Paleontologists Stunning Conclusion: 2.5 Billion T. Rexes Roamed North America Over the Cretaceous Period

Tyrannosaurus T-Rex Dinosaur

Paleontologists believe that about 20,000 adult T. rexes lived concurrently, with a margin of error around 10. This suggests that approximately 2.5 billion T. rex individuals existed and perished over the dinosaur’s 2.5 million years on Earth.

Analysis of what’s known about the dinosaur leads to conclusion there were 2.5 billion over time.

How many Tyrannosaurus rexes roamed North America during the Cretaceous period?

That’s a question Charles Marshall pestered his paleontologist colleagues with for years until he finally teamed up with his students to find an answer.

What the team found, to be published this week in the journal Science, is that about 20,000 adult T. rexes probably lived at any one time, give or take a factor of 10, which is in the ballpark of what most of his colleagues guessed.

What few paleontologists had fully grasped, he said, including himself, is that this means that some 2.5 billion lived and died over the approximately 2 1/2 million years the dinosaur walked the earth.

Until now, no one has been able to compute population numbers for long-extinct animals, and George Gaylord Simpson, one of the most influential paleontologists of the last century, felt that it couldn’t be done.

Marshall, director of the University of California Museum of Paleontology, the Philip Sandford Boone Chair in Paleontology and a UC Berkeley professor of integrative biology and of earth and planetary science, was also surprised that such a calculation was possible.

T. rex Cast at UC Berkeley

A cast of a T. rex skeleton on display outside the UC Museum of Paleontology at the University of California, Berkeley. The original, a nearly complete skeleton excavated in 1990 from the badlands of eastern Montana, is at the Museum of the Rockies in Bozeman, Montana. Credit: Keegan Houser, UC Berkeley

“The project just started off as a lark, in a way,” he said. “When I hold a fossil in my hand, I can’t help wondering at the improbability that this very beast was alive millions of years ago, and here I am holding part of its skeleton — it seems so improbable. The question just kept popping into my head, ‘Just how improbable is it? Is it one in a thousand, one in a million, one in a billion?’ And then I began to realize that maybe we can actually estimate how many were alive, and thus, that I could answer that question.”

Marshall is quick to point out that the uncertainties in the estimates are large. While the population of T. rexes was most likely 20,000 adults at any give time, the 95% confidence range — the population range within which there’s a 95% chance that the real number lies — is from 1,300 to 328,000 individuals. Thus, the total number of individuals that existed over the lifetime of the species could have been anywhere from 140 million to 42 billion.

“As Simpson observed, it is very hard to make quantitative estimates with the fossil record,” he said. “In our study, we focused in developing robust constraints on the variables we needed to make our calculations, rather than on focusing on making best estimates, per se.”

He and his team then used Monte Carlo computer simulation to determine how the uncertainties in the data translated into uncertainties in the results.

The greatest uncertainty in these numbers, Marshall said, centers around questions about the exact nature of the dinosaur’s ecology, including how warm-blooded T. rex was. The study relies on data published by John Damuth of UC Santa Barbara that relates body mass to population density for living animals, a relationship known as Damuth’s Law. While the relationship is strong, he said, ecological differences result in large variations in population densities for animals with the same physiology and ecological niche. For example, jaguars and hyenas are about the same size, but hyenas are found in their habitat at a density 50 times greater than the density of jaguars in their habitat.

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

As part of the calculations, Marshall chose to treat T. rex as a predator with energy requirements halfway between those of a lion and a Komodo dragon, the largest lizard on Earth.

The issue of T. rex’s place in the ecosystem led Marshall and his team to ignore juvenile T. rexes, which are underrepresented in the fossil record and may, in fact, have lived apart from adults and pursued different prey. As T. rex crossed into maturity, its jaws became stronger by an order of magnitude, enabling it to crush bone. This suggests that juveniles and adults ate different prey and were almost like different predator species.

This possibility is supported by a recent study, led by evolutionary biologist Felicia Smith of the University of New Mexico, which hypothesized that the absence of medium-size predators alongside the massive predatory T. rex during the late Cretaceous was because juvenile T. rex filled that ecological niche.

What the fossils tell us

The UC Berkeley scientists mined the scientific literature and the expertise of colleagues for data they used to estimate that the likely age at sexual maturity of a T. rex was 15.5 years; its maximum lifespan was probably into its late 20s; and its average body mass as an adult — its so-called ecological body mass, — was about 5,200 kilograms (11,500 pounds), or 5.2 tons. They also used data on how quickly T. rexes grew over their life span: They had a growth spurt around sexual maturity and could grow to weigh about 7,000 kilograms (15,000 pounds), or 7 tons.

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

Then, estimating that the total geographic range of T. rex was about 2.3 million square kilometers (0.9 million square miles), and that the species survived for roughly 2 1/2 million years, they calculated a standing population size of 20,000. Over a total of about 127,000 generations that the species lived, that translates to about 2.5 billion individuals overall.

With such a large number of post-juvenile dinosaurs over the history of the species, not to mention the juveniles that were presumably more numerous, where did all those bones go? What proportion of these individuals have been discovered by paleontologists? To date, fewer than 100 T. rex individuals have been found, many represented by a single fossilized bone.

“There are about 32 relatively well-preserved, post-juvenile T. rexes in public museums today,” he said. “Of all the post-juvenile adults that ever lived, this means we have about one in 80 million of them.”

“If we restrict our analysis of the fossil recovery rate to where T. rex fossils are most common, a portion of the famous Hell Creek Formation in Montana, we estimate we have recovered about one in 16,000 of the T. rexes that lived in that region over that time interval that the rocks were deposited,” he added. “We were surprised by this number; this fossil record has a much higher representation of the living than I first guessed. It could be as good as one in a 1,000, if hardly any lived there, or it could be as low as one in a quarter million, given the uncertainties in the estimated population densities of the beast.”

Marshall expects his colleagues will quibble with many, if not most, of the numbers, but he believes that his calculational framework for estimating extinct populations will stand and be useful for estimating populations of other fossilized creatures.

“In some ways, this has been a paleontological exercise in how much we can know, and how we go about knowing it,” he said. “It’s surprising how much we actually know about these dinosaurs and, from that, how much more we can compute. Our knowledge of T. rex has expanded so greatly in the past few decades thanks to more fossils, more ways of analyzing them, and better ways of integrating information over the multiple fossils known.”

The framework, which the researchers have made available as computer code, also lays the foundation for estimating how many species paleontologists might have missed when excavating for fossils, he said.

“With these numbers, we can start to estimate how many short-lived, geographically specialized species we might be missing in the fossil record,” he said. “This may be a way of beginning to quantify what we don’t know.”

Reference: “Absolute abundance and preservation rate of Tyrannosaurus rex” by Charles R. Marshall, Daniel V. Latorre, Connor J. Wilson, Tanner M. Frank, Katherine M. Magoulick, Joshua B. Zimmt and Ashley W. Poust, 16 April 2021, Science.
DOI: 10.1126/science.abc8300

Marshall’s co-authors are UC Berkeley undergraduate Connor Wilson and graduate students Daniel Latorre, Tanner Frank, Katherine Magoulick, Joshua Zimmt and Ashley Poust, who is now a postdoctoral fellow at the San Diego Natural History Museum.

37 Comments on "Paleontologists Stunning Conclusion: 2.5 Billion T. Rexes Roamed North America Over the Cretaceous Period"

  1. Joseph A. Bachofen | April 15, 2021 at 2:21 pm | Reply

    That works out to roughly 30 in any given year. My guess is there had to be more in order to find a mate.

    • Clyde Spencer | April 15, 2021 at 4:44 pm | Reply

      How did you come up with your number 30? 2.5 x 10^9 individuals over a period of 2.5 x 10^6 years is an average of 1,000 individuals per years. That might still be a little low for a breeding population over the entire earth, but it is still more than 33 times your estimate.

  2. The numbers mite be right t.rex was not the top tyrannosaur spinosauridae is the top tyrannosaur . they are so many tyrannosaur In Florida Louisiana and South America today since control skin trade .gator handle mammal pretty well dinosaur cheated when into water turn into super reptile the death roll is the most superior dinosaur technology spinosauridae was first to developed it and become king of dinosaur .motor back was too much for rest of dinosaur was not the meterorite it was the advance dinosaur the mesoeucrocodylia tyrannosaur allso call spinosauridae and modern crocodilian .modern crocodilian is one of most dangerous animal ever walk the earth even more dangerous what media says .clearly inside body very high tech warm bloodedish even more advance than mammal that is a fact .heart kidney spleen.they have cat eyes that glow cat is one of the great modern predator .modern crocodilian had hard time getting rid of early land gator type mesoeucrocodylia early land mesoeucrocodylia did most of damage against dinosaur since the gator line is semi aquatic allso did some damage too the land mesoeucrocodylia made it to human time .it’s scientific impossible for dinosaur to be advance like modern crocodilian .gator can crush metal.

  3. So stupid. Who cares.

  4. Matthew Ferrantino | April 16, 2021 at 6:20 am | Reply

    Interesting. Of course theoretical branches of knowledge are easy to postulate but it seems like someone actually got off their ass to apply an idea to check to see if some data could have an estimate marked down and highlighted for further study. Anyone can talk about population statistics but did anyone publish anything peer reviewed about
    TRex population? That’s why what this guy did is something new. Theory and application are different.

  5. Earth is less than 7,000 years old. So all of this is crap.

  6. Shawn Williams | April 16, 2021 at 6:56 am | Reply

    Ah yes. Oversimplification on the part of a non-life sciences guy.

    It only looks like “just multiplication when one article points out the minutiae of one species. When they discovered it applied to all living life…that may have been a better pizza party at The Hut.

  7. Ah theories… they never cease to amuse.

  8. Fake.

    Obviously there weren’t 2.5 billion tyrannosaurs, given that nearly every fossil excavation is mere feet below ground level, we would be buried in dinosaur bones every time a new building was constructed or an oil was drilled. You can count on one hand the fossils found drilling for petroleum or mining for coal or diamonds or rare earth minerals. No dinosaurs exist preserved in cave formations.

    Dinosaur bones were never found until after freemasons started looking for them in the 1850s to prove the theory of evolution.

    It’s all a big hoax. And any level of rational scrutiny tears apart the big lie.

    We are created in God’s image and this world is made for us to enjoy. The wealthy people who run the world want to deny our divinity and our birthright in order to accumulate wealth and power for themselves.

    • Clyde Spencer | April 16, 2021 at 9:04 am | Reply

      You can come to any conclusion you want when you make up your own facts. Dinosaur bones that are exposed by erosion quickly weather and break down into unrecognizable rubble. They are often unrecoverable after only a couple of years of exposure to the elements. And, it takes exceptional circumstances for bones to be preserved! We don’t find find dinosaur bones in caves because limestone caves are created in a marine environment, where dinosaurs didn’t live, and there were few even closely related species. Interestingly, megafauna marine animals in general are rarely preserved in limestone, although many limestone formations are actually made of marine calcifiers, such as coccoliths in the White Cliffs of Dover, and the famous nummalitic limestones from which the Great Pyramids are constructed.

      Ranting will not make converts from those with a science education reading this site. I suggest that you take your theology dogma to an echo chamber of like-minded who will happily shout hallelujah and affirm your piety.

  9. Once again they come out with numbers that give them enough leeway that anybody could have probably guessed that somewhere between 1300 and 328,000 would be the number of T-Rexes alive at one time. Now when I go to a carnival they’ve got dudes there that can guess and age within 3 years or a month within 2 months or even your weight within 5 lb. And those guys usually aren’t even educated, but these so-called scientific Masterminds try to manipulate everyone by using big numbers with big margins, and all they’re doing is wasting big dollars to prove absolutely nothing. They always try to claim theories as facts but they have not run a scientific method experiment to prove anything, they just wasted time and money trying to figure this out and it really doesn’t matter for us in this day and age to know how many T-Rex is there were or how many brontosauruses there were or whether or not red Flintstone was driving around in a stone car working at a quarry when those T-Rexes were walking the Earth. So for the total they say somewhere between $140 million and 42 billion…. What kind of a margin of error to these guys need, to try to skew numbers. I mean that’s the difference of just do 1 billion would be a difference of 860 million, plus 42 billion so they gave themselves a leeway of 42 billion 860 million T-Rexes. Why don’t they find the Bones from let’s say 20 T-Rexes. Then we can start there and say that there was 20 of them and when they find more they can decide that there was more. These researchers waste our time, our money, and the best they can come up with is somewhere within practically 43 billion… That is not a solid number. When will people realize that science doesn’t use actual scientific method anymore they just make a guess throw out some big numbers and then people give them money to do research so that they can put it in their pocket and tell everybody how smart they are again. Don’t feed into the LIE these guys are funny as hell and they should be on late night TV telling their theories to all of us and then we can laugh it’s called stand up comedy… And when I was in biotech and genetics, back in the 80s I could see this coming all these liars in upper echelon science acting like they have half a brain. But common Sense shoots down their theories quicker than they can make them up, and meanwhile people believe their lies and consider them some type of new religion and they don’t realize that these are just the nerds that were in their class that didn’t have any girlfriends that just makes it up to make themselves sound smart because they don’t have anything else going for themselves. I know I’m one of those people and I think it’s hilarious but come on guys get within a million years do something for once halfway accurate. Yes are disgusting and so stupid that you shouldn’t even have a degree need to retake some tests buddy get a brain in there. What are you making meth and back I don’t know what they’re doing. It’s beyond me sorry I won’t b**** anymore I’m just having fun hope you got a kick out of this but it’s the truth these guys don’t know anything and they can’t prove it. Just like that stupid guy in his damn wheelchair spent all that time trying to figure out the algorithm for everything, what good would that do anybody nothing but what he should do or should have done was used his mind to figure out how to fix the disease he had called Lou Gehrig’s disease and got his ass out of that chair and helped other people to get out of theirs. But he was too busy trying to waste our time trying to figure out things that don’t matter like The Big bang theory who really cares how the universe started what could that possibly do for us we can’t make it happen, and we can’t prove that it happened so they just throw out some numbers and tell other people they’re stupid and then write it down and books and teach it as truth when they have no idea they can’t replicate it a bunch of dummies God they piss me off sorry I’m still bitching I didn’t mean to be but damn I want to just kick him in the teeth buy him a drink and say let’s figure this out for real.

  10. I thought April Fool’s Day was on the 1st? Just posting any kind of nonsense these days.

  11. Checkmate atheist

  12. Must be fun to make up stuff.

  13. So how many other dinosaurs did they all eat? That has to be an even larger number.

  14. James Gross MD | April 16, 2021 at 11:32 am | Reply

    Maybe there are not that many bones all of the various species because in a portion of the animal kingdom they kill their mates the mates Kill The Offspring and maybe the bones get disintegrated or they get spread out in various ways and that’s why you only find one- or two and only in a tar pit where they are trapped will you find an entire spasm and it’s a pretty big stretch from Lion to Komodo dragon since the Komodo dragon bites its victim poisons its victim and then wait till it dies.

  15. Linda Hardiman | April 16, 2021 at 11:38 am | Reply

    How ludicrous.

  16. One problem. T-Rex has been found with soft tissue in the bones. Soft tissue cannot last 65 my.
    Second problem. T-Rex fossils are cabon 14 datable. That puts a top age of T-rex at less than 65 thousand years old.
    Third problem. There are man made renditions, drawings, etchings, models, and stories, of known dinosaurs.
    The belief that dinosaurs died out 65 my ago is just that.
    A belief devoid of observable or testable evidence.

  17. And global warming killed them all because of their farts. Its repeating itself. Noooooooooooo

  18. Im just here to read all the keyboard paleintologists in the comment sections. What degrees do you have? What group studies of dinosaurs have u been in amongst other scientists in ur field?? That’s the real question.

  19. Atik is right.

  20. And this is newsworthy why?

  21. You crazy Christians realize you look just as crazy as the Muslim fundamentalists. Nobody gives a damn about your religious opinions

  22. Yeah because Christians make it a habit of chopping people’s heads off or blowing them up with pipe bombs. Wow, here’s your sign 🤪

  23. What Zabinski said. Lol.

  24. My car goes 200 mph.
    I did 50 mph last Monday, 50 mph on Tuesday, 50 mph on Thursday and 50 mph yesterday.
    Yup. That’s 200mph.

  25. Duane Johnson | April 16, 2021 at 5:23 pm | Reply

    A pointless article that illustrates much that is wrong with science reporting.

  26. Bradley Barber | April 16, 2021 at 6:58 pm | Reply

    I say this in reply: “The fool, in his heart, says there is no God.”. Only the Creator God invented all that we see, know and haha even guess about. DNA in the quadrillions was invented by the Lord God Almighty…. So I ask the question: Are you a fool?

  27. False. The t-rex is a composite of various other dinosaurs. Its actually an interesting story. A museum was looking to bring in more customers. And a dig site was like we found something with alot of teeth but we were only able to find the creatures skull. The t-rex is is simply a marketing ploy.

  28. I have never commented on the Internet before in my life. I have no idea why this article showed up in my unwanted Google home news feed, nor why I clicked on it, nor why I read the entire thing, nor why I continued scrolling to see the comments. But now that I have? I forever want in on this massive sh*t show.

  29. Im an expert in most things and I noticed the Flood wasnt mentioned as a selective pressure on Trex evolution

  30. Jebediah Bibblethumpergh | April 16, 2021 at 10:39 pm | Reply

    God created the dinosaurs when he created the beasts of the field. He knew that there would be a lot of fields to plow before Jesus would later create the farm tractor and before he put the gasoline in the ground to run them. Men and dinosaurs coexisted for the better part of our entire human history which runs about 6000 years. Sadly they couldn’t fit on Noah’s ark and drowned in the flood. God bless the dinosaurs . I thank them for helping to make America Great way back when.

  31. Kinda makes me curious if you could extrapolate this information and get a rough representation of how much oil we have in our world. Of course, it would take a vast amount of additional data but if it was possible, that would be really cool.

  32. Sorry but this world is not millions of years old.
    Get your facts straight.

  33. Shelly L Cooper | April 17, 2021 at 8:17 am | Reply

    I’m curious to know more about this research. Questions like 1. Did the shells change based on ocean chemistry nearer the end of the Cretaceous period?
    Ex: was the chemistry of the ocean so changed at that point that they had to range farther for food sources? In our current oceanic situation we arr witnessing chemical changes due to CO2 absorption. There are currently brine rivers moving along the floor of the Gulf of Mexico, these may likely reach the deep oceanic belt somewhere to mix.
    2. Did the shells change due to predation? With changing ocean chemistry right before their extinction, perhaps they had to create more bouncy to range higher or lower or both due to food sources and predators. Which predators did they avoid? Perhaps due to losses in numbers of other species of animals for food, the predators began a more focused routine that caused an increase in shell construction.
    3. With the ability to hold water on the shell surface aside from the ability to use the chambers of the structure, this to my mind implies a deeper bouncy. Perhaps the water was cooler. We are currently seeing coral reefs developing at deeper levels due to warming oceans.

  34. Curious George | April 17, 2021 at 11:42 am | Reply

    Just curious… Is there a FULL skeleton anywhere? Not a pieced together one, but a full skeleton from a single site?

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