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How many tyrannosaurs have ever lived?

It was one of the largest land-dwelling carnivores of all time and is the epitome of the top predator: Tyrannosaurus Rex. No other dinosaur posthumously achieved such fame as this member of the Theropoda, who, with a body length of twelve meters, a weight of up to nine tons and a bite force of at least 35,000 Newtons, was undoubtedly an extremely terrifying hunter. Even his stunted little arms, which T. rex now and again make a popular target of ridicule, were deadly tools. Not only could they lift 200 kilograms, but they were also effective slotting tools.

T. Rex is probably the most present of all dinosaurs in children's rooms around the world. How many of these powerful animals once made the Cretaceous earth unsafe before they disappeared around 66 million years ago is unclear. Fossil finds come mainly from western North America, but they only show a small and coincidental part of the population history of these giants. Scientists around Charles Marshall from the University of California at Berkeley have now tried a statistical estimate to find out how many tyrannosaurs there could have been.

21,000 contemporaries

For their study in the journal "Science", Marshall and colleagues used the so-called Damuth Law, which is named after the American ecologist John Damuth and allows the population density of animals to be derived from physiological information and their ecological position. The larger an animal is and the higher its energy requirements, the fewer specimens can make a living in the same space. In the case of carnivores, the sustainable population density is even lower than that of vegetarians, as can also be observed in recent animal species (including Homo sapiens): In order to get enough prey, carnivores need significantly more space.

The result sounds impressive at first glance: there could have been around 2.5 billion copies in the course of the late Cretaceous. Distributed over the entire range of the T. rex and over 127,000 generations, however, on average there must never have been more than 21,000 individuals at the same time. So you should have been unlucky to meet a T. rex.

The scientists evaluated numerous studies on the physiology of the predatory dinosaurs and incorporated the data into their calculation model. For example, the sexual maturity of the animals was estimated at 15.5 years and the maximum lifespan at around 30 years. The average body mass of the dinosaurs was given as 5.2 tons, for animals that reached sexual maturity as seven tons. In their model, Marshall and colleagues calculated an average generation time of around 19 years - and came up with a distribution of one T. rex per 110 square kilometers. Accordingly, no four specimens would have found sufficient habitat on the area of ​​Vienna.

Great uncertainties, an exciting start

Extrapolated to the entire known range of T. rex, which covers around 2.3 million square kilometers, the coexistence of just under 21,000 of these predatory dinosaurs would be assumed. Over the period of around 2.4 million years in which T. Rex lived according to the fossil data, there could have been 127,000 generations and a total of around 2.5 billion individuals.

The emphasis in all the figures mentioned is on "could": The scientists expressly note that their calculation is associated with great uncertainties and, above all, should form a basis for further investigations. In any case, the researchers hope that their study will initiate a discussion about the potential of population models based on Damuth's law for paleontology. "That could be a way of quantifying what we don't know," Marshall said. (David Rennert, April 17, 2021)