Tyrannosaurus rex is unquestionably the most famous dinosaur, probably because of its awesome size and appearance, and its apparent strength, agility and prowess. In spite of its fame, the anatomy and relationships of Tyrannosaurus and its kin remain poorly understood. By Late Cretaceous times (about 65 to 100 million years ago), carnivorous dinosaurs (theropods) had reached their peak of diversity in the Northern Hemisphere, and there were many specialized species of all sizes.