Analysis of what is known about the dinosaur leads to the conclusion that there were 2.5 billion in time.
How many Tyrannosaurus rex roamed North America during cretaceous period?
This is a question that Charles Marshall bothered his paleontological colleagues for years until he finally teamed up with his students to find an answer.
What the team found, to be published in the journal this week Science, is that about 20,000 adults of T. rexes probably lived at some point, give or take a factor of 10, which is at the stage of what most of his colleagues guessed.
What few paleontologists have fully understood, he said, including himself, is that this means that about 2.5 billion have lived and died in about 2 1/2 million years since the dinosaur went to earth.
So far no one was able to send in the perfect solution, which is not strange. George Gaylord Simpson, one of the most influential paleontologists of the last century, felt that this could not be done.
Marshall, director of the University of California’s Museum of Paleontology, Philip Sandford Boone Department of Paleontology and professor of integrative biology at UC Berkeley and earth and planetary science, was also surprised that such a calculation was possible.
A distribution of a T. rex skeleton on display in front of the UC Museum of Paleontology at the University of California, Berkeley. The original, an almost complete skeleton excavated in 1990 from the hardlands of eastern Montana, is in the Rock Museum in Bozeman, Montana. Credit: Keegan Houser, UC Berkeley
“The project just started out as weird, in a way,” he said. “When I hold a fossil in my hand, I can’t help but wonder about the improbability of this beast being alive millions of years ago, and here I hold part of its skeleton – it seems so unlikely. The question popped into my head: “How unlikely 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 could actually estimate how many were alive, and so I could answer that question. ”
Marshall quickly noticed that the uncertainties in the estimates were high. While the population of T. rexes was most likely 20,000 adults at any one time, the 95% confidence interval – the range of the population in which there is a 95% chance that the actual number is – is from 1,300 to 328,000 individuals. Thus, the total number of individuals that have existed during the life of the species could have been between 140 and 42 billion.
“As Simpson noted, it is very difficult to make quantitative estimates with fossil records,” he said. “In our study, we focused on developing strong constraints on the variables we needed to do our calculations, rather than focusing on making the best estimates, in and of themselves.”
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 greatest uncertainty in these figures, Marshall said, revolves around questions about the exact nature of dinosaur ecology, including how hot T. rex was. The study is based on data published by John Damuth of UC Santa Barbara, which links body mass to population density for live animals, a relationship known as Damuth’s Law. Although the relationship is strong, he said, ecological differences lead to large variations in population density for animals with the same physiology and ecological niche. For example, jaguars and hyenas are about the same size, but hyenas are found in their habitat at a density 50 times higher than the density of jaguars in their habitat.
“Our calculations depend on this relationship for live animals between body mass and population density, but the uncertainty of the relationship is 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 in the paleontological data we used.”
As part of the calculations, Marshall chose to treat T. rex as a predator with energy requirements halfway between that of a lion and a Komodo dragon, the largest lizard on Earth.
The problem of the place of T. rex in the ecosystem led Marshall and his team to ignore juvenile T. rexes, which are underrepresented in the fossil record and which, in fact, would have lived apart from adults and pursued various prey. As T. rex matured, his jaws became stronger by an order of magnitude, allowing him to crush the bone. This suggests that young and adults ate different prey and were almost like different species of predators.
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 during the late Cretaceous was due to the fact that T. rex juvenile has filled that ecological niche.
What the fossils tell us
UC Berkeley scientists exploited the scientific literature and peer expertise for the data they used to estimate that the probable age at sexual maturity of a T. rex was 15.5 years; the maximum life expectancy was probably in the late 1920s; and the average body weight as an adult – the so-called ecological body mass – was about 5,200 kilograms, or 5.2 tons. They also used data on how fast T. rexes grew during their lifetime: they had an increase around sexual maturity and could reach a weight of about 7,000 kilograms or 7 tons.
From these estimates, they also calculated that each generation lasted about 19 years and that the average population density was about one dinosaur per 100 square kilometers.
Then, estimating that the total geographical area of T. rex was about 2.3 million square kilometers and that the species survived about 2 1/2 million years, they calculated a population size of 20,000. Over a total of about 127,000 generations that the species has lived, which translates to about 2.5 billion individuals as a whole.
With so many post-young dinosaurs throughout the species’ history, not to mention the young ones that were probably more numerous, where did all those bones go? What proportion of these individuals have been discovered by paleontologists? To date, less than 100 T. rex individuals have been found, many represented by a single fossilized bone.
“There are about 32 relatively well-preserved post-juvenile rexes in public museums today,” he said. “Of all the post-minor adults who have ever lived, this means that we have about one in 80 million of them.”
“If we restrict the analysis of the fossil recovery rate to where the T. rex fossils are most common, a portion of Montana’s famous Hell Creek, we estimate that we recovered about one in 16,000 of the T. rexes that lived in that area.” region at the time the rocks were deposited, “he added.” We were surprised by this number; this fossil record has a much larger representation of the living than we first guessed. It could be as as good as one in 1,000, if he barely lived there, or could be as small as one in a quarter of a million, given the uncertainties about the animal’s estimated population densities. ”
Marshall expects his colleagues to talk a lot, if not most of the numbers, but believes his calculation framework for estimating extinct populations will stand and be useful for estimating the populations of other fossilized creatures.
“In some ways, it was a paleontological exercise in how much we can know and how we strive to know it,” he said. “It’s amazing how much we actually know about these dinosaurs and, from this, how much more we can calculate. Our knowledge of T. rex has expanded so much in recent decades due to more fossils, more ways to analyze them, and better ways to integrate information over multiple known fossils. ”
The framework, which the researchers made available in computer code, also lays the groundwork for estimating how many paleontologists could have missed it when digging for fossils, he said.
“With these figures, we can begin to estimate how many short-lived, geographically specialized species we may be missing from fossil records,” he said. “This can be a way to start quantifying what we don’t know.”
Marshall’s co-authors are Connor Wilson, a UC Berkeley student, and graduate students Daniel Latorre, Tanner Frank, Katherine Magoulick, Joshua Zimmt and Ashley Poust, who is now a postdoctoral fellow at the San Diego Museum of Natural History.