Tyrannosaurs (from the Greek τυράννος tyrannos, “tyrant”, and σαῦρος sauros, “lizard”) are a family of theropod dinosaurs. The family includes well-known genera such as Tyrannosaurus, Tarbosaurus and Alioramus, although the exact definition of the family has long been a matter of debate.
The oldest known tyrannosaur is Dilong, which lived in what is now China during the early Cretaceous Period about 126 million years ago. However, this genus may not actually belong to Tyrannosauridae but rather its own separate family, Dilongidae. The most recent common ancestor of all tyrannosaurs lived approximately 168 million years ago during the mid-Jurassic Period.
Tyrannosaurs were large predators, with some members growing to over 12 m (40 ft) in length and weighing up to 6 metric tons (13,000 lb). They had long skulls loaded with sharp teeth for tearing flesh, powerful hind limbs for fast running, and muscular forelimbs with three clawed fingers on each hand. Their tails were stiffened by ossified tendons running down their length which helped them balance while running at high speeds.
Most tyrannosaurs were bipedal; however, some early members of the group such as Xiaotingia were quadrupeds. All known tyrannosaurs share a number of unique features which distinguish them from other theropods: an S-shaped neck profile in side view; an ilium (upper hip bone) which projects forwards and upwards above the pubis (lower hip bone); ribs which are fused at their tips; air sacs connected to their bones; and an arctometatarsus – a type of joint between the metatarsals (foot bones) and tarsals (ankle bones) which allows for greater mobility in the foot than seen in other dinosaurs.
It is thought that tyrannosaurs evolved from earlier carnivorous dinosaurs such as Coelophysis and Dilophosaurus through a series of gradual steps over millions of years. This transition can be seen clearly in fossils belonging to intermediate forms such as Proceratosaurus and Guanlong which show characteristics intermediate between earlier carnosaurians and later tyrant dinosaurs.
The first truly “tyrannical” form was probably Eotyrannus from Early Cretaceous England; however, this genus has often been considered either a member of Tyrannosauridae or its own separate family Eotyrannidae depending on how narrowly defined one chooses to make Tyrannosauridae.
The most famous representatives of Tyrannosauridae are undoubtedly Tyrannosaurus rex from Late Cretaceous North America and Asia and Tarbosaurus bataar from Late Cretaceous Mongolia – two closely related genera often popularly known simply as “T-rex”. These massive predators reached lengths of up to 12 m (40 ft) and weights approaching 7 metric tons (15,000 lb). In addition to being among the largest terrestrial carnivores ever known they also had some of the strongest bites ever recorded – up to 3500 psi(240 bar), comparable to that of an adult saltwater crocodile!
Other notable members include Albertosaurus from Late Cretaceous Alberta & Montana; Daspletosaurus from Late Cretaceous Alberta & Montana; Gorgosaurus from Late Cretaceous Alberta & Montana; Nanuqsaurus hoglundi from Late Cretaceous Alaska; Rapator Oweni from Early Cretaceous Australia; Shanxia tianyiensis from Early Cretaceous China