How many ‘Tyrannosaurus rex’ have walked the earth?

The research letter is a brief overview of 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 lived at any given time, according to a new calculation method we described in a paper published in the journal Science on April 15, 2021.

To estimate the population, our team of paleontologists and scientists had to combine the extraordinarily comprehensive existing research on it T. rex with an ecological principle linking population density to body size.

From microscopic growth patterns in bones, researchers deduced this T. rex first mated at 15 years old. With growth records, scientists can also survive curves – an estimate of a T. rextheir chances of living to a given age. Using these two numbers, our team estimated it T. rex generations took 19 years. Finally, T. rex existed as a species for 1.2 to 3.6 million years. With all this information we calculate it T. rex existed 66,000 to 188,000 generations.

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

In ecology, there is an established link between body mass and population density, which is called Damuth’s law. Larger animals need more space to survive – one square kilometer of grassland can carry many more rabbits than elephants. This ratio is also dependent on metabolism – animals that burn more energy require more space.

Paleontologists have a series of good estimates of T. rexbody mass and also estimated its metabolism – slower than mammals, but somewhat faster than a large modern lizard, the Komodo dragon. With Damuth’s law we then estimated that the ancient world had about one T. rex every 42.4 square miles (109.9 square km). It is about two individuals in the entire area of ​​Washington, DC

Now we had all the pieces we needed. Multiply the population density by the area in which T. rex lived gives us an estimate of 20,000 individuals per generation.

Why it matters

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

This number shows how incomplete the fossil record is and makes researchers ask how rare a species can be without disappearing completely from the fossil record.

In addition to calculating the T. rex with the rate of fossilization, our new method can be used to calculate the population size for other extinct species.

What is not yet known

Estimates of extinct animals always contain some uncertainty. Our estimate of T. rex population density varies from one individual for every 7 square kilometers (7 square kilometers) to one for every 665.7 square kilometers (1,724 square kilometers). But surprisingly, the biggest source of this uncertainty comes from Damuth’s law. There is a lot of variation in modern animals. North foxes and Tasmanian devils, for example, have a similar body mass, but demons have six times the population density.

Further study of living animals can give us estimates on T. rex.

We also do not know the fossilization rate of other long-extinct dinosaurs. If we have many fossils of one species, it means that they were more common than T. rex, or do we simply recycle their fossils more often?

What’s next

This study may lead to other hidden facts about T. rex biology and ecology.

For example, we can learn whether T. rex populations alternated up and down with Triceratops – similar to predatory and prey relationships between wolf and elk. However, most other dinosaurs do not yet have the incredibly rich data of decades of careful fieldwork that has enabled our team to come together. T. rex.

If scientists want to apply this powerful technique to other extinct animals, we still have to dig.