With fossils few and far between, paleontologists have shied away from estimating the size of extinct populations. But scientists decided to try, focusing on the North American predator T. rex. Using data from the latest fossil analyses, they concluded that some 20,000 adults likely roamed the continent at any one time, from Mexico to Canada. The species survived for perhaps 2.5 million years, which means that about 2.5 billion lived and died overall.