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, published this month 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.