Period: Late Cretaceous
Order, Suborder, Family: Saurischia, Segnosauria, Segnosauridae
Location: Asia (Mongolia)
Length: Estimated 20 feet (6 meters)
One of the important finds of the Joint Soviet-Mongolian Paleontological Expeditions of the 1970s was the discovery of a whole new group of dinosaurs, the segnosaurians. Segnosaurus galbinensis, or "slow lizard from Galbin" (a region of the Gobi Desert), was first described by Mongolian paleontologist Altangerel Perle in 1979. It was an unusual saurischian that he classified in its own family, the Segnosauridae.
It showed an unusual combination of features of ornithischians, theropods, and prosauropods. Its pelvis looked much like the pelvis of the dromaeosaurids, although it was much larger. Segnosaurus had feet with long, slender theropodlike claws and ankles, although it had four toes instead of three on each foot. The teeth, although there were many and they were small, resembled those of some theropods.
The pelvis of Segnosaurus was very wide, giving the animal a broad back and a "pot belly." Theropods had pelves (plural of pelvis) that were slender. Its feet were not really theropod feet. Its relative Erlikosaurus had a prosauropodlike beak, so Segnosaurus probably did, too. After much study, it was found that the teeth of Segnosaurus had more in common with prosauropod teeth than with theropod teeth. These and other features suggest that it was more closely related to the sauropods and prosauropods than to the theropods.
Segnosaurians have been found in several locations in Mongolia and China, and there is a possible Erlikosaurus specimen from Alberta. The puzzling dinosaur Therizinosaurus cheloniformis, known only from front limbs and hands with claws up to three feet long, was first classified as a turtle, but is probably a huge segnosaurian. Most segnosaurians (Segnosaurus, Erlikosaurus andrewsi, and Enigmosaurus mongoliensis) came from early to middle Late Cretaceous rocks. Nanshiungosaurus brevispinus from China may be younger, and Therizinosaurus was even younger.
What the lifestyle of Segnosaurus and its relatives was like is not known. The teeth show they were probably plant-eaters. They may have had a large gut for digesting plants. They may have looked like large, wide-bodied prosauropods, and they probably walked on all four legs most of the time. They may have been bearlike or like extinct Ice Age mammals called ground sloths. Segnosaurians were probably not fast runners. If their hands did have claws like Therizinosaurus, they would have been used as weapons against predators, including Alectrosaurus, which probably hunted Segnosaurus.