A total of 2.5 billion Tyrannosaurus rex probably existed during the lifespan of the species, researchers have calculated – suggesting that very few survived as fossils. Charles Marshall at the University of California, Berkeley, and his colleagues used body mass and population density to estimate how many T. rex once lived. Larger animals tend to have a larger individual range, because they need more food to support their body mass than smaller animals, meaning body mass is inversely correlated with population density – a rule known as Damuth’s law. Previous analysis of T. rex fossils shows that the average body...