Segnosaurus (‘slow lizard’) is a genus of herbivorous theropod dinosaur belonging to the Therizinosauridae from the Cretaceous of Mongolia.
Segnosaurus was a rather large therizinosaurid. In 2010 Gregory S. Paul estimated the body length at 6 metres (19.5 ft), the weight at 1.3 tonnes. Segnosaurus had an elongated head, large clawed hands, a somewhat elevated torso, a broad strong pelvis, stocky legs and a short tail.
Segnosaurus can be distinguished from all other therizinosaurs on the basis of two unique derived traits (autapomorphies). The in total forty-eight mandibular teeth are markedly peg-like and only slightly recurved: the front or mesial edge is curved and the back or distal edge is straight. The second autapomorphy is that the claws of the hand are rather flat instead of very narrow. In the same formation the closely related Erlikosaurus is found; Segnosaurus can be distinguished from this species by its moderate transverse compression of the pedal unguals or foot claws. Also the latero-dorsal shelf on the dentary, a flat bone surface at the upper outside of the lower jaw, starts at the fourteenth dentary tooth position and runs backwards for half the length of the lower jaw, unlike the shelf in Erlikosaurus, which starts at the fifth tooth position. This would have indicated that Segnosaurus did not have as extensive ‘cheeks’ as Erlikosaurus is believed to have had.
Exactly what use this strange combination of features was to Segnosaurus is widely debated. It has been suggested that it was a plant-eater descended from a meat-eating ancestor or, perhaps, a specialist termite hunter that used its huge claws to rip open termite nests. Alternatively, it may have been a specialist fish-hunting dinosaur, hooking fish out of the water with its claws.
Segnosaurus is a relatively new dinosaur, described in 1979, and known only from fragments and isolated bones. This makes it difficult to understand what it was really like. Mongolia and China have produced many unusual theropods, such as Segnosaurus, from late Cretaceous deposits. These groups are not found anywhere else, indicating that what is now central Asia was isolated from the rest of the world by mountains and seas for most of the later Mesozoic era.