How many Tyrannosaurus rex roamed North America during the Cretaceous period? That’s the question that dogged Charles Marshall for years, until he and his students found the answer—about 20,000 adult T. rex lived at one point in time.
That means about 2.5 billion tyrannosaurs lived on Earth in 2.5 million years, said Marshall, curator of the UC Museum of Paleontology and professor of integrative biology and of Earth and planetary sciences at UC Berkeley. “I think the results have high accuracy, but low precision. We’ve worked hard to make sure all the input variables are real, and I think we’ve been successful in that,” he said.
Impossible calculations Dinosaurs in the
traditional sense appeared in the middle of the Triassic period, flourished in the Jurassic and Cretaceous periods, but disappeared suddenly at the end of the Cretaceous period. They have multiplied on the earth for 160-170 million years.
Previously, no one had been able to count the number of long-extinct animals. One of the most influential paleontologists of the 20th century, George Gaylord Simpson, thought that was impossible.
”He had much less information then than we have now. People couldn’t estimate the population size and age of fossil animals at the time of death, whereas we can now take that data and draw dinosaur growth and survival curves,” Marshall said. “We also have climate models that can predict where the dinosaurs lived. We also have an incredible amount of fossils,” he said.
In fact, he too was surprised that such calculations were possible.
Why count dinosaurs? Curiosity is definitely not the only reason.
”Because we’re fascinated by dinosaurs and want to see how much we really know about the past. Also, it will help us estimate how species disappeared from the fossil record,” Marshall said.
When this project first started, it may have just been a “flash of inspiration”.
”I was holding a fossil and I couldn’t help but say, this animal lived millions of years ago, and I’m holding a bone of it now – it seemed unbelievable. A thought popped into my head, ‘this What’s the probability, one in a thousand, one in a million, or even one in a billion?'” Marshall said, “and then I realized that it might be possible to estimate how abundant dinosaurs were at that time.”
From uncertainty Looking for certainty
In fact , Marshall’s team’s estimates are subject to great uncertainty.
”As Simpson observed, it is difficult to make quantitative estimates from the fossil record,” Marshall said
. Caro computer simulations that measure how uncertainty in the data translates into uncertainty in the results.
The greatest uncertainty in these numbers centers on the exact nature of the dinosaur, including how warm the T. rex’s blood was, the researchers said. They relied primarily on data published by John Darmouth of the University of California, Santa Barbara – linking the weight of live animals to population density, a relationship known as “Darmouth’s law”.
Darmouth also acknowledges that, while the association is strong, ecological differences can lead to large variations in population densities of animals with the same physiology and ecological niche. For example, jaguars and hyenas are about the same size, but hyenas are 50 times denser than jaguars.
So Marshall chose to think of T. rex as a carnivore with energy requirements between that of a lion and the Komodo dragon, the largest lizard on Earth. At the same time, Marshall’s team ignored juvenile T. rex—which are underrepresented in the fossil record and likely lived separately from adults, chasing different prey. The researchers believe that when it reached maturity, the jaws of Tyrannosaurus rex were an order of magnitude stronger and capable of crushing bones. This suggests that they ate different foods as juveniles and adults, and thus were almost different predators.
This possibility is supported by a new study by Felicia Smith, an evolutionary biologist at the University of New Mexico in the US. The study hypothesized that in the late Cretaceous, there were not many medium-sized carnivorous dinosaurs, because the cubs and sub-adults of large carnivorous dinosaurs occupied these ecological niches.
The researchers estimate that the probable age of sexual maturity of Tyrannosaurus rex is 15.5 years, the longest life span may be in the 20s, and the average adult weight (the so-called ecological weight) is about 5200 kg. They also used data on the growth rate of Tyrannosaurus rex: there was a rapid growth period before and after sexual maturity, which could reach a weight of about 7000 kilograms. From these estimates, the scientists calculated that each generation of T. rex lasted about 19 years, with an average population density of 1 T. rex per 100 square kilometers.
They then estimated that Tyrannosaurus rex had a total geographic range of about 2.3 million square kilometers, lived for about 2.5 million years, and had a “resident population” of 20,000. Together they lived for about 127,000 generations, or 2.5 billion individuals in total.
Where did the bones go? With so many tyrannosaurs in
history , not to mention the likely higher number of juveniles, where did their bones go? So far, paleontologists have found fewer than 100 T. rex species, many of which have only a single bone fossil.
”Today, there are about 32 relatively well-preserved adult T. rex in public museums.” Marshall said, “the proportion of all T. rex adults is about 1 in 80 million.”
”If we put the fossil The recovery analysis was restricted to where T. rex fossils are most common, parts of the Hale Creek Formation in Montana, USA, and estimated to have found about 1 in 16,000 T. rex.” He added, “Our understanding of this Surprised by the numbers, the fossil record is more representative than I initially guessed. But there is uncertainty about the population density of this animal, which could be as high as 1 in 1,000, or as low as 250,000 One.”
Marshall expects peers to quibble with most of the numbers, but he believes the computational framework for estimating extinct species is feasible and useful for estimating the numbers of other fossil organisms as well.
”In a way, this is a palaeontological study of how much we can know and how.” He said the framework the researchers provided in computer code is also useful for estimating how much time paleontologists will have when they dig up fossils. How many species may have been missed laid the groundwork.
”With these numbers, we can start to estimate how many short-lived, geographically specialized species might be lost in the fossil record,” he said, “and that could be a way to start quantifying what we don’t know. ”
