Quirks & Quarks

# Billions and billions of tyrannosaurs walked the Earth

Scientists arrived at the estimate by using calculations based on size, metabolism, and the populations of animals today.

## At any one time in the Cretaceous, something like 20,000 animals roamed their range

Anyone who has ever been fascinated by dinosaurs has wondered what it would be like to encounter one, and maybe even how many there were on the Earth at any one time. This is especially true of the iconic Tyrannosaurus rex.

Charles Marshall, a professor of integrative biology at the University of California Berkeley, and his colleagues decided to figure that number out.

## Demuth's Law meets T.rex

By using a formula to determine the population density of living animals, Marshall now estimates that 20,000 T.rex lived on the Earth at any one time. That is roughly the current population of African lions. They also calculated an "all-time" number of adult tyrannosaurs and that number is an impressive 2.5 billion.

What began as a bit of a lark became a published research study once the team applied a formula called Demuth's Law. It uses the body mass of an animal to determine population density. For example, said Marshall, you can estimate how many elephants exist based on known factors including physical size, the size of their range, their longevity, as well as their energy requirements.

## T.rex by numbers

Although only on the order of 100 T.rex fossils exist, they can provide most of that information. Paleontologists calculate an average body mass somewhere between 5,200 and 7,000 kilograms. The range of T.rex, which was limited to North America, was estimated to be 2.3 million square kilometres. Their average lifespan was likely into the late 20s.

However, Marshall and his colleagues had to guess at the energy requirements of T-rex. For this part of the equation they chose to place Tyrannosaurus rex somewhere between an adult lion and a komodo dragon. With all of these numbers in place, Marshall concluded that about 20,000 roamed North America at any one time, one for every square kilometre of its range.

Because T-rex existed for about 2.5 million years during the Cretaceous, Marshall used the lifespan estimate to calculate a total of 127,000 generations over that period of time. This is how he arrived at the all time number of 2.5 billion.

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