Over their entire late-Cretaceous reign, the total number of Tyrannosaurus rex that ever lived on Earth was roughly 2.5 billion individuals, according to a new study, which leveraged the relationship between body mass and population density observed in living animals to estimate the population traits of the iconic, long-extinct species. According to the authors, the framework they developed could be applied to any extinct creature given the appropriate data. It opens the door for a host of new investigations into other palaeoecological and taphonomic questions. Much can be learned from the fossil record about extinct species like the dinosaurs. However, due to the record's fragmented nature, understanding ecological variables such as population density and abundance remains challenging. In living species, one way to understand more about these variables is by using the established relationship between population density and body mass, or Darmuth's Law, which states that the average population density of a species decreases with body size at a predictable rate. Using Darmuth's Law and the large body of paleontological data for T. rex - one of the largest carnivorous theropods known - Charles Marshall and colleagues estimated population-level traits and the rate of fossil preservation for the species. Their findings suggest that nearly 20,000 T. rex lived concurrently at any given time during their tenure on Earth and persisted for ~127,000 generations, totaling 2.5 billion over their entire existence. What's more, the authors estimate that the population density of the species equated to 3,800 T. rex in an area the size of California - or just two in an area the size of Washington D.C. The results also allowed Marshall et al. to determine that only about 1 in 80 million T. rex survived the eons as fossilized remains. Although the uncertainties of the estimations span over two orders of magnitude, the authors largely attribute the potential error to variance in the density-body mass relationship underlying Damuth's Law rather than to variance in the palaeobiological data for T. rex.
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Science