How many Tyrannosaurus rex ever lived ?

 Tyrannosaurus rex is one of the most well known dinosaur species popularized by the Jurassic park movies. It was the integral part of the story line of the Jurassic park. However, what makes T. rex special is not because it had a central role in movies, but it was indeed one of the most iconic predators that have ever lived on Earth. 

Life expectancy of a T. rex was about 28 years. One generation of T. rex is estimated to about 19 years. It lived as species for about 1.2 to 3.6 million years in the late Cretaceous (68 to 66 million years ago). 

Using the data from living mammal species and the T. rex fossil record, a study was recently published to estimate how many T. rex lived at any time or how many of them ever lived. For this purpose they used Damuth's law which says that the population density of a species is correlated with its body mass. Generally the correlation is negative, meaning, the population density declines as the body mass of the species increases. Therefore, larger animals would tend to have lower population densities.

So, from the data from an area of about thousand square kilometers and a time period of about 1.2 million years, it was estimated that there had been about 610 thousand T. rex lived. At any given time in that period, about 9 individuals roamed this area. They estimate that in all of its range, about 20 thousand T. rex roamed the Earth. During its existence of about 127 thousand generation, about 2.5 billion individual Tyrannosaurus lived and died.

PS: During writing this article I found another article that was written by the authors of the original study. It would certainly be more informative to read what they themselves wrote.