“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. (Chart courtesy of John Damuth, UC Santa Barbara) rex’s ecological niche using a plot, called Damuth’s Law, of body mass versus population density for living mammals. 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.Ī critical part of the analysis was estimating T. 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. 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. 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. He and his team then used Monte Carlo computer simulation to determine how the uncertainties in the data translated into uncertainties in the results. “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.” “As Simpson observed, it is very hard to make quantitative estimates with the fossil record,” he said. Thus, the total number of individuals that existed over the lifetime of the species could have been anywhere from 140 million to 42 billion. rexes was most likely 20,000 adults at any given 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. Marshall is quick to point out that the uncertainties in the estimates are large. The original, a nearly complete skeleton excavated in 1990 from the badlands of eastern Montana, is on display at the Smithsonian Institution in Washington, DC. rex skeleton on display outside the UC Museum of Paleontology in the Valley Life Sciences Building. 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.”Ī cast of a T. “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 project just started off as a lark, in a way,” he said. 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. 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. 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. 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 the team found, to be published this week in the journal Science, is that about 20,000 adult T. 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. How many Tyrannosaurus rexes roamed North America during the Cretaceous period? (Image by Julius Csotonyi, courtesy of Science magazine) Over approximately 2.5 million years, North America likely hosted 2.5 billion Tyrannosaurus rexes, a minuscule proportion of which have been dug up and studied by paleontologists, according to a UC Berkeley study.
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