Tyrannosaurus rex: about 2.5 billion walked on Earth


Tyrannosaurus rex: The little dinosaur Hora, from the comic strip of Monica’s Gang, is a Tyrannosaurus rex puppy in search of his mother and his peers. His loneliness has now been quantified: according to paleontologist Charles Marshall, director of the Museum of Paleontology at the University of California, up to 20,000 animals divided the North American continent at the same time; about 2.5 billion of these prehistoric giants walked the Earth before extinction.

For years, Marshall wondered if it was possible to calculate how many Tyrannosaurus rex had lived in the period between 1.2 and 3.6 million years that dominated the Earth. There were few bones left of these animals, and that was all he and his team of graduate students had – and they knew how to make good use of them.

“The project started out as a joke, in a way. When I hold a fossil in my hand, I can’t help wondering about the improbability that this same beast has been alive for millions of years, and here I am holding part of its skeleton. The question kept coming up in my head: ‘How unlikely is that? Is it one in a thousand, in a million, one in a billion? ‘And then I started to realize that maybe we could really estimate how many were alive,’ said the paleontologist.

Thousands of generations

The first step was to estimate the biological cycle of T. rex, using microscopic patterns of growth in the bones and stipulating that a new generation was born every 19 years – in 2.4 million years of existence on Earth, there were from 66 thousand to 188 thousand generations. It remained to be determined how many individuals there were in each generation.


“In ecology, there is a relationship between body mass and population density, Damuth’s law: larger animals with a higher metabolism need more space to survive. With good estimates of T. rex body mass and metabolism (slightly lower than that of a lion), we estimate that there were about one T. rex per 109.9 square kilometers, ”said paleontologist Ashley Poust, coauthor of the study, published in the journal Science.

According to him, “multiplying the population density by the area where T. rex lived, this gave us an estimate of 20 thousand individuals per generation”.

Marshall himself points out that the uncertainties in the calculation are great: the so-called confidence interval (the one within which there is a 95% chance that the real number is) is between 1,300 and 328,000 individuals – this means that the total number of T rex that walked the Earth varies between 140 million to 42 billion.

Lost fossils

The way in which Marshall and his team arrived at this T. rex population can be used both to estimate extinct populations and to calculate how many paleontologists may have lost when excavating fossils.

“With the average population size, we calculated the rate of T. rex fossilization – the chance that a single skeleton would survive to be discovered 66 million years later. The answer: about one in 80 million. In other words, for every 80 million T. rex adults, there is only one specimen in a museum, ”said paleontologist Daniel Varajão de Latorre, co-author of the study.


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