotta Jaws!
Researchers at the University of California, Berkeley calculated how many T. rex lived over 127,000 generations using estimates focused on body size, sexual maturity, and the creatures' energy needs. It's a first-of-its-kind figure, but it's just an estimation with a T. rex-sized margin of error.
"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," he told the Associated Press.
T. rex population density was low at any given time since the species roamed North America for 1.2 million to 3.6 million years. The report said there would be about two in a city the size of Washington, D.C., or 3,800 in California.
"Probably like a lot of people, I literally did a double-take to make sure that my eyes hadn't deceived me when I first read that 2.5 billion T. rexes have ever lived," said Macalester College paleobiologist Kristi Curry Rogers, who wasn't part of the study.
2.5 billion Tyrannosaurus rex Dinosaurs: What Now?
The calculation, according to Marshall, aids scientists in determining the preservation rate of T. rex fossils and emphasizes how fortunate the world is to know about them at all.
Experts found around 100 T. rex fossils. Around 32 of them contained enough material to determine that they are adults. Marshall said they would have never known about T. rex if there were 2.5 million instead of 2.5 billion.
Marshall's team measured the population density by applying a general biology rule of thumb that states that the larger the species, the less dense the population.
The researchers chose to treat T. rex as a predator with energy needs halfway between those of a lion and those of a Komodo dragon, the world's largest living lizard.
The researchers also chose to disregard juvenile T. rexes, who are belittled in the fossil record. These young creatures may have lived separately from adults, hunted different prey, and acted more like a different predator group.
Given the unknowns about the generation length, range, and how long they roamed, the Berkeley team estimated that the total population could be as low as 140 million or as high as 42 billion, with 2.4 billion being the midpoint.
The researchers also noticed that the differences they discovered differed over time. They noted gaps being smaller in Jurassic communities 200-145 million years ago and larger in Cretaceous communities 145-65 million years ago.
Professor Marshall expects that most of his colleagues, if not all, contended the team's estimates.
The researchers have made the computer code they used to estimate T. rex numbers accessible to other scientists. They claimed that it might pave the groundwork for calculating how many organisms are still unknown to science.
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