Tyrannosaurus rex dinosaur, artwork. T. rex was a carnivorous theropod dinosaur that existed in the Upper Mastrichtian stage of the Cretaceous Period, between 65 and 67 million years ago. It was one of the largest known carnivorous dinosaurs, measuring 12.4 metres long and between 4.6 to 6 metres tall. Its skull was around 1.6 metres long and the eye sockets about 10 centimetres wide. Adults may have weighed around 7 tonnes. T. rex walked on two legs, and may have reached a running speed of 24 kilometres per hour. It is thought that T. rex may have been both a predator and a scavenger, having characteristics that suggest both possibilities.