The analysis of what is known about the dinosaur leads to the conclusion that there were 2.5 billion over time.
How Many Tyrannosaurus Rexes Wandered During North America Chalk period?
This is a question that Charles Marshall plagued his colleagues of the paleontologist for years until he was finally with his students to find an answer.
What the team found will be published in the magazine this week Science, is that about 20,000 adult T. rexes probably lived at any given time, give or take a factor of 10, which is in the ballpark of what most of his colleagues guessed.
What few paleontologists fully understand, he said, and himself included, is that it means that about 2.5 billion lived and died during the approximately 2 1/2 million years that the dinosaur walked on earth.
Until now, no one could calculate the population numbers for long extinct animals, and 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 Museum of Paleontology, the Philip Sandford Boone Chair in Paleontology and a UC Berkeley professor of integrative biology and earth and planetary science, was also surprised that such a calculation was possible.
An occupation of a T. rex skeleton outside the UC Museum of Paleontology at the University of California, Berkeley. The original, an almost complete skeleton excavated in 1990 from the badlands of eastern Montana, is in the Museum of the Rockies in Bozeman, Montana. Credit: Keegan Houser, UC Berkeley
“The project started in a way just like a lark,” he said. ‘If I hold a fossil in my hand, I can not ask myself about the improbability that this animal lived millions of years ago, and here I am holding a part of the skeleton – it seems so unlikely. The question just came to mind: ‘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 much life there was, and so that I could answer the question. ‘
Marshall quickly points out that the ambiguities in the estimates are large. While the population of T. rexes at any given time was probably 20,000 adults, the 95% confidence – the population range within which the 95% chance of the actual number lies – is from 1,300 to 328,000 individuals. Thus, the total number of individuals that existed during the lifetime of the species could be about 140 million to 42 billion.
“As Simpson noted, 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 make our calculations, rather than focusing on the best estimates on their own.”
He and his team then used Monte Carlo computer simulation to determine how the uncertainties in the data are translated into the results in uncertainties.
The greatest uncertainty in these numbers, Marshall said, focuses on questions about the exact nature of the dinosaur’s ecology, including how warm-blooded T. rex was. The study is based on data published by John Damuth of UC Santa Barbara, related to body mass and population density for live animals, a relationship known as Damuth’s Law. While the ratio is strong, he said, ecological differences lead to large variations in population densities for animals with the same physiology and ecological niche. Jaguars and hyenas, for example, are the same size, but hyenas are found in their habitat with a density 50 times greater than the density of jaguars in their habitat.
“Our calculations depend on this relationship for live animals between their body mass and their population density, but the uncertainty in the ratio extends over two orders,” Marshall said. “It is surprising that the uncertainty in our estimates is dominated by this ecological variability and not by the uncertainty in the paleontological data we have used.”
As part of the calculations, Marshall decided to treat T. rex as a predator with energy needs halfway between that of a lion and a Komodo dragon, the largest lizard on earth.
The issue of the place of T. rex in the ecosystem has led to Marshall and his team of youths ignoring T. rexes, who is underrepresented in the fossil record, and that they actually lived separately from adults and pursued different prey . As T. rex progressed to adulthood, his jaws became stronger in an order of force, making it possible to crush bone. This suggests that juveniles and adults ate different prey and were almost like different predators.
This possibility is supported by a recent study, led by evolutionary biologist Felicia Smith of the University of New Mexico, who suspected the absence of medium-sized predators along with the massive predatory T. rex during the late Cretaceous because the youthful T. rex filled it. that ecological niche.
What the fossils tell us
The UC Berkeley scientists exploited the scientific literature and expertise of colleagues for data they used to estimate that the probable age at sexual maturity of a T. rex was 15.5 years; its maximum lifespan was probably in the late 20s; and his average body mass 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 a growth storm around sexual maturity and could weigh 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 for every 100 square kilometers.
When they estimated that the total geographical extent of T. rex was about 2.3 million square kilometers and that the species survived about 2 1/2 million years, they calculated a permanent population size of 20,000. Over a total of about 127,000 generations the species has lived, that means about 2.5 billion individuals.
With such a large number of post-juvenile dinosaurs about the history of the species, not to mention the juveniles that were presumably more, where did all the bones go? What part of these individuals were discovered by paleontologists? To date, less than 100 T. rex individuals have been found, many of which are represented by a single fossilized bone.
“There are about 32 relatively well-preserved T. rexes in public museums today,” he said. “Of all the adults after the youth who ever lived, that means we have about one in 80 million of them.”
‘If we limit our analysis of the fossil recovery rate to where T. rex fossils are most common, part of the famous Hell Creek Formation in Montana, we estimate that we have about one in every 16,000 of the T. rexes lived in it, recycled. area during that time interval that the rocks were laid down, ”he added. ‘We were surprised by this number; this fossil record has a much higher version of the living than I first guessed. It can be as good as one in 1,000, if almost no one lives there, or it can be as low as one in a quarter of a million, given the uncertainty in the estimated population density of the animal. ‘
Marshall expects his colleagues to quarrel with many, if not most of the numbers, but he believes his computational framework for estimating extinct populations will remain and be useful for estimating populations of other petrified creatures.
“In some ways, it was a paleontological exercise in how much we can know and how we go about knowing it,” he said. ‘It’s amazing how much we actually know about these dinosaurs, and from that how much we can still calculate. Our knowledge of T. rex has expanded so strongly over the past few decades thanks to more fossils, more ways to analyze it, and better ways to integrate information about the multiple fossils. ”
The framework, which the researchers made available as a computer code, also lays the foundation for estimating how many species paleontologists would have missed when digging for fossils.
“With these numbers, we can begin to estimate how many short-lived, geographically specialized species we could miss in the fossil record,” he said. “It could be a way to start quantifying what we do not know.”
Marshall’s co-authors are Connor Wilson, undergraduate students at UC Berkeley, and Daniel Latorre, Tanner Frank, Katherine Magoulick, Joshua Zimmt and Ashley Poust, who is now a doctor at the San Diego Natural History Museum.