Named and described by Polish paleontologist Maria Magdalena Borsuk-Białynicka in 1977, Opisthocoelicaudia first was thought to be a new member of the Camarasauridae, but now is considered a derived member of the Titanosauria. Its exact relationships within Titanosauria are contentious, but it may had 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 suggested that Nemegtosaurus and Opisthocoelicaudia may represent one and the same species. Sauropod footprints from the Nemegt Formation, which include skin impressions, can probably be referred to either Nemegtosaurus or Opisthocoelicaudia, because there are no other sauropods from the formation.
Opisthocoelicaudia was relatively small for a sauropod. The nearly complete reconstructed skeleton represented an individual that measured over 11 m (36 ft) from the head to the tip of the tail. The body mass was estimated at 8.4 tonnes (8.3 long tons; 9.3 short tons), 10.522 tonnes (10.356 long tons; 11.599 short tons), 22 tonnes (22 long tons), and 13 tonnes (13 long tons; 14 short tons) in separate studies.
Skull and neck are not preserved, but the reconstruction of the nuchal ligament indicates the possession of a neck of medium length of roughly five meters. 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 vertebra 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 usually were 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.
The hips were composed of three bones each, namely the ilium, ischium, and pubis bones. As in many other titanosaurs, the ischium was relatively short, measuring only 2/3 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 is normally present between these bones (the thyroid fenestra), distinguishing Opisthocoelicaudia from other titanosaurs. The limbs were proportionally short, as seen in other titanosaurs. The forelimbs measured 1.87 metres (6.1 ft) in height in the nearly complete specimen, approximately two thirds the length of the hindlimbs, which were reconstructed at 2.64 metres (8.7 ft) height. As in other titanosaurs, the limbs were slightly spreaded outwards rather than standing vertically under the body, while the forelimbs were more flexible and mobile compared to other sauropods.
The manus (hand) 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 also were 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 probably was 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 digit bones (phalanges) beginning with the innermost digit, is 2-2-2-1-0. The foot anatomy is completely preserved in Opisthocoelicaudia – to date, only two additional complete titanosaur foot skeletons are known, which show an aberrant phalangeal formula.
In 10 of the over 40 known titanosaur genera osteoderms were found, bony plates that covered the animals bodies. The lack of osteoderms in the nearly complete Opisthocoelicaudia skeleton indicates that this genus indeed lacked osteoderms. Within the Titanosauria, osteoderms probably have evolved independently several times.
Discovery and speciesEdit
The type specimen was discovered by geologist Ryszard Gradzinski in 1965, between the 10th and 23 June, during a joined Polish-Mongolian expedition. This find, belonging to an old individual, was lacking only the head and neck and is by far the most complete find of this dinosaur. The transport of the specimen out of very rough terrain caused major technical problems: Because the skeleton lay embedded in a very hard sandstone, large blocks of stone and bones had to be budged on sledges some 580 m to the next place that was accessible for trucks. Together, these blocks weighed about 12 tons. On the 9th of July, the packing of the skeleton into 35 crates started in order for transportation to Dalanzadgad, once packed, many of the crates weighed over a ton. The site of discovery, Altan Ula IV, is located in Ömnögovi Province in southern Mongolia and belongs to the Nemegt Formation, the youngest of the three geological formations of the Nemegt Basin. Altan Ula IV is famous for its abundant vertebrate fossils, other important dinosaur finds from this locality include the troodontid Borogovia and the ankylosaur Tarchia.
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. The genus name, hinting at the unusual opisthocoel condition of the tail vertebrae, means "posterior cavity tail". It is derived from the Greek οπισθή, opisthe [back], κοιλος, koilos [hollow], and Latin cauda [tail]. The specific name honors Mr. Wojceich Skarzynski, the person who prepared the specimen ZPAL MgD-Ij48, the holotype. Opisthocoelicaudia was only the third sauropod from Asia known from a postcranial skeleton, after Euhelopus and Mamenchisaurus. Today, the skeleton is part of the collection of the Institute of Geology of the Mongolian Academy of Sciences in Ulaanbaatar.