page_title: Opisthocoelicaudia
section_title: Relationship to Nemegtosaurus
hierarchical_section_title: Opisthocoelicaudia / Classification / Relationship to Nemegtosaurus
caption_attribution_description: Polski: Rekonstrukcja czaszki Nemegtosaurus na szkielecie Opisthocoelicaudia. Muzeum Ewolucji w Warszawie English: Cast of the skull of Nemegtosaurus, on a mounted Opisthocoelicaudia skeleton, Museum of Evolution of Polish Academy of Sciences, Warsaw Italiano: Calco del teschio di un nemegtosauro, montato su uno scheletro di Opisthocoelicaudia (Museo dell'Evoluzione, Varsavia).
context_page_description: 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 a part of a shoulder and a fragmentary tail. A relatively small sauropod, Opisthocoelicaudia measured about 11.4–13 metres 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.
context_section_description: Another sauropod of the Nemegt-Formation, Nemegtosaurus, is known only from a skull. 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. According to the International Code of Zoological Nomenclature (ICZN), the oldest name has priority over younger synonyms – if Opisthocoelicaudia would be shown to be a synonym of Nemegtosaurus, the name Nemegtosaurus would remain valid while Opisthocoelicaudia would become invalid.
  Both Opisthocoelicaudia and Nemegtosaurus where discovered during the 1965 joint Polish-Mongolian expedition. Before the remains were prepared and described, the expedition crew believed both finds to belong to the same species of sauropod. In 1977, Borsuk-Białynicka deemed Opisthocoelicaudia and Nemegtosaurus to represent separate genera because Nemegtosaurus was at this time considered to be a member of the Dicraeosauridae, while Opisthocoelicaudia seemed to be a representative of a different group, the Camarasauridae.
  Currently, both Opisthocoelicaudia and Nemegtosaurus are classified within the Titanosauria, and Jeffrey Wilson stated in 2005 that synonymy cannot be ruled out. Currie and colleagues, in 2003 and 2017, argued that a synonymy is very probable in the light of new fossil discoveries in the Nemegt Formation. After relocating the original Nemegtosaurus quarry, these researchers excavated postcranial bones of the Nemegtosaurus holotype including the centrum of a caudal vertebra and hind limb bones, which allowed, for the first time, a direct comparison between the Nemegtosaurus and Opisthocoelicaudia type specimens based on overlapping elements. These postcranial elements were found to be very similar to the corresponding parts of the Opisthocoelicaudia holotype. Most importantly, the discovered caudal centrum is opisthocoelous – a diagnostic feature of Opisthocoelicaudia – suggesting that both genera were either closely related or synonymous. Furthermore, these authors noted that none of the 32 known sauropod localities of the Nemegt Formation revealed evidence for the presence of more than one species of sauropod. In 2019, Alexander O. Averianov and Alexey V. Lopatin reported Nemegt sauropod vertebrae discovered in 1949 and some femora that diferred from the same bones of Opisthocoelicaudia, and stated they probably belonged to Nemegtosaurus, thereby supporting thatn the two genera were distinct.
  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. Currie and colleagues, however, stressed in 2018 that the dinosaur fauna of the Nemegt Formation was fundamentally different, as larger dinosaurs were present with only few species per clade, indicating a harsh and geographically restricted habitat. 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.
caption: Cast of the skull of Nemegtosaurus, a possible senior synonym, mounted on the Opisthocoelicaudia skeletal restoration in Warsaw
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