Tyrannosaurus rex was a massive theropod dinosaur that lived during the Late Cretaceous period, around 68 to 66 million years ago. Measuring up to 12 meters (40 feet) in length and weighing up to 9 metric tons, it was one of the largest land predators ever. T. rex had a massive skull filled with large, serrated teeth designed for crushing bone, and powerful hind legs that allowed it to move swiftly. Despite having relatively small arms, they were incredibly strong. Fossils of T. rex have been primarily found in North America, providing extensive insights into its biology, behavior, and the ecosystems of the Late Cretaceous.