Billions of T. rexes were there!

With fossils few and far between, palaeontologists have shied away from estimating the size of extinct populations. But scientists decided to try, focusing on the North American predator T. rex.

Using data from the latest fossil analyses, they concluded that some 20,000 adults likely roamed the continent at any one time, from Mexico to Canada. The species survived for perhaps 2.5 million years, which means that about 2.5 billion lived and died overall.

The study relies on data that relates body mass to population density for living animals, a relationship known as Damuth's Law. While the relationship is strong, he said, ecological differences result in large variations in population densities for animals with the same physiology and ecological niche.

For example, jaguars and hyenas are about the same size, but hyenas are found in their habitat at a density 50 times greater than the density of jaguars in their habitat.