Opisthocoelicaudia

Opisthocoelicaudia was a genus of sauropod dinosaur of the Late Cretaceous Period discovered in the Gobi Desert of Mongolia. The only species is Opisthocoelicaudia skarzynskii. A well pr eserved skeleton was unearthed in 1965 by Polish and Mongolian scientists, lacking only the head and neck, making Opisthocoelicaudia one of the best known sauropods from the late Cretaceous. Tooth marks on this skeleton indicate that large carnivorous dinosaurs had fed on the carcassand possibly had carried away the now missing parts. To date, only two additional, much less complete specimens are known, including a part of a shoulder and a fragmentary tail. A relatively small sauropod, Opisthocoelicaudia measured approximately 11 metres (36 ft) in length. Like other sauropods, it would have been characterised by a small head sitting on a very long neck and a barrel shaped trunk carried by four column-like legs. The name Opisthocoelicaudia means "posterior cavity tail", alluding to the unusual, opisthocoel condition of the anterior tail vertebrae that were concave on their posterior sides. This and other skeletal features lead researchers to propose that Opisthocoelicaudia was able to rear on its hindlegs.

Named 1977, Opisthocoelicaudia first was thought to be a new camarasaurid, but now is considered a titanosaur. Its exact relationships within Titanosauria are contentious, but it may had been close to the North American Alamosaurus. All Opisthocoelicaudia fossils stem from the Nemegt Formation. Despite being rich in dinosaur fossils, the only other sauropod from this rock unit is Nemegtosaurus, which is known from a single skull. Since the skull of Opisthocoelicaudia remains unknown, several researchers suggested that Nemegtosaurus and Opisthocoelicaudia may be synonymous; in that case, the name Opisthocoelicaudia would be scrapped in favor of the earlier Nemegtosaurus. Skeletally, it is generally similar to most other titanosaurs. It has, however, unique joints in the tail vertebrae which account for its unusual name. Each of the thirty four tail vertebrae has a large dome on the front and a deep hemisperical socket on the posterior, creating a series of strong ball-and-socket joints. There is also notable evidence of immense ligament and muscle tissue attached to the tail. The tail thus tends to slant slightly upwards, not downwards as in o

ther sauropods. One possible explanation is that the tail was used as a prop to form a tripod with its back limbs, thus allowing the animal to reach high foliage