Tyrannosaurus Rex was one of the largest meat-eating dinosaurs that ever lived. Everything about this ferocious predator, from its thick, heavy skull to its 4-foot-long jaw, was designed for maximum bone-crushing action. Tyrannosaurus Rex lived in forested river valleys in North America during the late Cretaceous period. It became extinct about 65 million years ago in the Cretaceous. Fossil evidence shows that Tyrannosaurus was about 40 feet long and about 15 to 20 feet tall. It's strong thighs and long, powerful tail helped it move quickly, and its massive 5-foot-long skull.