How many Tyrannosaurus Rex walked the Earth?

National

Stan, one of the largest and most complete Tyrannosaurus rex fossil discovered, is on display, Tuesday, Sept. 15, 2020, at Christie’s in New York. The T. rex named after the paleontologist who first found the skeleton’s partially unearthed hip bones, will be auction on Oct. 6, 2020 and will be on public view from Sept. 16 – Oct. 21, 2020 to pedestrians through Christie’s floor-to- ceiling gallery windows and a limited number of in-gallery viewings by appointment. Stan’s head on the display is a casting of the original, which is too heavy for the display. (AP Photo/Mary Altaffer)

(THE CONVERSATION) The Research Brief is a short take about interesting academic work.

The big idea

During 2.4 million years of existence on Earth, a total of 2.5 billion Tyrannosaurus rex ever lived, and 20,000 individual animals would have been alive at any moment, according to a new calculation method described in a paper published on April 15, 2021, in the journal Science. To estimate population, our team of paleontologists and scientists had to combine the extraordinarily comprehensive existing research on T. rex with an ecological principle that connects population density to body size.

From microscopic growth patterns in bones, researchers inferred that T. rex first mated at around 15 years old. With growth records, scientists can also generate survivorship curves – an estimate of a T. rex‘s chances of living to a given age. Using these two numbers, a team of researchers estimated that T. rex generations took 19 years. Finally, T. rex existed as a species for 1.2 to 3.6 million years. With all of this information, it was calculated that T. rex existed for 66,000 to 188,000 generations.

From the fossil record alone, a T. rex turnover rate was generated. If the team could estimate the number of individuals in each generation, they would know how many T. rex ever lived.

In ecology, there is a well-established relationship between body mass and population density called Damuth’s law. Larger animals need more space to survive – one square mile of grassland can support a lot more bunnies than elephants. This relationship is also dependent on metabolism – animals that burn more energy require more space.

Paleontologists have come up with a range of good estimates of T. rex’s body mass and have also estimated its metabolism – slower than mammals but somewhat faster than a large modern lizard, the Komodo dragon. With Damuth’s law, it was then estimated that the ancient world held about one T. rex every 42.4 square miles (109.9 square km). That’s about two individuals in the entire area of Washington, D.C. Multiplying the population density by the area in which T. rex lived gave an estimate of 20,000 individuals per generation.

Why it matters

Once the average population size was figured out, we were able to calculate the fossilization rate for T. rex – the chance that a single skeleton would survive to be discovered by humans 66 million years later. The answer: about 1 in 80 million. That is, for every 80 million adult T. rex, there is only one clearly identifiable specimen in a museum.

This number highlights how incomplete the fossil record is and allows researchers to ask how rare a species could be without disappearing entirely from the fossil record. Beyond calculating the T. rex fossilization rate, this new method could be used to calculate population size for other extinct species.

What still isn’t known

Estimates about extinct animals always include some amount of uncertainty. The estimate of T. rex population density ranges from one individual for every 2.7 square miles (7 square km) to one for every 665.7 square miles (1,724 square km). But surprisingly, the largest source of this uncertainty comes from Damuth’s law. There is a lot of variation in modern animals. For example, Arctic foxes and Tasmanian devils have similar body mass, but devils have six times the population density.

Further study of living animals could tighten up estimates on T. rex. What is also unknown are fossilization rates of other long extinct dinosaurs. If one has many fossils of one species, does that mean they were more common than T. rex, or are their fossils recovered more often?

What’s next

This study might lead to other hidden facts about T. rex biology and ecology. For instance, we might be able to learn whether T. rex populations fluctuated up and down with Triceratops – similar to wolf and moose predator and prey relationships today. However, most other dinosaurs do not yet have the incredibly rich data from decades of careful fieldwork that allowed the research team to tally up T. rex.

If scientists want to apply this powerful technique to other extinct animals, there’s more digging to be done.

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