Around 2.5 billion Tyrannosaurus rex ever walked the Earth
A total of 2.5 billion Tyrannosaurus rex probably existed during the lifespan of the species, researchers have calculated – suggesting that very few survived as fossils.
Charles Marshall at the University of California, Berkeley, and his colleagues used body mass and population density to estimate how many T. rex once lived.
Larger animals tend to have a larger individual range, because they need more food to support their body mass than smaller animals, meaning body mass is inversely correlated with population density – a rule known as Damuth’s law.
Previous analysis of T. rex fossils shows that the average body mass of an adult was about 5200 kilograms. The team also used climate models and the locations of T. rex remains to estimate that the total geographic range of the species was about 2.3 million square kilometers across North America.
Using these figures and data from living species, the team estimated that there was around one T. rex for every 100 square kilometers in North America. “This would mean there was about 20,000 adult T. rex at any given time,” says Marshall.
Previous research shows T. rex lived into its late 20s and, using this figure, the team estimates that 2.5 billion T. rex spanning 127,000 generations graced our planet between 69 and 66 million years ago, the lifespan of the species.
Estimates of population size for long-extinct animals are rare because there are so few fossils. This estimate for adult T. rex suggests a very low fossil incidence rate – it would mean only one in 80 million T. rex survived as fossilized remains.
“This question has been in my head for years,” says Marshall. “I would ask the question every time I held a fossil in my hand.”
Marshall and his colleagues acknowledge that their estimates could vary because there are some uncertainties in the data – there could have been anywhere from 140 million to 42 billion individuals over the time period they existed. There are animals with similar body masses that have very different population densities. “For example, spotted hyenas have the same body mass as jaguars, but are about 50 times more densely populated,” says Marshall.
The team is now looking to apply the same method to all the dinosaurs that lived during the Cretaceous to model the entire dinosaur ecosystem.
“This method is necessary for estimating how many dinosaur species might actually be going unrecognised due to the lack of fossilisation,” says Holly Woodward Ballard at Oklahoma State University.
Journal reference: Science, DOI: 10.1126/science.abc8300