Tyrannosaurus (meaning "tyrant lizard") is an extinct genus of tyrannosaurid theropod dinosaur that lived in North America during the Late Cretaceous period around 73-66 million years ago. The genus includes two valid species; Tyrannosaurus rex and Tyrannosaurus mcraeensis. However, two additional species, Tyrannosaurus imperator and Tyrannosaurus regina, have been proposed, though paleontologists near-universally agree upon their invalidity. Often credited as the king of the dinosaurs, Tyrannosaurus rex is one of the contenders for the largest theropod, with the largest specimens measuring 43 feet (13.1 meters), standing 13 feet (4 meters) tall, and weighing up to 24,000 pounds (11,000 kilograms). However, recent studies have speculated that Tyrannosaurus could have grown larger than previously thought, possibly reaching lengths of up to 50 feet (15 meters) long, growing to stand 16 feet (5 m) tall, and weighing 33,000 pounds (15,000 kg). Since its description in 1905, Tyrannosaurus has become one of, if not the number one, most famous and well-studied dinosaur in the world