|Tyrannosaurus skeletal mount, Carnegie Museum of Natural History|
|Gorgosaurus skeletal mount, Royal Tyrrell Museum of Palaeontology|
Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family of coelurosaurian theropod dinosaurs that comprises two subfamilies containing up to thirteen genera, including the eponymous Tyrannosaurus. The exact number of genera is controversial, with some experts recognizing as few as three. All of these animals lived near the end of the Cretaceous Period and their fossils have been found only in North America, Europe and Asia.
Although descended from smaller ancestors, tyrannosaurids were almost always the largest predators in their respective ecosystems, putting them at the apex of the food chain. The largest species was Tyrannosaurus rex, one of the largest and most massive known land predators, which measured over 12.3 metres (40 ft) in length and according to most modern estimates 8.4 metric tons (9.3 short tons) to 14 metric tons (15.4 short tons) in weight. Tyrannosaurids were bipedal carnivores with massive skulls filled with large teeth. Despite their large size, their legs were long and proportioned for fast movement. In contrast, their arms were very small, bearing only two functional digits.
Unlike most other groups of dinosaurs, very complete remains have been discovered for most known tyrannosaurids. This has allowed a variety of research into their biology. Scientific studies have focused on their ontogeny, biomechanics and ecology, among other subjects.
The tyrannosaurids were all large animals, with all species capable of weighing at least 1 metric ton. A single specimen of Alioramus of an individual estimated at between 5 and 6 metres (16 and 20 ft) long has been discovered, although it is considered by some experts to be a juvenile. Albertosaurus, Gorgosaurus and Daspletosaurus all measured between 8 and 10 metres (26 and 33 ft) long, while Tarbosaurus reached lengths of 12 metres (39 ft) from snout to tail. The massive Tyrannosaurus reached 12.3 metres (40 ft) in the largest specimen, FMNH PR2081.