How many T. rexes ever lived? Billions

Paleontologists have crunched the numbers to estimate just how many Tyrannosaurus rexes lived and died – and the answer could be in the billions.

The T. rex is perhaps the most famous dinosaur species in the world and holds a unique place in modern culture, appearing in dozens of films from Jurassic Park to the Land Before Time, as well as in TV shows, books and even on postage stamps.

This new study, published in Science, estimates that about 20,000 adult T. rexes lived at any one time and that the species persisted for about 127,000 generations – meaning at least 2.5 billion walked the Earth in total.

But the authors stress that this is merely an estimate. Calculating population numbers for long-extinct species is no easy feat and is rife with uncertainties.

Lead author of the study, Charles Marshall, says the project started off as a “lark”.

“When I hold a fossil in my hand, I can’t help wondering at the improbability that this very beast was alive millions of years ago, and here I am holding part of its skeleton,” says Marshall, a professor at UC Berkeley, US, and director of the University of California Museum of Paleontology.

“The question just kept popping into my head, ‘Just how improbable is it? Is it one in a thousand, one in a million, one in a billion?’ And then I began to realize that maybe we can actually estimate how many were alive, and thus, that I could answer that question.”

Although Marshall and colleagues estimate 20,000 adult T. rexes existed at any time, large uncertainties mean that the real number could have been anywhere between 1,300 to 328,000. This would drastically change the total population over the lifetime of the species – from as low as 140 million or as high as 42 billion.

This uncertainty mainly comes from our lack of knowledge of the dinosaur’s ecology – which is understandable, given that fewer than 100 T. rex individuals have been found in the fossil record.

To formulate their estimates, the team used data that focuses on living animals, relating body mass to population density. But even very similarly sized animals can have vastly different population densities due to ecological differences – jaguars and hyenas, for example, are roughly the same size and yet hyenas have a population density 50 times greater.

To achieve an estimate, Marshall and colleagues treated T. rex as a predator with energy requirements halfway between those of a lion and a Komodo dragon (per unit mass). They also used data from previous studies to put numbers on T. rex’s age of sexual maturity, maximum lifespan, average body mass, and rate of growth. (Note that the team discounted juveniles in this study to simplify the calculations.)

This allowed them to calculate how long each generation lasted and how dense the population was (about one dinosaur for every 100 square kilometres). Then by estimating the geographic range of the dinosaur (2.3 million square kilometres) and the length of its reign (2.5 million years), they reached their answer.

So, if 2.5 billion T. rexes existed during their tenure on the Earth, give or take a few billion, where are all their remains? Just how improbable is it for Marshall to hold a fossil in his hands?

Considering just the 32 well-preserved adult specimens contained in museums, this means just one in 80 million of all T. rexes survives as fossilised remains.

But if the analysis is restricted to places where T. rex fossils are most, common like the Hell Creek Formation in Montana, Marshall says “we have recovered about one in 16,000 of the T. rexes that lived in that region over that time interval that the rocks were deposited”.

Though these calculations will undoubtedly be challenged by other paleontologists, the framework for estimating extinct populations could be useful to apply to other species.

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

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