If you traveled back in time to ancient Montana 67 million years ago, you would enter the realm of a tyrant: the iconic predator Tyrannosaurus rexBut before you venture into that lost world, you might want to know: How close is the nearest on average T. rex to you?
That may sound like an impossible thing to know, but after crawling on for two decades T. rex Research provides a new study estimates of the animal’s population density. In all likelihood, one T. rex would be within 25 miles of you if not much closer.
The new study, published last Thursday in Science, this population density also translates into estimates for how much T. rex ever lived. On average, researchers estimate that there are about 20,000 T. rex once lived and that about 127,000 generations of the dinosaurs lived and died. Those averages imply a total of 2.5 billion T. rex lived in the species’ native North America, possibly as far north as Alaska and as far south as Mexico, for a period of two to three million years.
This research isn’t the first time scientists have attempted to estimate T. rex numbers. In fact, the average population density in the new newspaper is about one T. rex every 42 square miles – is very similar to an earlier estimate published in 1993. But the new study uses the latter T. rex biology research to try to establish very accurate upper and lower limits for the total population.
After running millions of computer simulations, each with a slightly different mix of the possible values, the study found that the total T. rex the number can be as low as 140 million and as high as 42 billion, averaging around 2.5 billion. Likewise anywhere from 1,300 to 328,000 T. rex could ever have been alive, averaging 20,000.
“It’s really exciting that someone is trying to … use everything we know T. rex to find out population dynamics, ”said Holly Woodward, a paleontologist at the Oklahoma State University Center for Health Sciences who was not involved in the new study. “It’s interesting and fun that this hasn’t been done on this kind of scale.” (Read more about Woodward’s research on how T. rex spent his teenage years.)
T. rex accountancy
Over the past 20 years, researchers have discovered an extraordinary amount about it T. rexincluding how long it lived (about 28 years), when it was sexually mature (about 15.5 years old), and how much it weighed when fully grown (about 15,000 pounds on average). Scientists calculate these data T. rexthe estimated generation time – 19 years, give or take – and the average body mass of T. rex at any time.
Reach T. rexs population numbers, researchers took advantage of a relationship between body weight and population density among living animals. As body weight increases by a factor of 10, population density decreases on average by more than four-fifths – a pattern known as Damuth’s law.
Damuth, an ecologist at the University of California, Santa Barbara, first discovered this pattern by gathering 30 years of ecological data on living mammals. Damuth’s law is not very strong, however, as animals differ greatly in their specific lifestyles and habitats. Spotted hyenas and jaguars, for example, have a similar body weight, and both are predators, but the hyena’s population density is about 50 times higher.
When applied to T. rex (after correcting for the fact that T. rex is not a mammal), Damuth’s law implied that the dinosaur’s true total probably fell within the 140 million and 42 billion individual dinosaurs.
“In paleontology it is very difficult to estimate things… so what I started is thinking less about estimating something and more about putting it in parentheses. Can I put a robust upper and lower limit on it? says lead study author Charles Marshall, a paleontologist at the University of California, Berkeley.
From life to rock
Aside from getting a better idea of how many of these elephant-sized predators were stomping, Marshall and his team were able to crunch the numbers to better gauge how often fossils form. Would the chance of one T. rex fossilization are quantified, in the same way we can calculate the probability of being struck by lightning?
About a hundred copies are known T. rex, but about two-fifths of it is privately or commercially owned and cannot be reliably studied. So to establish a minimum fossil total for the purposes of the study, Marshall’s team limited the number to 32 fossils from post-juvenile T. rex which are held in public institutions.
If all T. rex that ever lived – the estimated 2.5 billion – yielded only those 32 fossils, then only about one in 80 million T. rex petrified after they died. Even if a higher percentage of the animals are fossilized and we have yet to find the remains, the sheer trifle of these odds underscores how rare it is for a carcass to be buried quickly enough and under the right chemical conditions to mineralize and form a fossil . . “If T. rex were a thousand times less abundant – if the total was not 2.5 billion but 2.5 million– we may never have found it, ”says Marshall.
The method outlined by Marshall’s team could be used for other extinct creatures as well. Among dinosaurs, researchers say one of the best candidates is the Cretaceous herbivore Maiasaura, which is known from hundreds of specimens, from newborns to adults.
For Woodward, one of the most exciting implications of the study is how rare dinosaur fossils really are. If these rates apply to other types then T. rex, researchers can even estimate how many dinosaur species don’t fossilize at all – and have now been irretrievably lost over time. “Being able to find out how much we are missing can be just as important as knowing how much we have,” she says.