T-rex dinosaur and polar bear. Illustration of the coelurosaurian theropod dinosaur Tyrannosaurus rex from 68 million years ago. Next to it, for comparison, is an adult male polar bear (Ursus maritimus). The Tyrannosaurus rex is about four metres tall at the hip, 12 metres long and weighs 6000 kilograms. The polar bear stands three metres tall and weighs 600 kilograms.