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DINOSAURS DOMINATED THE world’s land masses for 185 million years, from some 250 million years ago to around 65 million years ago. How many dinosaurs existed?

Charles R. Marshall and his colleagues give an answer for a well-known dinosaur, in “Absolute Abundance and Preservation Rate of Tyrannosaurus rex,” Science magazine, April 16, 2012, published by the American Association for the Advancement of Science.

Marshall and his colleagues are at the University of California, Berkeley, and the San Diego Natural History Museum. As described in their paper’s Abstract, they “used a relationship established between body size and population density in extant species to estimate traits such as density, distribution, total biomass, and species persistence for one of the best-known dinosaurs, Tyrannosaurus rex, revealing previously hidden aspects of its population ecology.”

Damuth’s Law. Laws in ecology are analogous to those in physics, generalized descriptions of how things behave. In particular, an allometry is a relationship expressed in a mathematical power that links different phenomena. The T. rex researchers used Damuth’s Law, given by the allometric equation d = aW-3/4, where d is the average density of a population, a is a constant, and W is the average body mass of the organism. 

John Damuth proposed this relationship in 1981. As an example described at, “A mammal that is 16 times larger than a second mammal will generally have an average population density 1/8 that of the second species.” Subsequent analyses have shown its validity in most cases for terrestrial vertebrates and invertebrates. 

An Abundance of T. rex. Marshall and his colleagues performed Damuth-law analyses of living species together with T. rex data obtained from fossil evidence. Extensive computer analyses estimated T. rex population characteristics through its 185 million years of dominance.

A fully grown  T. rex would have been 40 ft. long, 13 ft. high at the hips, and weighed as much as 30,000 lb. It’s estimated to have had the strongest bite of any terrestrial animal. No wonder its name: Tyrannosaur, Greek for “tyrant lizard;” rex, Latin for “king.” Illustration by Danielle Default, Royal Ontario Museum, from Science, April 16, 2021.

Researchers reported that T. rex “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.”

They continued, “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.”

To put the total in perspective, the world’s population of humans today is approaching 8 billion.

Putting It In Perspective. Michael Greshko offered another perspective in National Geographic, April 19, 2021, “Billions of T. rex Likely Roamed the Earth, Paleontologists Report.” He writes, “If you traveled back in time 67 million years ago to ancient Montana, you’d be entering the realm of a tyrant: the iconic predator Tyrannosaurus rex. Before you venture into that lost world, though, you might want to know: On average, how close is the nearest T. rex to you?”

Based on the work of Marshall et al, Greshko says of your imaginary ancient Montana presence, “In all likelihood, a T. rex would be within 15 miles of you, if not much closer.”

As Ogden Nash wrote of another predator, “… if called by a panther,/ Don’t anther.” ds 

© Dennis Simanaitis,, 2021 

One comment on “T. REX TALLY

  1. Valter Prieto Jr
    May 10, 2021

    And with this posted I wonder if it is real the oil is a renewable resource, as some guys have been saying for years.

    So many oil, so little dinosaurs.

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