Opisthocoelicaudia is a genus of sauropod dinosaur of the Late Cretaceous Period discovered in the Gobi Desert of Mongolia. The type species is Opisthocoelicaudia skarzynskii. A well-preserved skeleton lacking only the head and neck was unearthed in 1965 by Polish and Mongolian scientists, 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 carcass and possibly had carried away the now-missing parts. To date, only two additional, much less complete specimens are known, including part of a shoulder and a fragmentary tail. A relatively small sauropod, Opisthocoelicaudia measured about 11.4–13 m (37–43 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 and described by Polish paleontologist Maria Magdalena Borsuk-Białynicka in 1977, Opisthocoelicaudia was first thought to be a new member of the Camarasauridae, but is currently considered a derived member of the Titanosauria. Its exact relationships within Titanosauria are contentious, but it may have 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 have suggested that Nemegtosaurus and Opisthocoelicaudia may represent the same species. Sauropod footprints from the Nemegt Formation, which include skin impressions, can probably be referred to either Nemegtosaurus or Opisthocoelicaudia as these are the only known sauropods from this formation.
Discovery and specimens
The type specimen was discovered between June 10 and 23, 1965, during a joint Polish-Mongolian paleontological expedition led by Polish paleontologist Zofia Kielan-Jaworowska. The largest of a series of expeditions carried out in 1963–1971, this expedition involved 21 members, which at times were supported by additional hired Mongolian workers. The site of discovery is located in Ömnögovi Province in southern Mongolia in the Altan Uul area, which exposes some 100 km2 of badlands. The sediments exposed at Altan Uul belong to the Nemegt Formation, the youngest of the three geological formations of the Nemegt Basin. Opisthocoelicaudia was the first of several important dinosaur discoveries made by the 1965 expedition. The other finds, made at different localities, include several skeletons of the tyrannosaurid Tarbosaurus as well as the type specimens of the giant ornithomimosaur Deinocheirus, the sauropod Nemegtosaurus, and the pachycephalosaur Homalocephale.
On the fifth day of fieldwork, Ryszard Gradziński, the geologist of the expedition, found a concretion of well-preserved bones which promised to belong to a fairly complete skeleton. Excavation starting the next day revealed a nearly complete skeleton lacking only the head and neck. Until today, this specimen remains by far the most complete finding of this dinosaur. The transport of the specimen out of rough terrain caused major technical problems. Stone blocks containing the fossils had to be moved some 580 meters, dragged on improvised metal sledges made out of petrol drums, before they could be loaded onto trucks. Because the skeleton was embedded in very hard sandstone layers, several blocks weighed over a ton. On July 9, the packing of the skeleton into 35 crates started for transportation to Dalanzadgad; together, the crates weighed about 12 tons. The type specimen belonged to an aged individual. Its taphonomy is unusual as it was found lying on its back – in contrast, most other nearly complete dinosaur skeletons of the Nemegt Formation usually are found lying on their sides. The specimen was found encased in cross-bedded sandstone deposited by a river. Most of the discovered vertebrae were still connected together, forming a continuous series of eight dorsal, six sacral and thirty-four caudal vertebrae. An additional three vertebrae were found isolated from the series and may belong to the transitional area between back and neck. The remaining parts of the skeleton were shifted slightly out of their original anatomical position. Both the left limb and rib bones were found on the right side of the body, while conversely the right limb and rib bones were found on the left side. Bite marks have been identified on the skeleton, particularly in the pelvis and the thigh bone, showing that predators had fed on the carcass. The skull and neck are missing, indicating that the carnivores might have carried away these body parts. The completeness of the remains indicate that the individual had died near the discovery site. A flooding event might have transported the carcass a short distance and subsequently covered it with sediment, even before the soft tissue had decayed entirely.
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