Name: Opisthocoelicaudia ‭(‬Posterior cavity tail‭)‬.
Phonetic: Oh-pis-tho-see-lih-caw-dee-ah.
Named By: Borsuk-Białynicka‭ ‬-‭ ‬1977.‭
Classification: Chordata,‭ ‬Reptilia,‭ ‬Dinosauria,‭ ‬Saurischia,‭ ‬Sauropodomorpha,‭ ‬Sauropoda,‭ ‬Titanosauria,‭ ‬Antarctosauridae.
Species: O.‭ ‬skarzynskii‭ (‬type‭)‬.
Diet: Herbivore.
Size: Estimated at‭ 11 to 13‭ ‬meters long.
Known locations: Mongolia‭ ‬-‭ ‬Nemegt Formation.
Time period: Campanian to Maastrichtian of the Cretaceous.
Fossil representation: Almost complete post cranial skeleton.‭ ‬Skull and cervical vertebrae however are still unknown.

       Opisthocoelicaudia is a bit of a mouthful to say but is based upon the structure of the caudal vertebrae.‭ ‬The vertebrae connect in a ball and socket fashion,‭ ‬and while the socket part is usually found to point forwards,‭ ‬it‭’‬s actually reversed to point backwards in Opisthocoelicaudia.‭ ‬Since this initial discovery however the same arrangement has been found in other titanosaurids.‭
       When first named Opisthocoelicaudia was thought to be similar to Camarasaurus,‭ ‬but later study has since found it to be closer to Saltasaurus.‭ ‬The post cranial remains for Opisthocoelicaudia are actually surprisingly complete despite the lack of neck and head.‭ ‬Aside for the back to front orientation of the vertebrae,‭ ‬the caudal‭ (‬tail‭) ‬vertebrae also have well developed attachments for strong muscles and ligaments.‭ ‬Restoration resulted in an upwards curving tail that would have been very rigid,‭ ‬and is thought to have possibly been used as a support for when Opisthocoelicaudia reared up on its hand legs to feed on high growing vegetation.‭ ‬This would have allowed Opisthocoelicaudia to adopt a more stable tripod posture that would have enabled it to feed more comfortably.
       The absence of the head and neck as well as tooth marks upon the pelvis and a femur have led to the suggestion that the remains were fed upon by tyrannosaurs,‭ ‬possibly Tarbosaurus which is known from the same area and time.‭ ‬Unfortunately it is not possible to say if Opisthocoelicaudia had been killed or just scavenged by these predators‭ (‬both answers may be applicable‭)‬.
       Another titanosaurid named Nemegtosaurus is known from the same Formation as Opisthocoelicaudia,‭ ‬but is only known from a partial skull which has led to speculation that it may be the missing head to Opisthocoelicaudia.‭ ‬If ever proved to be correct then Opisthocoelicaudia would become a junior synonym to Nemegtosaurus.

Further reading
- A new camarasaurid sauropod Opisthocoelicaudia skarzynskii gen. n., sp. n. from the Upper Cretaceous of Mongolia. - Palaeontologia Polonica 37:5-64. - M. Borsuk-Bialynicka - 1977.
- Considerations on the phylogenetic relationships of Opisthocoelcaudia skarzynskii (Sauropoda) from the Upper Cretaceous of Mongolia. translated by: Jeffrey A. Wilson - University of Chicago Department of Organismal Biology & Anatomy June, 1997. - L. SAlgado & R. A. Coria - 1993.
- Rediscovery of the type localities of the Late Cretaceous Mongolian sauropods Nemegtosaurus mongoliensis and Opisthocoelicaudia skarzynskii: Stratigraphic and taxonomic implications. - Palaeogeography, Palaeoclimatology, Palaeoecology. 494: 5–13. - Philip J. Currie, Jeffrey A. Wilson, Federico Fanti, Buuvei Mainbayar & Khishigjav Tsogtbaatar - 2018.


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