How many T. rexes were there? Billions

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. (Image by Julius Csotonyi , courtesy of Science magazine) 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. 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. "The project just started off as a lark, in a way,” he said.