Ever wondered how many Tyrannosaurus rex ever roamed Earth? The answer is 2.5 billion over the two million or so years for which the species existed, according to a calculation published today in Science1. The figure has allowed researchers to estimate just how exceedingly rare it is for animals to fossilize.
Palaeontologists led by Charles Marshall at the University of California, Berkeley, used a method employed by ecologists studying contemporary creatures to estimate the population density of T. rex during the late Cretaceous period.
“You hold a fossil in your hand and you know it’s rare. The question is, how rare?” says Marshall. “To know that, you need to know how many of them existed.”
To do that, he and his co-authors turned to a method used to estimate the population density of living animals from their body mass and the geographic ranges that they occupy. Damuth’s Law stipulates that the average population density of a species decreases in a predictable way as body mass increases; for example, there are fewer elephants than mice in a given area.
Chances of being fossilized vanishingly small
The team used its estimates of the total range of T. rex across modern North America, combined with estimates of the dinosaur’s body mass, to calculate that, at any one time, around 20,000 T. rex would have been alive on the planet. That translates to around 3,800 T. rex in an area the size of California, or just two T. rex patrolling Washington DC.
Calculating that T. rex survived for about 127,000 generations before becoming extinct, the researchers came up with a figure of 2.5 billion individuals over the species’ entire existence. Only 32 adult T. rex have been discovered as fossils, so the fossil record accounts for just one in about every 80 million T. rex. This means that the chances of being fossilized — even for one of the largest-ever carnivores — were vanishingly small.
These numbers suggest that fossils in general are exceedingly rare, and hint that many species that were much less widespread than T. rex were probably never preserved, says Marshall, who adds: “The fossil record is our only direct knowledge of these completely unimaginable past histories of our planet.”
Thomas Holtz, a vertebrate palaeontologist at the University of Maryland in College Park, calls the calculation an “interesting speculation”, adding that “we always knew that the chance of any individual becoming a fossil was exceedingly rare, but we lacked the calculation to figure out how rare”.
But he says it would be good “to see someone ground-truth these kinds of estimations against living species to get a better sense of accuracy”. He’d also like to see comparable studies made on extinct species with more abundant fossils, such as woolly mammoths, Neanderthals and dire wolves, which might allow us to better understand historical ecosystems.