Absolute abundance and preservation rate of Tyrannosaurus rex

Abstract

Although much can be deduced from fossils alone, estimating abundance and preservation rates of extinct species requires data from living species. Here, we use the relationship between population density and body mass among living species combined with our substantial knowledge of Tyrannosaurus rex to calculate population variables and preservation rates for postjuvenile T. rex. We estimate that its abundance at any one time was ~20,000 individuals, that it persisted for ~127,000 generations, and that the total number of T. rex that ever lived was ~2.5 billion individuals, with a fossil recovery rate of 1 per ~80 million individuals or 1 per 16,000 individuals where its fossils are most abundant. The uncertainties in these values span more than two orders of magnitude, largely because of the variance in the density–body mass relationship rather than variance in the paleobiological input variables.

Population sizes of T. rex cannot be precisely estimated

“…Undaunted, Marshall et al (2021) recently estimated the population density, and total population size of Tyrannosaurus rex across their entire range, and not only at a single point in time but across the duration of the existence of this most charismatic of dinosaurs, thus calculating the total number of T. rex that have ever lived. This bold and ambitious attempt earned them a place in the pages of Science.…”

“…To estimate T. rex population size, Marshall et al (2021) estimated its body size, diet, and metabolic rate. They used a 1980's cross-taxon allometry of population density on body size to ascribe a density value for T. rex (Damuth 1987).…”

“…All these estimates are associated with substantial uncertainties, some were considered, others neglected. Thus, for example, Marshall et al (2021) "assumed a physiology midway between… mammalian carnivores and… large varanid lizards". The metabolic rate of T. rex is unknown.…”

“…The metabolic rate of T. rex is unknown. It may have been similar to that of extant reptiles, or extant birds and mammals (had there been anything in these clades approaching its size), and it is likely to have occupied a position somewhere between these extremes (e.g., Benton 2021, Legendre and Davesne 2020), and a midpoint position (Marshall et al 2021) is as likely as any. But "midway" is a point estimate that has no direct support and using a range of values would have been appropriate.…”

“…Density estimates vary greatly with detectability, which is influenced by e.g., habitat, weather, time of day, and survey methods. Marshall et al (2021) acknowledged some of this variability, used the confidence interval around density estimates at a single body size (from Damuth 1987, neglecting the uncertainty in body size, see below) to estimate a 241-fold 95% confidence interval for population densities (0.00058 to 0.14 individuals/km 2 ). In a recent comprehensive dataset or tetrapod densities (TetraDENSITY; Santini et al 2018) 12 large (>10kg) mammalian carnivore species (T. rex analogs; Marshall et al 2021), have > 10 density estimates each.…”