Over about 2.5 million years, north America likely organized 2.5 exchange rate Tyrannosaurus rexes, a minuscule relationship of which have actually been dug up and studied by paleontologists, follow to a UC audioeditorfree.com study. (Image by Julius Csotonyi, courtesy of science magazine)

How numerous Tyrannosaurus rexes roamed north America during the Cretaceous period?

That’s a concern Charles Marshall pestered his paleontologist colleagues v for years until he finally teamed up with his student to discover an answer.

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What the team found, come be published this week in the audioeditorfree.compaper Science, is that about 20,000 adult T. Rexes more than likely lived at any one time, provide or take a variable of 10, i beg your pardon is in the ballpark the what many of his partner guessed.

What couple of paleontologists had totally grasped, the said, consisting of himself, is the this method that some 2.5 billion lived and died over the around 2 1/2 million year the dinosaur to walk the earth.

Until now, no one has actually been able come compute populace numbers because that long-extinct animals, and George Gaylord Simpson, among the most significant paleontologists of the critical century, felt that it couldn’t it is in done.

Marshall, manager of the college of California Museum the Paleontology, the Philip Sandford Boone Chair in Paleontology and a UC audioeditorfree.com professor the integrative biology and also of earth and planetary science, was additionally surprised that such a calculation to be possible.

“The project just started off as a lark, in a way,” that said. “When I hold a fossil in my hand, i can’t aid wondering at the improbability the this really beast to be alive numerous years ago, and also here ns am holding part of the skeleton — it appears so improbable. The question simply kept popping into my head, ‘Just just how improbable is it? Is it one in a thousand, one in a million, one in a billion?’ and then I began to establish that probably we have the right to actually estimate how plenty of were alive, and thus, the I could answer that question.”


A actors of a T. Rex skeleton on display outside the UC Museum the Paleontology in the sink Life scientific researches Building. The original, a practically complete skeleton excavated in 1990 native the badlands of eastern Montana, is on display at the Smithsonian institution in Washington, DC. (UC audioeditorfree.com photograph by Keegan Houser)

Marshall is fast to suggest out the the unpredictabilities in the approximates are large. When the population of T. Rexes to be most most likely 20,000 adult at any type of give time, the 95% confidence range — the population range within which there’s a 95% possibility that the genuine number lies — is native 1,300 to 328,000 individuals. Thus, the total variety of individuals that existed end the lifetime of the varieties could have been everywhere from 140 million to 42 billion.

“As Simpson observed, it is an extremely hard to do quantitative approximates with the fossil record,” that said. “In ours study, we concentrated in occurring robust border on the variables we necessary to do our calculations, fairly than on focusing on making ideal estimates, per se.”

He and also his team then supplied Monte Carlo computer system simulation to determine exactly how the unpredictabilities in the data analyzed into unpredictabilities in the results.

The best uncertainty in this numbers, Marshall said, centers approximately questions about the specific nature the the dinosaur’s ecology, including exactly how warm-blooded T. Rex was. The study counts on data released by john Damuth the UC Santa Barbara the relates human body mass to population density for living animals, a relationship known as Damuth’s Law. While the relationship is strong, the said, ecological differences result in big variations in populace densities for animals with the same physiology and ecological niche. For example, jaguars and hyenas are around the very same size, however hyenas are uncovered in their habitat in ~ a density 50 times higher than the density of jaguars in their habitat.


A vital part that the analysis was estimating T. Rex’s ecological niche making use of a plot, dubbed Damuth’s Law, of body mass versus population density because that living mammals. (Chart courtesy of john Damuth, UC Santa Barbara)

“Our calculations depend on this relationship for living animals between their body mass and also their populace density, but the uncertainty in the connection spans about two assignment of magnitude,” Marshall said. “Surprisingly, then, the skepticism in our estimates is dominated by this ecological variability and not native the skepticism in the paleontological data we used.”

As part of the calculations, Marshall chose to act T. Rex together a predator with power requirements halfway in between those the a lion and a Komodo dragon, the largest lizard top top Earth.

The issue of T. Rex‘s location in the ecosystem led Marshall and his team to neglect juvenile T. Rexes, which room underrepresented in the fossil record and also may, in fact, have lived apart from adults and also pursued various prey. As T. Rex crossed right into maturity, the jaws ended up being stronger through an order of magnitude, permitting it come crush bone. This says that juveniles and adults ate different prey and were nearly like various predator species.

This possibility is supported by a recent study, led by evolution biologist Felicia smith of the college of brand-new Mexico, i beg your pardon hypothesized that the lack of medium-size predators alongside the substantial predatory T. Rex during the late Cretaceous was because juvenile T. Rex to fill that ecological niche.

What the fossils phone call us

The UC audioeditorfree.com scientists mined the clinical literature and the expertise of colleagues because that data they supplied to estimate that the likely age at sex-related maturity that a T. Rex was 15.5 years; its maximum lifespan to be probably into its late 20s; and also its average body mass as an adult — the so-called ecological body mass, — was around 5,200 kilograms, or 5.2 tons. They likewise used data ~ above how quickly T. Rexes flourished over your life span: They had actually a development spurt approximately sexual maturity and could prosper to weigh about 7,000 kilograms, or 7 tons.


From this estimates, they likewise calculated that each generation lasted about 19 years, and that the average populace density was about one dinosaur for every 100 square kilometers.

Then, estimating that the full geographic variety of T. Rex was about 2.3 million square kilometers, and also that the varieties survived for approximately 2 1/2 million years, they calculated a standing population size that 20,000. Over a complete of around 127,000 generations that the species lived, that converts to about 2.5 billion people overall.

With such a large number of post-juvenile dinosaurs over the background of the species, not to mention the juveniles that were presumably an ext numerous, wherein did every those skeleton go? What proportion of this individuals have been discovered by paleontologists? to date, fewer 보다 100 T. Rex individuals have been found, numerous represented through a solitary fossilized bone.

“There are about 32 relatively well-preserved, post-juvenile T. Rexes in public museum today,” that said. “Of every the post-juvenile adults that ever before lived, this means we have around one in 80 million of them.”

“If us restrict our evaluation of the fossil recovery rate to where T. Rex fossils are most common, a section of the famed Hell Creek development in Montana, we calculation we have actually recovered around one in 16,000 that the T. Rexes that resided in that region over the time interval the the rocks to be deposited,” that added. “We to be surprised by this number; this fossil record has a much greater representation of the living than I an initial guessed. It could be as good as one in a 1,000, if hardly any kind of lived there, or it could be as low together one in a quarter million, provided the unpredictabilities in the estimated population densities that the beast.”

Marshall expects his colleagues will quibble through many, if not most, the the numbers, however he believes that his calculational frame for estimating extinct populaces will stand and be helpful for estimating populaces of various other fossilized creatures.


The tooth of a tyrannosaur – no a T. Rex – whereby Charles Marshall found it in Montana in 2019. When T. Rex is an solely North American dinosaur, several other tyrannosaur types have been uncovered in north America and also Asia as well. (UC audioeditorfree.com photo by Charles Marshall)

“In some ways, this has actually been a paleontological practice in how much we have the right to know, and also how we go around knowing it,” the said. “It’s surprising just how much we actually know about these dinosaurs and, from that, just how much an ext we have the right to compute. Our understanding of T. Rex has increased so greatly in the past couple of decades many thanks to more fossils, an ext ways of examining them and better ways of integrating info over the many fossils known.”

The framework, i beg your pardon the researchers have made accessible as computer system code, likewise lays the foundation for estimating exactly how many species paleontologists could have missed as soon as excavating because that fossils, that said.

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“With this numbers, we have the right to start to calculation how plenty of short-lived, geographically committed species we can be absent in the fossil record,” that said. “This might be a means of beginning to quantify what we don’t know.”

Marshall’s co-authors space UC audioeditorfree.com undergraduate Connor Wilson and graduate college student Daniel Latorre, Tanner Frank, Katherine Magoulick, Joshua Zimmt and also Ashley Poust, who is currently a postdoctoral fellow at the san Diego Natural background Museum.