Tyrannosaurus rex is a member of the saurischia or "lizard-hipped" dinosaurs. T.rex can then be classed in the theropods, carnivores that have many hollow bones and walk on their hind legs with three or fewer working toes on each foot. T.rex was one of the biggest theropods, a group that ranged from the chicken sized Compsognathus to dinosaurs as big as forty feet long. T.rex is then placed in the Coelurosaurus group, the closest descedants to modern birds.