This group includes all the known carnivorous dinosaurs as well as the birds. No obviously adapted herbivores are recognized in the group, but some theropods, notably the toothless oviraptorids and ornithomimids, may well have been relatively omnivorous like today’s ostriches. Mesozoic Era theropods ranged in size from the smallest known adult Mesozoic nonavian dinosaur, the crow-sized Microraptor, up to the great Tyrannosaurus and Giganotosaurus, which were 15 or more metres (50 feet) long, more than 5 metres (16 to 18 feet) tall, and weighed 6 tons or more. Theropods have been recovered from deposits of the Late Triassic through the latest Cretaceous and from all continents.
Theropods may be defined as birds and all saurischians more closely related to birds than to sauropods. They have a carnivorous dentition and large, recurved claws on the fingers. They also share many other characteristics, such as a distinctive joint in the lower jaw, epipophyses on the neck vertebrae, and a unique “transition point” in the tail where the vertebrae become longer and more lightly built. Other similarities include the reduction or loss of the outer two fingers, long end joints of the fingers, and a straplike fibula attached to a crest on the side of the tibia.
Herrerasaurus and several fragmentary taxa from South America, including Staurikosaurus and Ischisaurus, from the Middle to Late Triassic of Argentina are carnivores that have often been included in the Dinosauria, specifically in Theropoda. Whereas these animals closely resemble dinosaurs and have many carnivorous features, they also lack a number of features present in dinosaurs, saurischians, and theropods. For example, they have only two sacral vertebrae, unlike dinosaurs; their hips are more primitive than those of saurischians, as are their wrists; and the second finger is not the longest, unlike those of all saurischians. It remains probable that the features they seem to share with theropod dinosaurs are simply primitive and related to carnivory, the general habit of archosaurs. Future discoveries and analyses may help to resolve these questions.