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 part of a shoulder and a fragmentary tail. A relatively small sauropod, Opisthocoelicaudia measured about 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.
Discovery and specimens
The type specimen was discovered between June 10 and 23, 1965, during a joint Polish-Mongolian paleontological expedition led by Polish paleontologist Zofia Kielan-Jaworowska. The largest of a series of expeditions carried out in 1963–1971, this expedition involved 21 members, which at times were supported by additional hired Mongolian workers. The site of discovery is located in Ömnögovi Province in southern Mongolia in the Altan Uul area, which exposes some 100 km2 of badlands. The sediments exposed at Altan Uul belong to the Nemegt Formation, the youngest of the three geological formations of the Nemegt Basin. Opisthocoelicaudia was the first of several important dinosaur discoveries made by the 1965 expedition. The other finds, made at different localities, include several skeletons of the tyrannosaurid Tarbosaurus as well as the type specimens of the giant ornithomimosaur Deinocheirus, the sauropod Nemegtosaurus, and the pachycephalosaur Homalocephale.
On the fifth day of fieldwork, Ryszard Gradziński, the geologist of the expedition, found a concretion of well-preserved bones which promised to belong to a fairly complete skeleton. Excavation starting the next day revealed a nearly complete skeleton lacking only the head and neck. Until today, this specimen remains by far the most complete finding of this dinosaur. The transport of the specimen out of rough terrain caused major technical problems. Stone blocks containing the fossils had to be moved some 580 meters, dragged on improvised metal sledges made out of petrol drums, before they could be loaded onto trucks. Because the skeleton was embedded in very hard sandstone layers, several blocks weighed over a ton. On July 9, the packing of the skeleton into 35 crates started for transportation to Dalanzadgad; together, the crates weighed about 12 tons.
The type specimen belonged to an aged individual. Its taphonomy is unusual as it was found lying on its backin contrast, most other nearly complete dinosaur skeletons of the Nemegt Formation usually are found lying on their sides. The specimen was found encased in cross-bedded sandstone deposited by a river. Most of the discovered vertebrae were still connected together, forming a continuous series of eight dorsal, six sacral and thirty-four caudal vertebrae. An additional three vertebrae were found isolated from the series and may belong to the transitional area between back and neck. The remaining parts of the skeleton were shifted slightly out of their original anatomical position. Both the left limb and rib bones were found on the right side of the body, while conversely the right limb and rib bones were found on the left side. Bite marks have been identified on the skeleton, particularly in the pelvis and the thigh bone, showing that predators had fed on the carcass. The skull and neck are missing, indicating that the carnivores might have carried away these body parts. The completeness of the remains indicate that the individual had died near the discovery site. A flooding event might have transported the carcass a short distance and subsequently covered it with sediment, even before the soft tissue had decayed entirely.
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 , [behind, at the back], , [hollow], and Latin [tail].Hentschel, E., & Wagner, G. (1990). Zoologisches Wörterbuch. Tiernamen, allgemeinbiologische, anatomische, physiologische Termini und biographische Daten (4th edition). Stuttgart: Gustav Fischer. The specific name honors Wojciech Skarżyński, the person who prepared the type specimen. Opisthocoelicaudia was only the third sauropod from Asia known from a postcranial skeleton, after Euhelopus and Mamenchisaurus. After its discovery, the holotype skeleton became part of the collection of the Institute of Paleobiology in Warsaw, but later was handed back to its country of origin, now being housed at the Institute of Geology of the Mongolian Academy of Sciences in Ulaanbaatar under the catalog number MPC-D100/404. Besides the type specimen, Borsuk-Białynicka described a shoulder blade and coracoid (ZPAL MgD-I/25c) from the same locality. These bones were not yet fused to each other, indicating a juvenile individual.
By 2017, sauropod fossils had been recovered from a total of 32 localities within the Nemegt Formation, and possibly belong to either Opisthocoelicaudia or Nemegtosaurus. At least two finds from the Nemegt localitya fragmentary tail (MPD 100/406) and a pair of clawsshow features diagnostic for Opisthocoelicaudia and can be referred to the latter. Field crews led by Philip Currie attempted to relocate the Opisthocoelicaudia holotype quarry in 2006 and 2008, but became successful only in 2009 thanks to additional data provided by Gradziński. Although a prospection for additional bone material was not possible as the quarry had been filled by windblown sand, the quarry could be determined to fall within the lower portion of the Nemegt Formation.
Description
Like other sauropods, Opisthocoelicaudia 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 to 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 . Borsuk-Białynicka, in her 1977 description, noted the presence of eleven dorsal vertebrae. Gregory Paul in 2019, however, argued that the known part of the vertebral column actually includes the first cervical (neck vertebra), leaving only ten dorsals, typical of titanosaurs. 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 absentin 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, Opisthocoelicaudia was the most derived while showing the fewest phalanges, indicating a progressive reduction in the phalangeal count during titanosaur evolution. The claw of the first digit was not larger than that of the second digit, as in other sauropods, but roughly equal in size.
Osteoderms (bony plates formed in the skin) have been found with at least 10 titanosaur genera. The lack of osteoderms in the nearly complete Opisthocoelicaudia skeleton might indicate they are absent in this genus. However, the closely related Alamosaurus was found to have osteoderms nearly a century after its discovery, in addition to several other closely related titanosaurs, thus raising the possibility that Opisthocoelicaudia had them as well.
Classification