How Many Tyrannosaurus Rexes Ever Lived on Earth? Here’s a New Clue.

An estimation of the iconic predator’s total population can teach us things about dinosaurs that fossils cannot.

Before they were killed off by a meteor that hit Earth 66 million years ago, some 20,000 adults of the iconic ferocious dinosaur predator — Tyrannosaur rex — roamed North America at any given time, researchers have calculated. That’s not a precise number, and the correct total could be far lower or higher because of uncertainties like how long they lived, how quickly they grew and matured and the rate of their metabolisms. Still, the research, published on Thursday by the journal Science, opens doors in studying long-extinct dinosaurs beyond what can be gleaned from individual fossils. Skeletal features can tell a lot about an animal. For example, someone looking at a human tooth could infer that it is suited for chewing both plants and meat, and the shape of the skeleton could yield an estimate of how fast a person can run. But the physical attributes cannot tell you how many people live in New York City.