New Tyrannosaurus rex study shows 20k lived at any one time, or 2.5 billion in total

Tyrannosaurus rex model in Jurapark, Bałtów (Poland). By Marcin Polak.

How many Tyrannosaurus rexes were alive at any one moment? A new study suggests the answer is about 20,000.

The study, published today in the journal Science, agrees in broad outlines with previous research. But what’s new is that lead author Charles Marshall concluded that this means roughly 2.5 billion Tyrannosaurus rexes lived and died over the 2.5 million years that the dinosaur existed.

Until now, scientists had been unable to calculate population numbers for long-extinct animals. Some famous paleontologists even thought such estimates were impossible.

Tyrannosaurus rex energy requirements

The study relies on data that relates body mass to population density for living animals. Ecological differences result in large variations in population densities for animals with the same physiology and ecological niche.

For example, jaguars and hyenas are about the same size. Nonetheless, hyenas are found in their habitat at a density 50 times greater than that of jaguars.

As part of the calculations, Marshall chose to treat T. rex as a predator with energy requirements halfway between those of a lion and a Komodo dragon, the largest lizard on Earth.

Youthful T. rexes a matter apart

In their research, Marshall and his team ignored younger T. rexes. That’s because they are underrepresented in the fossil record, and may have lived apart from adults and pursued different prey.

As a T. rex crossed into maturity, its jaws became about 10x stronger, enabling the dinosaur to crush bone. This suggests that juveniles and adults ate different prey, and in that sense were almost different species.

This possibility concurs with recent research suggesting that the absence of medium-size predators alongside the massive predatory T. rex during the late Cretaceous period was because the juvenile T. rexes filled that ecological niche.

What the fossils tell us about Tyrannosaurus rex

Marshall and his team estimated that the average age of a T. rex at sexual maturity was 15.5 years, and its maximum lifespan was probably close to 30. years.

Its average adult body mass was about 5,200 kilograms, with some Tyrannosaurus rexes weighing up to 7,000 kilograms.

From these estimates, they calculated that each generation lasted about 19 years, and that the average population density was about one dinosaur for every 100 square kilometers.

Then, estimating that the total geographic range of T. rex was about 2.3 million square kilometers, and that the species survived for roughly 2.5 million years, they calculated a standing population size of 20,000.

Over the total of about 127,000 generations that the species existed, that translates to about 2.5 billion individuals overall.

Could have been as few as 140 million or as many as 42 billion

Marshall points out that these new figures are by no means rock solid. Though the population of Tyrannosaurus rexes was most likely 20,000 adults at any given time, the study’s 95% confidence interval means the total number of T. rexes that existed over the species’ lifetime could have been anywhere from 140 million to 42 billion.

He expects that colleagues will quibble with many, if not most, of the numbers. Nonetheless, he believes his calculational framework for estimating extinct populations will stand, and will be useful for estimating populations of other fossilized creatures.

The framework, which the researchers have made available as computer code, also lays the foundation for estimating how many species paleontologists might have missed when excavating for fossils, he said.

“With these numbers, we can start to estimate how many short-lived, geographically specialized species we might be missing in the fossil record,” he said. “This may be a way of beginning to quantify what we don’t know.

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