T. rex: How many have ever existed?

Very few of the many specimens of T. rex that once existed left us with a fossil trace, one study shows. (Keegan Houser, UC Berkeley)

Questioning look at the king of the predatory dinosaurs: What population density was Tyrannosaurus rex and how many individuals could have lived in total? Paleontologists are now presenting an assessment: within the framework of the existence of the species, there could have been 2.5 billion individuals on earth. This extrapolation is in turn based on a calculation of the former population based on the relationship between body mass and population density. As the researchers emphasize, however, their assessments are associated with a large factor of uncertainty. The method can at least give a rough impression and is also applicable to other extinct animal species, say the paleontologists.

“The project grew out of mind games during my work,” says lead author Charles Marshall of the University of California at Berkeley. “Whenever I hold a fossil in my hand, I have always wondered how likely it was that the remains have been preserved,” says the paleontologist. The fundamental question that arises is how many individuals there once were of a species. “So the idea came up that we might actually be able to estimate how many once existed,” says Marshall. This is how the study developed, in which Marshall and his colleagues took the most famous of all predatory dinosaurs as an example: Tyrannosaurus rex, which ruled large parts of present-day North America for around 2.5 million years towards the end of the Cretaceous Period.

Paleontological arithmetic games

Marshall and his colleagues used the so-called Damuth law as a basis to obtain information about the possible population densities of T. rex in its habitat. In today’s animal species, it describes the relationship between population density and body mass, as well as physiology. The following applies: The average number of individuals in an area decreases with increasing size of a species. The rate is predictable to a certain extent. However, characteristics of an animal’s ecology and its energy consumption also play an important role. Nevertheless, the Damuth law can provide at least a rough basis for assessments of population densities.

As one of the bases of the calculation, Marshall and his colleagues decided to treat T. rex as a predator whose energy requirements were somewhere between that of a warm-blooded mammal and a cold-blooded reptile. They estimated the average body mass of the adult animals to be 5.2 tons. Based on this data, they calculated that the average population density of the adult animals could have been around one specimen per 100 square kilometers. That means there may have been around 3,800 individuals populating an area the size of California, the researchers say. Based on this, further extrapolations based on the estimated distribution area of ​​the predatory dinosaurs were possible. According to fossil records, it covered about 2.3 million square kilometers. This brought the paleontologists to a population size of 20,000 specimens, each of which existed at the same time.

2.5 billion individuals possible – with a lot of plus / minus

In the next step, the researchers integrated assumptions about the generation sequence in T. rex and the length of existence of the species into their calculations. On the basis of earlier assessments of the age at sexual maturity and the life expectancy of the predatory dinosaurs, they estimated a time of 19 years for the generation change. In combination with common assumptions about the “reign” of T. rex of around 2.5 million years, they then came to the possible total number of individuals that have ever existed: The approximately 127,000 generations could therefore have comprised 2.5 billion specimens, write the scientists. Applied to the previous fossil finds of T. rex, this means: “We have discovered about one in 80 million,” says Marshall.

As he and his colleagues expressly emphasize, the projections in particular can only form a kind of basis for discussion. Because they are based on the rather rough assessments on the basis of the Damuth Law, they are associated with a great uncertainty factor. “We assume that the uncertainty of our results can extend over about two orders of magnitude,” says Marshall.

He also expects some aspects of the study to be criticized by his colleagues. But he sees the study above all as an illustration of the basic potential of the calculation method for population densities in paleontology. Because the system developed by the researchers can be applied to many extinct animal species. “The study shows a way of quantifying what we do not yet know,” the paleontologist concludes.

Source: University of California, Article: Science, doi: 10.1126 / science.abc8300

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