Opisthocoelicaudia

Opisthocoelicaudia Temporal range: Late Cretaceous, 70 Ma PreЄ Є O S D C P T J K Pg N ↓
Skeleton restoration in the Museum of Evolution of Polish Academy of Sciences, Warsaw
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Dinosauria
Order:	Saurischia
Suborder:	†Sauropodomorpha
Clade:	†Neosauropoda
Clade:	†Macronaria
Clade:	†Titanosauria
Family:	†Saltasauridae
Subfamily:	†Opisthocoelicaudiinae
Genus:	†Opisthocoelicaudia Borsuk-Białynicka, 1977
Species:	†O. skarzynskii
Binomial name
Opisthocoelicaudia skarzynskii Borsuk-Białynicka, 1977

Opisthocoelicaudia /ɒˌpɪsθoʊsɪlᵻˈkɔːdiə/ is 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-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 and 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 about 11.4 metres (37 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 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.