Long-term followers of Earth-logs and its predecessor Earth-pages News will have observed my general detachment from the dinosaur hullabaloo, which just runs and runs. That is, except for real hold-the-front-page items. One popped up in the 16 April 2021 issue of Science (Marshall, C.R. et al. 2021. Absolute abundance and preservation rate of Tyrannosaurus rex. Science, v. 372, p. 284-287; DOI:10.1126/science.abc8300). For over two million years in the Late Cretaceous, just before all dinosaurs – except for birds – literally bit the dust, the authors estimated a lot of the dinosaurian poster-childTyrannosaurus rex lurking in North America. I write ‘lurking’ because ‘tyrant lizard the king’ when fully grown was so big that if it ran and fell over, it would have been unable to get up! Tangible evidence from trackways suggests that it ambled from place to place. The leg bones of a 7-tonner would probably have shattered at speeds above 18 km per hour, and accelerating to the speed of a human jogger would, anyhow, have exhausted its energy reserves, But it was agile enough to be an ambush predator; it could even pirouette! And it could crush bones so well that it was able to consume prey entirely. It has been suggested that T. rex may have been a scavenger, at least in old age. Whatever, how is it possible to estimate numbers of any extinct species, let alone dinosaurs?

The stumbling block to getting a result that is better than guesswork is the fossil record of a species. First, it is incomplete, secondly the chance of finding a fossil varies from area to area, depending on all kinds of factors. These include the degree of exposure of sedimentary rock formed by the environment in which they thrived, as well as the vagaries of preservation due to post-mortem scavenging, erosion and water transport. In life the population density of a particularspecies varies between different ecosystems and from species to species. For instance, more lions can thrive in open rangeland than in wooded environments, whereas the opposite holds for tigers: probably because of different hunting strategies. Many factors such as these conspire to thwart realistic estimates of ancient populations. Studies of living species, however, suggest that population density of an animal species is inversely related to the average body mass of individuals. Take British herbivores: there are many more rabbits than there are deer. On the grasslands of East Africa hyenas and wild dogs outnumber lions. This mass-population relationship (Damuth’s Law) outlined by US ecologist John Damuth also depends on where a species exists in the food chain (its trophic level) as well as its physiology. Yet for living species, populations of flesh-eating mammals of similar mass show a 150-fold variation; a scatter that results from their different habits and habitats and also their energy requirements. Because they are warm-blooded (endothermic), small carnivorous mammals need a greater energy intake than do similar sized, cold-blooded reptiles, which need to eat far less. But not all living reptiles are ectothermic, especially the bigger ones. The Komodo dragon is mesothermic, midway between the two, and uses about a fifth of the energy needed by a similar-sized mammal carnivore. Population densities of dragons in the Lesser Sunda Islands are more than twice those of physiologically comparable mammalian predators.

A number of features suggest that the metabolism of carnivorous dinosaurs lay midway between those of large predatory mammals and big lizards like the Komodo dragon. This is the basic assumption for the analysis by Charles Marshall and colleagues. They did not focus on the biggest T. rex specimens, but on the average, estimated body mass of adults. There are numerous smaller specimens of the beast, but clearly some of these would have been sexually immature. It has been estimated that adulthood would have been achieved by around 15 years. The size data seem to show that achieving sexual maturity was accompanied by a 4 to 5 year growth spurt from the 2 to 3 tonnes of the largest juveniles to reach >7 t in the largest known adults which may have lived into their early 30s. The authors used this range to estimate a mean adult mass of 5.2 t. Taking this parameter and much more intricate factors into account, using intricate Monte Carlo simulations Marshall et al. came up with an estimate of 20 thousand T. rex adults across North America at any one time: but with an uncertainty of between 1,300 to 328,000. Spread over the 2.3 million km² area of Late Cretaceous North America that lay above sea level their best-estimated population density would have been about 1 individual for every 100 square kilometres. An area the size of California could have had about 3800 adult Tyrannosaurus rex, while there may well have been two in Washington DC. Lest one’s imagination gets overly excited, were tigers and lions living wild today in North America under similar ecological conditions there would have been 12 and 28 respectively in the US capital. Yet those two adult Washingtonian T. rexs would have been unable to catch anything capable of a sustained jog, without keeling over. The juveniles weighing in at up to 3 tonnes would probably have been the real top predators; the smaller, the swifter and thus most fierce. Which leaves me to wonder, “Did the early teenagers catch the prey for their massive parents to chow-down on?”

See also: How many T. rexes were there? Billions. (ScienceDaily 15 April 2021)