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Therizinosaur, group of theropod dinosaurs that lived during the Late Cretaceous (roughly 100 million to 66 million years ago) in Asia and North America and were characterized by their relatively small skulls, leaf-shaped teeth, and extended fingers with extremely long and robust claws. Therizinosaurs also lacked teeth in the front half of their upper jaws, and they had long necks, wrist bones similar to those of birds, widely spaced hips, a backward-pointing pubis bone, and four widely spread toes similar to those of sauropod dinosaurs. Fossil specimens have been known since the 1950s, but their unusual combination of skeletal features (especially their teeth, hips, and toes) made their relationships to other dinosaur groups contentious. By the mid-1990s, the discovery of new, more complete specimens had confirmed their theropod ancestry. Therizinosaurs are divided into five genera (Beipiaosaurus, Falcarius, Alxasaurus, Erlikosaurus, and Therizinosaurus).
Unlike most other theropods, therizinosaurs were most likely herbivorous. It is likely that the transition from carnivory to herbivory occurred early in the evolution of the group. The transition involved changes in dentition and changes to the hips and hind limbs—which allowed more room and better support for the larger gut needed to digest plants. The most primitive therizinosaur, Falcarius, has been described as a transitional species because it has herbivorous dentition and wider hips; however, it also possessed a pubis bone and legs that resembled those of its running, carnivorous ancestors.
Some therizinosaur fossils show remarkable preservation. For example, Beipiaosaurus specimens show large patches of featherlike integument on the chest, forelimbs, and hind limbs. Several embryonic therizinosaur skeletons have been found inside fossilized eggs. These embryos show several unambiguous theropod characteristics that are lost by adulthood; they provide insight into the order of bone formation in dinosaurs.