2.5 billion T. Rexes roamed North America during the Cretaceous Period

Analysis of what is known about the dinosaur leads to the conclusion that there were 2.5 billion over time.

How many Tyrannosaurus rex roamed North America during the Cretaceous period?

It’s a question that Charles Marshall harassed his fellow paleontologists for years until he finally teamed up with his students to find an answer.

What the team found, to be published this week in the newspaper Science, is that about 20,000 adult T. rexes probably lived at any given time, a factor of 10, or about 10, which is what most of his colleagues guessed.

What few paleontologists fully understood, he said, including himself, is that this means that some 2.5 billion lived and died during the roughly 2 1/2 million years. years in which the dinosaur roamed the earth.

Until now, no one has been able to calculate the number of populations of long-extinct animals, and George Gaylord Simpson, one of the most influential paleontologists of the last century, felt that this could not be made.

Marshall, director of the University of California Museum of Paleontology, the Philip Sandford Boone Chair of Paleontology and Professor of Integrative Biology and Terrestrial and Planetary Science at UC Berkeley, was also surprised that such a calculation was possible. .

“The project started out like a lark, in a way,” he said. “When I hold a fossil in my hand, I cannot help but wonder about the improbability that this very beast was alive millions of years ago, and here I am holding part of its skeleton – this seems so unlikely. The question kept coming to my mind: “ How improbable is this? Is it one in a thousand, one in a million, one in a billion? And then I started to realize that maybe we could estimate how many were alive, and so I could answer that question.

Marshall quickly points out that the uncertainties in the estimates are significant. While the population of T. rexes was probably 20,000 adults at any given time, the 95% confidence range – the population range in which there is a 95% chance that the true number will be found – is 1,300. to 328,000 individuals. Thus, the total number of individuals that existed during the life of the species could have been between 140 million and 42 billion.

“As Simpson observed, it is very difficult to make quantitative estimates with the fossil record,” he said. “In our study, we focused on developing robust constraints on the variables we needed to perform our calculations, rather than focusing on the best estimates per se.”

He and his team then used the Monte Carlo computer simulation to determine how the uncertainties in the data translated into uncertainties in the results.

The greater uncertainty in those numbers, Marshall said, centers on questions about the exact nature of the dinosaur’s ecology, including how the warm-blooded T. rex was. The study builds on data published by John Damuth of UC Santa Barbara which relates body mass to the population density of living animals, a relationship known as Damuth’s Law. While the relationship is strong, he said, ecological differences result in large variations in population densities of animals with the same physiology and ecological niche. For example, jaguars and hyenas are roughly the same size, but hyenas are found in their habitat 50 times the density of jaguars in their habitat.

“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 of our estimates is dominated by this ecological variability and not by the uncertainty of the paleontological data that we used.”

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

The question of T. rex’s place in the ecosystem has led Marshall and his team to ignore juvenile T. rexes, which are under-represented in the fossil record and may, in fact, have lived separately from adults and pursued different prey. As T. rex matured, its jaws strengthened by an order of magnitude, which allowed it to crush bones. This suggests that juveniles and adults ate different prey and were almost like different predator species.

This possibility is supported by a recent study, led by evolutionary biologist Felicia Smith of the University of New Mexico, which hypothesized that the absence of medium-sized predators alongside the massive predator T. rex at the Late Cretaceous was due to the fact that the juvenile T. rex was filled. this ecological niche.

What the fossils tell us

Scientists at UC Berkeley drew on the scientific literature and the expertise of their colleagues for the data they used to estimate that the probable age at sexual maturity of a T. rex was 15.5 years. ; her maximum lifespan was probably in her late twenties; and his average adult body mass – his so-called ecological body mass – was about 5,200 kilograms, or 5.2 tons. They also used data on how fast T. rexes grew over their lifetime: they had a growth spurt around sexual maturity and could reach a weight of around 7,000 kilograms, or 7 tons.

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

Then, estimating that the total geographic range of T. rex was about 2.3 million square kilometers and that the species had survived for about 2 1/2 million years, they calculated a size of permanent population of 20,000 people. Out of a total of approximately 127,000 generations that the species has lived, this translates to approximately 2.5 billion individuals in total.

With such a large number of post-juvenile dinosaurs throughout the history of the species, let alone the juveniles which were probably more numerous, where did all those bones go? What proportion of these individuals has been discovered by paleontologists? To date, less than 100 individuals of T. rex have been found, many of which are represented by a single fossilized bone.

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

“If we limit our analysis of the rate of fossil recovery to where T. rex fossils are most common, part of the famous Hell Creek Formation in Montana, we estimate that we have recovered about one in 16,000 of the fossils. T. rex who lived in this area. region during this time interval where the rocks were deposited, ”he added. “We were surprised by this number; this fossil record has a much higher representation of the living than I had imagined. It could be as good as one in 1000, if there are hardly any, or it could be as low as one in a quarter of a million, given the uncertainties in the estimated population densities of the beast.

Marshall expects his colleagues to quibble with many, if not most, numbers, but he believes his computational framework for estimating extinct populations will be valid and useful for estimating populations of other fossilized creatures.

“In some ways it’s been a paleontological exercise in what we can know and how we are going to know it,” he said. “It’s surprising how much we know about these dinosaurs and, from there, how much more we can calculate. Our knowledge of T. rex has grown so much over the past few decades with more fossils, more ways to analyze them, and better ways to integrate information about the multiple known fossils.

The framework, which the researchers made available in the form of computer code, also lays the groundwork for estimating how many species paleontologists might have missed when searching for fossils, he said.

“With these numbers, we can begin to estimate the number of short-lived and geographically specialized species that might be missing from the fossil record,” he said. “This can be a way to start quantifying what we don’t know.”

Marshall’s co-authors are Connor Wilson, an undergraduate student at UC Berkeley, and graduate students Daniel Latorre, Tanner Frank, Katherine Magoulick, Joshua Zimmt and Ashley Poust, who is now a postdoctoral fellow at the Museum of Natural History from San Diego.