Theropods are a group of bipedal saurischian dinosaurs. Although they were primarily carnivorous, a number of theropod groups evolved herbivory, omnivory, and insectivory. Theropods first appeared during the Carnian age of the late Triassic period about 230 million years ago (Ma) and included the sole large terrestrial carnivores from the Early Jurassic until at least the close of the Cretaceous, about 65 Ma. Today, they are represented by the 9,900 living species of birds, which evolved in the Late Jurassic from small specialized coelurosaurian dinosaurs.
Among the features linking theropods to birds are the three-toed foot, a furcula (wishbone), air-filled bones and (in some cases) feathers and brooding of the eggs.
Tyrannosaurus was the largest and most popular theropod known to the general public for many decades. Since its discovery, however, a number of other giant carnivorous dinosaurs have been described, including Spinosaurus, Acrocanthosaurus, Carcharodontosaurus, and Giganotosaurus. The original Spinosaurus specimens (as well as newer fossils described in 2006) support the idea that Spinosaurus is larger than Tyrannosaurus, showing that Spinosaurus was possibly 6 meters longer and at least 3 metric ton heavier than Tyrannosaurus. There is still no clear explanation for exactly why these animals grew so much larger than the predators that came before and after them.
The smallest non-avialan theropod known from adult specimens is the troodontid Anchiornishuxleyi, at 110 grams in weight and 34 centimeters (1 ft) in length. When modern birds are included, the Bee Hummingbird Mellisuga helenae is smallest at 1.9 g and 5.5 cm (2.2 in) long.