Tyrannosaurus rex lived about 65 to 70 million years ago during the beginning of the Cretaceous period and end of the Mesozoic era. Many roamed around present-day western North America. They were carnivores - which means they hunted their dinosaur prey - and were able to eat many pounds a day. T. rex could grow to massive sizes, and scientists often call them the "kings" of all the dinosaurs.