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''Opisthocoelicaudia'' was 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 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 approximately 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. ==Description== Like other sauropods, ''Opistocelicaudia'' had a small head on a long neck, a barrel-shaped body on four columnar limbs, and a long tail. It was relatively small for a sauropod; the type specimen was estimated at over from the head to the tip of the tail.〔〔 The body mass has been estimated at ,〔 ,〔 ,〔 and 〔 in separate studies. The skull and neck are not preserved, but the reconstruction of the nuchal ligament indicates the possession of a neck of medium length of roughly .〔 As in other titanosaurs, the back was quite flexible due to the lack of accessory vertebral joints (hyposphene-hypantrum articulations),〔 while the pelvic region was strengthened by an additional sixth hip vertebra.〔 The anterior vertebrae of the tail were opisthocoelous, which means they were convex on their anterior sides and concave on their back sides, forming ball-and-socket joints.〔 These opisthocoelous tail vertebrae lend ''Opisthocoelicaudia'' its name and serve to distinguish the genus from all other titanosaurs.〔 Other titanosaurs were usually characterised by strongly procoelous anterior tail vertebrae, which were concave on their anterior sides and convex on their back sides.〔 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.〔 While unique in titanosaurs, this feature can be found in several other unrelated sauropods, including ''Diplodocus'' and ''Euhelopus'', where it evolved independently.〔 As in the hips of other titanosaurs, the ischium was relatively short, measuring only two-thirds the length of the pubis. The left and right ischium bones as well as the left and right pubis bones were ossified with each other over most of their length, closing the gap that in other sauropods is normally present between these bones.〔 The limbs were proportionally short, as seen in other titanosaurs.〔 The forelimbs measured in height in the nearly complete specimen, approximately two thirds the length of the hindlimbs, which were reconstructed at height.〔 As in other titanosaurs, the limbs were slightly spread outwards rather than standing vertically under the body,〔 while the forelimbs were more flexible and mobile compared to other sauropods.〔 The manus was composed merely of the five metacarpalia, which were orientated vertically and arranged in a semicircle. Carpal bones were missing, as in other titanosaurs.〔〔 Finger bones and claws were also completely absent – in most other titanosaurs, these bones were still present though extremely reduced in size. In the foot, the talus bone was strongly reduced as in other titanosaurs, while the calcaneus was probably completely absent in ''Opisthocoelicaudia''.〔 In contrast to the manus, the foot showed well developed digits and claws. The phalangeal formula, which states the number of phalanges (digit bones) beginning with the innermost digit, is 2-2-2-1-0. Foot skeletons of titanosaurs are rarely found, and besides ''Opisthocoelicaudia'', completely preserved examples are known only from ''Epachthosaurus'' and the unnamed La Invernada titanosaur, whose phalangeal formulas are 2-2-3-2-0 and 2-2-2-2-0, respectively. Of these three titanosaurs, ''Ophistocoelicaudia'' was the most derived while showing the fewest phalanges, indicating a progressive reduction in the phalangeal count during titanosaur evolution.〔 Osteoderms (bony plates formed in the skin) have been found with 10 of the over 40 known titanosaur genera. The lack of osteoderms in the nearly complete ''Opisthocoelicaudia'' skeleton indicates that they are indeed absent in this genus. Within the Titanosauria, osteoderms have probably evolved independently several times.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Opisthocoelicaudia」の詳細全文を読む スポンサード リンク
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