Opisthocoelicaudia /ɒˌpɪsθoʊsɪlɪˈkɔːdiə/ is a genus of sauropod dinosaur of the Late Cretaceous Period discovered in the Gobi Desert of Mongolia.

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.

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 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 site of discovery is located in Ömnögovi Province in southern Mongolia in the Altan Uul area, which exposes some 100 km2 of badlands.

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.

[2] 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.

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.

[4] 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.

[3] In 1977, Polish paleontologist Maria Magdalena Borsuk-Białynicka published her comprehensive description of the skeleton and named Opisthocoelicaudia skarzynskii as a new genus and species.

[3] By 2017, sauropod fossils had been recovered from a total of 32 localities within the Nemegt Formation, and possibly belong to either Opisthocoelicaudia or Nemegtosaurus.

At least two finds from the Nemegt locality – a fragmentary tail (MPD 100/406) and a pair of claws – show features diagnostic for Opisthocoelicaudia and can be referred to the latter.

[8][7] Field crews led by Philip Currie attempted to relocate the Opisthocoelicaudia holotype quarry in 2006 and 2008, but became successful only in 2009 thanks to additional data provided by Gradziński.

Gregory Paul in 2019, however, argued that the known part of the vertebral column actually includes the first cervical (neck vertebra), leaving only ten dorsals, typical of titanosaurs.

[16] As in other titanosaurs, the back was quite flexible due to the lack of accessory vertebral joints (hyposphene-hypantrum articulations),[17] while the pelvic region was strengthened by an additional sixth hip vertebra.

[19] Another unique feature can be found in the back vertebrae, which show bifurcated spinous processes, resulting in a double row of bony projections along the top of the spine.

[3] As in other titanosaurs, the limbs were slightly spread outwards rather than standing vertically under the body,[23] while the forelimbs were more flexible and mobile compared to other sauropods.

[3] A 1981 study by Walter Coombs and Ralph Molnar, on the other hand, considered it a member of the subfamily Camarasaurinae and therefore a close relative of Camarasaurus.

[18][30] Within the Lithostrotia, Opisthocoelicaudia has been found to be closely related to the genera Alamosaurus, Neuquensaurus, Rocasaurus and Saltasaurus, together forming the family Saltasauridae.

Opisthocoelicaudia, on the other hand, lacks both the skull and neck, precluding a direct comparison and leading to suspicions that it may represent a synonym of Nemegtosaurus.

Most importantly, the discovered caudal centrum is opisthocoelous – a diagnostic feature of Opisthocoelicaudia – suggesting both genera were either closely related or synonymous.

[7] In 2019, Alexander O. Averianov and Alexey V. Lopatin reported Nemegt sauropod vertebrae discovered in 1949 and some femora that differed from the same bones of Opisthocoelicaudia, and stated they probably belonged to Nemegtosaurus, thereby supporting that the two genera were distinct.

[35] In her 1977 description, Borsuk-Białynicka argued that different sauropod genera sharing the same habitat is nothing unusual, as is evident in the North American Morrison Formation.

Definitive proof for the suggested synonymy is, however, still missing, and additional overlapping elements would be required before Opisthocoelicaudia and Nemegtosaurus can be formally declared synonyms.

This was based on the reconstruction of the nuchal ligament, which runs atop of the cervical and dorsal vertebrae and serves to support the weight of the head and neck.

[36] With the vertebral column of the trunk and neck held in a relatively straight line, this would result in an elevated position of the head.

The tracks were left in the soft and wet mud of shallow or freshly dried up points along a river and subsequently filled up with sand.

[41] The fauna of the Nemegt Formation includes aquatic or amphibious animals such as fish, turtles, and crocodiles as well as birds and the abundant medium to large sized dinosaurs, while smaller terrestrial vertebrates like lizards and mammals are rare.

Ornithischians are represented by the "duck-billed" hadrosaurids (including the very common Saurolophus), the thick-skulled pachycephalosaurs, and the heavily armored ankylosaurs.