How Many T Rexes Roamed Earth? Billions.

Paleontologists relied on a new method of estimating extinct populations to peg the number of Tyrannosaurus rexes that terrorized Earth throughout the Cretaceous period at 2.5 billion.

A cast of a T. rex skeleton on display outside the UC Museum of Paleontology at the University of California, Berkeley. The original, a nearly complete skeleton excavated in 1990 from the badlands of eastern Montana, is at the Museum of the Rockies in Bozeman, Montana. (Keegan Houser, UC Berkeley)

(CN) — A breakthrough new study has satisfied the long unanswered question of how many T. Rexes lived on Earth in their time, finding that approximately 2.5 billion roamed the Earth over the span of 2½ million years.

In the study published Thursday in the journal Science, paleontologist Charles Marshall and a team of his students used statistics, computer simulations and an ecological formula to estimate the number of T. Rexes that lived at a given time.

The Tyrannosaurus rex, — with a name that means “king of the tyrant lizards” — was one of the most feared predators of the Cretaceous period. Its body was as long as a school bus, it weighed up to 8 tons, it walked on two giant, muscular legs and had a terrifying set of razor-sharp teeth with a bite force of up to 6 tons of pressure. It preyed on living animals, but if prompted would also eat scavenged prey and even each other.

Paleontologists have wondered how many of these apex predators lived, but until now it’s always been an impossible question to answer. Computing the population of any extinct animal had yet to be done, and leading experts had doubts about the concept. Even Marshall, who had been curious for years, was shocked when he and his team finally found an avenue.

“The project just started off as a lark, in a way,” said Marshall, director of the University of California Museum of Paleontology, the Philip Sandford Boone Chair in Paleontology, and UC Berkeley professor of integrative biology and of earth and planetary science, in a statement accompanying the study.

“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 — it seems so improbable,” he continued. “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.”

While the new formula is a huge step forward, Marshall acknowledges the figure is an estimate and has room for error. They believe that there were probably 20,000 adult T. Rexes at a given time, with a 95% confidence range that the true number lies between 1,300 and 328,000. Therefore, there could have been approximately 140 million to 42 billion T. Rexes across the entire Cretaceous period.

The team then used the Monte Carlo computer simulation, a mathematical technique that estimates probable outcomes, to examine how their uncertainties about the dinosaurs themselves would affect the results. Specifically, the biggest unknown factor had to do with the dinosaurs’ ecology, like how warm-blooded they were, as their method relied heavily on this question.

Specifically, they used a method recently published by John Damuth of the University of California, Santa Barbara, called Damuth’s Law that looks at how body mass affects population density in animals. While a significant correlation exists, it is also affected by differences in a species’ physiology and role in an ecosystem. The authors offer hyenas and jaguars as a prime example, since they are similar in stature but more hyenas can occupy an environment than jaguars can.

“Our calculations depend on this relationship for living animals between their body mass and their population density, but the uncertainty in the relationship spans about two orders of magnitude,” Marshall said. “Surprisingly, then, the uncertainty in our estimates is dominated by this ecological variability and not from the uncertainty in the paleontological data we used.”

To fill this knowledge gap, Marshall proposed that the T. Rex’s energy levels fell somewhere in between a lion and a Komodo dragon.

The team also acknowledged their research, like most existing fossil records, ignored juvenile T. Rexes. The species could live up to 28 years and were considered juveniles at 13 or younger, subadults at 14, young adults at 18 and reached adulthood at 23. Judging by the differences in jaw size and strength, as well as recent evidence, the scientists believe that juvenile and adult T. Rexes were likely unique predators from one another.

They estimated that the T. Rex most likely reached sexual maturity at 15½ years old and lived into its late 20s, with an average body mass of 5.2 tons. They also estimated the dinosaurs hit a growth spurt around the time of their sexual maturity, with some able to reach 7 tons during this time. Additionally, they found only one T. Rex typically lived within a radius of 38.6 square miles.

They also found a generation lasted about 19 years, and since the species lived for about 2½ million years and had an average population size of 20,000, they calculated there were a total of 127,000 generations — with a total of 2.5 billion dinosaurs.

The next question was if their estimate is correct and millions of T. Rexes lived throughout the Cretaceous, why do we only have the fossils of about 100 of them?

“There are about 32 relatively well-preserved, post-juvenile T. rexes in public museums today,” Marshall said. “Of all the post-juvenile adults that ever lived, this means we have about one in 80 million of them.”

He added: “If we restrict our analysis of the fossil recovery rate to where T. rex fossils are most common, a portion of the famous Hell Creek Formation in Montana, we estimate 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. We were surprised by this number; this fossil record has a much higher representation of the living than I first guessed. It could be as good as one in a 1,000, if hardly any lived there, or it could be as low as one in a quarter million, given the uncertainties in the estimated population densities of the beast.”

Marshall said he expects much dispute over the findings, but he is confident that his method will persevere and become very useful in estimating extinct populations.

“In some ways, this has been a paleontological exercise in how much we can know, and how we go about knowing it,” he said. “It’s surprising how much we actually know about these dinosaurs and, from that, how much more we can compute. Our knowledge of T. rex has expanded so greatly in the past few decades thanks to more fossils, more ways of analyzing them and better ways of integrating information over the multiple fossils known.”

Furthermore, the team believes the computer code they have developed will allow scientists to start to estimate how many fossils of species they have yet to uncover.

“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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