Billions of T. rexes walked on North America during the Age of the Dinosaurs

Paleontologist Charles Marshall had long wondered how many Tyrannosaurus rex dinosaurs populated the North American continent. A few years ago, finally tired of asking his colleagues for answers, Marshall decided to find out for himself.

With the help of his students, Marshall determined North America's T. rex population peaked at 20,000. That means that over the course of their 2.5 million-year reign atop the Cretaceous food chain, 2.5 billion lived and died on the continent.

Marshall and his research partners detailed their novel calculations in a new paper, published Thursday in the journal Science.

"Fossils are rare, but how rare?" Marshall told UPI in an email, speaking to the questions that inspired his investigation. "When I see a T. rex fossil, is it 1 in a million, a billion, a trillion?

"And then there is the more general issue of just how much can we know about extinct animals, and how do we go about knowing it," Marshall said. "Finally, why study T. rex in particular? Because it turns out it is one of the best understood dinosaurs!"

Though T. rex is one of the most well understood dinosaur species, the remains of only 100 adults have been discovered, and more than two-thirds of those are represented only by a single bone.

To figure out how many T. rex were living in North America at any given time, Marshall and his research partners couldn't simply make inferences based on the abundance -- or lack -- of T. rex fossils. Instead, the scientists used what they knew about the dinosaur's ecology to establish robust constraints on the variables relevant to the species' abundance.

Those variables included: average adult body weight, geographic range, generation time, geological longevity and physiology.

"Most important by far is the relationship among living animus between population density and body mass -- bigger animals are on average rarer," Marshall said. "This relationship is called Damuth's Law, and to apply you also have to know what its trophic level was (carnivore, herbivore) and its physiology, how warm-blooded it is. Without this relationship, the study would have been impossible -- we needed data from the living to make this go."

Body temperature dictates an animal's metabolism, and the higher a species' metabolism, the lower its population density. Lizards, for example, have a slow metabolism, and as a result, their population densities are often 30 to 35 times greater than similarly sized mammals.

How warm-blooded T. rex was remains a matter of debate, and thus, a significant source of uncertainty in Marshall's calculations.

"The general consensus is that T. rex was warm-blooded, but not as warm-blooded as lions or tigers," Marshall said. "Some have suggested it might be as 'cool' as Komodo dragons (which are hot-blooded compared with regular lizards), but many dinosaur workers disagree, so we split the difference between the average flesh-eating mammals and the Komodo dragon."

The new calculations provide interesting context for the few T. rex fossils that have been recovered from ancient rock deposits. If Marshall's estimates are accurate, that would mean paleontologists have found about 1 in 16,000 of the T. rexes that were perished and buried within Montana's Hell Creek Formation.

Even if Marshall's estimates are a bit off, the calculations offer a roadmap for analyzing the abundance of long-extinct species.

Moving forward, Marshall said he and his research partners plan to repeat their analysis for all of the dinosaur species that were part of the T. rex ecosystem.

"So we can see what that ecosystem looked like, and to begin to see if we can use the preservation rate to estimate how many species of dinosaur the fossil record might have missed."

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