Before they were killed by a meteor that struck Earth 66 million years ago, researchers estimated around 20,000 adults of the legendary wild dinosaur predator Tyrannosaur Rex roamed North America at one point in time.
This is not an exact number, and the correct sum could be far lower or higher due to uncertainties such as lifespan, the rate of growth and maturation, and the rate of their metabolism. However, the study published Thursday by the journal Science opens doors to study long-extinct dinosaurs beyond what can be extracted from individual fossils.
Skeletal features can tell a lot about an animal. For example, someone looking at a human tooth could conclude that it is suitable for both plant and meat chewing, and the shape of the skeleton could provide an estimate of how fast a person can run. However, the physical characteristics cannot tell you how many people live in New York.
“Studies like this one are the first step in restoring ancient ecosystems,” said Felisa A. Smith, a biology professor at the University of New Mexico who was not involved in the research. “We have to go beyond what fossils have been found and where the bigger picture goes: how the ecosystem works.”
Charles R. Marshall, a professor of integrative biology at the University of California at Berkeley who led the research, said the work began by making him wonder how rare it was when he found a T. rex fossil in the Hand held.
“Were there a million, a billion, a trillion T. rexes?” he said. “Is that one in a million, one in a billion, one in a trillion? How on earth could we know this number? We all know fossils are rare, but how rare are they? And so it really started with this question. “
Therefore, he and the students in his research group used tools that biologists use on modern animals and applied them to the dinosaur era. The core of their calculations depends on the observation that there are many more small animals than large, that rats are far more than elephants.
For living species, John Damuth, a biologist at the University of California at Santa Barbara, developed a mathematical relationship between an animal’s average body mass and its expected population density, known today as Damuth’s Law.
The relationship is not universal, but it generally applies to large classes of animals such as lizards or carnivorous mammals. So for Tyrannosaurus Rex, they not only had to pocket the dinosaur’s weight – about six tons, give or take a few – but also derive other legal numbers.
That includes the metabolism. It is no longer believed that dinosaurs are cold-blooded like modern lizards, but they probably weren’t as warm-blooded as mammals. So Dr. Marshall’s team suggested a physiology between that of carnivorous mammals and Komodo dragons. You also had to take into account some uncertainties about where the dinosaurs lived in North America. Paleontologists do not know whether T. rex’s range was limited to fossil sites in the western United States and Canada, or whether it extended to other locations with similar climates from Alaska to the east coast.
With so much unknown about dinosaurs, the scientists didn’t want to find a single definitive answer, but rather set boundaries on what they thought could have been a plausible number. “For most paleontological data, I don’t know how to guess a number,” said Dr. Marshall. “But I can tell you what is a good minimum and what is a good maximum.”
The calculations gave a most likely standing population of 20,000 adults with Tyrannosaurus rex. This would indicate a small distribution equivalent to two adults in an area the size of Washington, DC
However, the uncertainty surrounding this estimate was very great. The same computer simulations showed there was a 97.5 percent probability that at least 1,300 adults but no more than 328,000 adults were present.
Dr. Damuth described the new paper as a “good contribution”.
He added: “Despite the uncertainties associated with most of the model parameters, the paper gives a solid qualitative result.”
If the number of 20,000 were correct, a total of about 2.5 billion adults would have lived in the 2.4 million years that T. rex lived on Earth.
But even Dr. Marshall thinks the number of 20,000 is low. “It just seems inconceivable that you can last a few million years with these few people,” he said. “You just need some terrible plague or something and you’re gone.”
He said he thought the population could have been tens of thousands, or maybe 100,000 or 200,000. Much of the uncertainty is that Damuth’s law is not absolute. Jaguars and spotted hyenas are both carnivorous mammals of similar size, but the population density of hyenas is around 50 times higher.
“Using these methods is really the exciting part,” said Peter Makovicky, a University of Minnesota paleontologist who was not involved in the research. “I don’t think a lot of us who are skeletal-focused paleontologists, for example, thought it was possible, but I think it’s really eye-opening in that sense.”
Dr. Makovicky said reading the paper today he started looking for populations of large predators to compare to the estimate of 20,000 T. rexes. “There are fewer than 4,000 tigers in the wild, with a few more lions,” he said. “It didn’t sound like a large number, but it’s actually not that small.”
The study did not attempt to count younger T. rexes. In recent years, research suggests that juveniles were almost like a different species. Although they may be more numerous than adults, few fossils were found, suggesting that most survived this period of life or that adolescent bones were less likely to be preserved.
For Dr. Marshall was also able to get an answer to the question he started with: Only about one in 80 million T. rexes that ever existed was fossilized, although in the most populous regions the number may be as high as 1 in 16,000.