Segnosaurus
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Segnosaurus Temporal range: Late Cretaceous, 90 Ma |
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| Mandible in several views | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Order: | Saurischia |
| Suborder: | Theropoda |
| Family: | †Therizinosauridae |
| Genus: |
†Segnosaurus Perle, 1979 |
| Species: | †S. galbinensis |
| Binomial name | |
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Segnosaurus galbinensis Perle, 1979 |
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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.
Both mandibles of Segnosaurus are known, missing only small portions of the posterior bones, and the entire angular bone. The dentary is very complez compared to other therizinosaurs, with the strongest downwards curve in the anterior end of any known therizinosaur. The front 25.5 mm (1.00 in) of this curve is edentulous (toothless), which is 8% more than in Erlikosaurus. A toothless portion of the mandible is near absent in Jianchangosaurus, and present but unknown in percentage in Beipiaosaurus and Neimongosaurus. When articulated the mandibles form a U-shaped curve like in Erlikosaurus and Neimongosaurus.
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