# 2.5 billion T. rex dinosaurs once lived, study finds

For the first time, scientists have estimated how many Tyrannosaurus rex, the so-called king of the dinosaurs, ever roamed the Earth.

Because it is important: The number is staggering: 2.5 billion Tyrannosaurus rex lived and died during the roughly 2.4 million years the species survived on the planet, according to a new study to be published in the journal. Sciences on Friday.

The study can help contextualize the fossil record and the rarity of finding certain fossilized prehistoric organisms, according to lead researcher Charles Marshall, director of the Museum of Paleontology at the University of California.

• “I mean, to me, it’s just amazing that we could have found a number,” Marshall told Axios. “Some people have asked me, ‘How does your number compare to other numbers in the total that you have ever lived?’ The answer is that it is not like that because there were none. “

How does it work: The team of researchers could not use the limited fossil record to estimate the population of the species, so they instead used Damuth’s Law, which describes a relationship between population density and body mass.

• The relationship, used in population ecology, generally states that species with larger body sizes tend to have lower population densities.
• The researchers then calculated the average body mass of a T. rex, setting an average of 5,200 kilograms (approximately 11,460 pounds).
• Using body mass and population density, the team calculated that the species had a population density of about one individual per 40 square miles.

By the numbers: With this information and an estimated geographic area that the species occupied, the researchers were able to estimate that around 20,000 T. rex were alive at any given time that the species lived on the planet.

• To find the total number of T. rex that walked the Earth, the team multiplied the species’ standing population by the number of generations it spanned (about 127,000), which they determined by dividing how long the species survived by its age. estimated generation of 19 years.
• The researchers noted that their estimated population density for the species would translate to about 3,800 T. rex in an area the size of California and just two in an area the size of Washington, DC.

Yes, but: Marshall said that the precision of the analysis was “low” and this was mainly due to the uncertainty about the precision of the relationship between the body mass of live animals and their population density, rather than the paleontological data the team used.

James clark, a George Washington University biology professor who was not involved in the study, said the research did not reach a definitive conclusion, but showed difficulties in estimating the lives of extinct animals.

• “It’s an exercise in what can and cannot be said,” Clark said. “It gives you a chance to say, ‘Wow, there really were a lot of these things, and we’re not going to get a lot of them captured in the fossil record.’

Go deeper: How the meteor that killed the dinosaurs created modern forests

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