Studying the fossil record allows us to see the coming and going of species, the timeline of which is marked by mass extinction events and explosions of biodiversity. The Earth has witnessed countless species come and go – but without hunting down every last specimen, it’s been something of a puzzle trying to estimate the total population size of species from their emergence to their extinction.

A new study published in the journal Science set out to do just this for perhaps one of the most celebrated dinosaurs: Tyrannosaurus rex. Their calculations landed on the conclusion that roughly 2.5 billion individuals walked the Earth during their existence, and best of all, the researchers were able to reach that number without hacking into the fossil record.

To establish how best to calculate the total population of dead species, it’s sensible to first look at how we do this for living ones, and a little thing called Damuth’s Law comes into play here. The law states that average population density can be predicted based on body size, with bigger animals having smaller populations compared to the larger populations of little animals. Think of it like a herd of elephants versus a colony of ants.

T. rex is famous for its large body size, being one of the biggest carnivorous theropods known to science. Combining the paleontological data on this species with Damuth’s law, the researchers estimate that at any given time during their existence, there were around 20,000 T. rex alive, and they went about their carnivorous duties for roughly 127,000 generations. This brings their total population size from their emergence to their extinction to 2.5 billion T. rex.

It’s difficult to establish a population density from this information alone, as it’s not simply a matter of space and numbers. The pursuit of food grouped in flocks and herds may have made their distribution more “clumpy,” explained study author Charles Marshall in an email to IFLScience.

Unfortunately, extant carnivores are too fun-sized to glean many transferable insights here, but we can compare modern estimations with those for extinct T. rex. “The density drops with body mass – bigger animals are rarer, and there are no terrestrial carnivores today the size of T. rex,” continued Marshall. “The biggest living carnivore is the polar bear – now food is scarce up there in the Arctic, so it turns out that its population density is only about 2 x our best T. rex estimate. Tigers are about 6 times more dense, lions about 14 times denser.”

The approach has big potential, according to the paper’s authors, who believe the framework could be used to estimate the total number of any extinct species – provided we have the necessary information. According to Marshall, this includes the species’ body mass, whether it ate meat or vegetation, and if it was cold or warm-blooded. This information is then combined with data on its geographic range and how long it was alive as a species.

The researchers hope to expand their work to all the dinosaurs known to exist within T. rex’s ecosystem. They also hope that their results on total population compared to known specimens may give an idea as to how many dinosaurs that once walked the Earth were too rare to make it into the accessible fossil record.