How many T-Rex roamed North America during the Cretaceous Period? Dr Charles Marshall harassed his colleagues for years until he finally teamed up with his students to find an answer. Their work was published this Thursday in the journal Science.
It all started with questions from Charles R. Marshall, professor of integrative biology at the University of California at Berkeley, holding a T-Rex leftover in his hand: ” how rare is this fossil? Were there a million, a billion, a trillion T-Rex on Earth?“
Until now, no one had been able to calculate the number of populations of long-extinct animals. George Gaylord Simpson, one of the most influential paleontologists of the last century, believed that this could not be done. As part of this work, Marshall and his team tried anyway.
For living species, John Damuth, a biologist at the University of California at Santa Barbara, proposed a mathematical relationship. This “Damuth’s law”, as it is called, makes it possible to estimate the density of an expected population as a function of the average body mass of an animal representing that population. Other data, such as the trophic level (where they are in the food chain) and their physiology must also be taken into account.
Note that this relationship is not universal. Witness the example of jaguars and spotted hyenas. These two species of carnivorous mammals are similar in size. However, the population density of hyenas is about fifty times higher.
The data taken into account
Coming back to the T-Rex, Charles R. Marshall and his team therefore sought to integrate certain data related to their metabolism into their equations. One of the uncertainties concerned the animal’s blood.
Dinosaurs are no longer thought to be cold-blooded like modern-day lizards, but they probably weren’t warm-blooded like mammals either. Dr. Marshall’s team therefore assumed a physiology between that of carnivorous mammals and that of Komodo dragons, the largest lizard on the planet.
The question of T-Rex’s place in the ecosystem has also led researchers to ignore juveniles. On the one hand, because they are under-represented in the fossil record. And on the other hand, they might have lived separately from adults and pursued different prey. In other words, the researchers considered them to be a different species.
Drawing on the expertise of their colleagues, the researchers then estimated that:
- the probable age of sexual maturity of a T-Rex was 15.5 years
- its maximum lifespan was probably around 30 years
- his average body mass as an adult was around 5.2 tons
From these estimates, they calculated that each generation spanned about 19 years and that the average population density was about one dinosaur per 100 square kilometers.
Note that the researchers here had to account for some uncertainty about where the dinosaurs lived. Paleontologists, for example, are uncertain whether the T-Rex range was limited to areas where their fossils were found (western United States and Canada), or whether it extended to others. regions with similar climates at the time.
Billions of T-Rex
Ultimately, estimating that the total geographic range of T-Rex was approximately 2.3 million square kilometers, and that the species had survived for approximately 2.5 million years ago, the researchers calculated a permanent population size of 20,000 specimens. Out of a total of approximately 127,000 generations developed over the course of their history, researchers therefore estimate that 2.5 billion the number of individuals in total.
Despite the uncertainties associated with most of the model parameters, Dr Damuth noted a “solid qualitative result“.
However, even Dr Marshall thinks that number of 20,000 is probably low. “It just seems inconceivable that you could last a few million years with these few individuals.», He explains. According to him, there were perhaps tens of thousands, or even maybe 100,000 or 200,000 individuals at any given moment.
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 have all those bones gone then? What proportion of these individuals has been discovered by paleontologists?
“There are around 32 relatively well preserved post-juvenile T-Rexes in public museums today”, notes Dr. Marshall.“Of all the post-juvenile adults who have ever lived, this means that we have about one in 80 million ”. This is the answer to his first question.