Opisthocoelicaudia

*Opisthocoelicaudia skarzynskii*

***Opisthocoelicaudia*** (/oʊpɪsθjɛkɔɪloʊskɔːdɑː/ meaning "posterior cavity tail") 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 was unearthed in 1965 by Polish and Mongolian scientists, lacking only the head and neck, 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 11 metres (36 ft) 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* 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.

## Discovery and specimens

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.

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*.

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 an juvenile individual.[5] More recently, Philip Currie and colleagues (2003) mentioned a fragmentary tail (MPD 100/406) that also can be attributed to *Opisthocoelicaudia*. This tail comes from the Nemegt locality, were the skull of the related titanosaur *Nemegtosaurus* was also discovered.

## Description

*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.[8] 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.

## Classification

Originally, Opisthocoelicaudia was classified as a member of the family Camarasauridae, together with Camarasaurus and Euhelopus. This classification was based on several shared features of the skeleton, most importantly the forked neural spines of the back vertebrae. In 1977, Borsuk-Białynicka considered Opisthocoelicaudia closer to Euhelopus than to Camarasaurus, placing it in the subfamily Euhelopodinae. A 1981 study by Walter Coombs and Ralph Molnar, on the other hand, considered it a member of the subfamily Camarasaurinae and therefore a close relative of Camarasaurus. Today, both Euhelopus and Opisthocoelicaudia are classified outside the Camarasauridae. In 1993, Leonardo Salgado and Rodolfo Coria showed Opisthocoelicaudia to represent a titanosaur and classified it within the family Titanosauridae. The name Titanosauridae is currently considered invalid by many scientists; instead, the name Lithostrotia is often used as an equivalent.
Within the Lithostrotia, Opisthocoelicaudia has been found to be closely related to the genera Alamosaurus, Neuquensaurus, Rocasaurus and Saltasaurus, together forming the family Saltasauridae. Interrelationships of these genera are contested. Many scientists considered Opisthocoelicaudia to be most closely related to Alamosaurus, with both genera forming a monophyletic group, the Opisthocoelicaudiinae. Other scientists concluded that the Opisthocoelicaudiinae is paraphyletic (not forming a natural group). Contradicting most other studies, Upchurch and colleagues in 2004 argued that Alamosaurus has to be placed outside the Saltasauridae as a close relative of Pellegrinisaurus, and therefore is not related to Opisthocoelicaudia at all.

This cladogram, based on Calvo and colleagues (2007), shows a monophyletic Opisthocoelicaudiinae:

### Relationship to Nemegtosaurus

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 were 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 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 differed from the same bones of Opisthocoelicaudia, and stated they probably belonged to Nemegtosaurus, thereby supporting that 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.

## Paleobiology

### Posture

Originally, Borsuk-Białynicka assumed that in standard position the neck was horizontal or slanted slightly downward. This was based on the reconstruction of the nuchal ligament, which runs atop of the cervical and dorsal vertebrae and serves to support the weight of the head and neck. Although an S-curved, swan-like ascending neck was envisaged in several subsequent reconstructions following similar depictions of better known sauropods, recent studies argue that sauropod necks were relatively straight and were carried more horizontally.
The back was also reconstructed in a more or less horizontal orientation by Borsuk-Białynicka, which was followed by most subsequent depictions. In a 2007 study, Daniela Schwarz and colleagues suggested that the back dipped towards the rear. According to these researchers, the shoulder blade would have been inclined at a horizontal angle of 55–65°, much steeper than previously thought, resulting in an elevated shoulder region. With the vertebral column of the trunk and neck held in a relatively straight line, this would result in an elevated position of the head.

### Rearing stance

Opisthocoelicaudia may have been able to rear up on its hindlimbs for foraging, using its tail as a third leg. In 1977, Borsuk-Białynicka cited several skeletal features that might have been related to rearing, including the opisthocoelous vertebrae of the anterior part of the tail, which, according to this author, would have made the tail more flexible than in other sauropods. Features of the pelvis, such as the thickened shelf of the acetabulum, the flaring ilia, and the fused pubic symphysis, may have allowed the pelvis to withstand the stress of rearing.
Heinrich Mallison in 2011 argued that Opisthocoelicaudia may have been able to angle the anterior part of the tail against the posterior part, producing a buckle in midsection. Thus, the anterior part would have been more straight during rearing than in other sauropods. In 2005, Wilson assumed that rearing was an innovation not only of Opisthocoelicaudia but also of related genera within the subfamily Saltasaurinae. Common features of these genera, such as the shortened tail, may have evolved as adaptations to rearing.

### Footprints

Footprints from the Nemegt Formation were unknown until 2003, when several examples had been described from the Nemegt locality by Currie and colleagues. Most of these footprints belonged to hadrosaurids (probably Saurolophus), while two have been left by a large theropod (probably Tarbosaurus) and yet another two by the hindfoot of a sauropod. The sauropod tracks were assigned to Opisthocoelicaudia, which, according to these authors, showed a matching hind foot morphology and was probably the only known sauropod (and, thus, the only potential trackmaker species) from the Nemegt Formation when Nemegtosaurus is regarded a synonym. The tracks were left in the soft and wet mud of shallow or freshly dried up points along a river and subsequently filled up with sand. Today only the sand infill remains, with the encasing mudstone having been eroded away. The best-preserved footprint measures 63 cm (25 in) across, so it was probably created by an individual larger than the type specimen. Although the surface of the underside is hard to obtain, the vertical surfaces are very well preserved, making this track one of the best preserved sauropod tracks known. Four digital impressions can be distinguished, with two or three showing claw impressions. The toes were almost perpendicular. Even a skin impression has been preserved above the impression of the first toe, which shows the non-overlapping scales, each with an average diameter of 14 mm (0.55 in). The foot of the track creator was probably a little longer than wide. The second track is much shallower than the first, but shows well-preserved digit impressions with a high degree of detail, including at least two deep claw impressions that are rotated outwards, and a well-preserved impression of a fleshy toe pad behind the middle claw.
Although number of additional sauropod tracks were reported in subsequent accounts, they continued to be rare in relation to the much more common hadrosaurid and theropod tracks. Brennan Stettner and colleagues, in 2017, reported on footprints discovered during a 2007 expedition to the Nemegt locality. The best preserved of these, a very large, 76 cm (30 in) long impression of a hindfoot, features a very well preserved underside showing digital pads and four outwards directed digits, the first three of which showing claws. Also in 2017, Judai Nakajima and colleagues described a kidney-shaped impression as the first sauropod manus (forefoot) impression discovered in the formation.

## Paleoecology

The Nemegt Formation is a late Cretaceous aged site about 77-66 million years old and in this location Opisthocoelicaudia lived with the ankylosaurs Saichania and Tarchia. The Alvarezsaurs Mononykus and Nemegtonykus. The dromaeosaur Adasaurus. The hadrosaurs Barsboldia and Saurolophus. The ornithomimids Anserimimus, Gallimimus and Deinocheirus. The Oviraptorids Avimimus, Conchoraptor, Elmisaurus, Gobiraptor, Nemegtomaia, Nomingia, Oksoko and Rinchenia. The pachycephalosaurs Homalocephale and Prenocephale. The fellow sauropod Nemegtosaurus. The therizinosaur Therizinosaurus. The Troodontids Borogovia, Tochisaurus and Zanabazar. And the tyrannosaurs Alioramus, Bagaraatan, Raptorex and Tarbosaurus.

## JPInstitute.com Description

Scientists have unearthed most of this big sauropod's skeleton, but they have not yet found its head and neck. This makes it very difficult to determine exactly how long it was or if it is even a new specimen. Opisthocoelicaudia got its name from the way its tailbones are shaped. It may have been possible for this dinosaur to have easily stood on its back legs and rest on its tail so it could reach high into the trees for its food.

Because the neck and head have not been found, there is debate as to which family that this creature belongs. There are definite similarities with Camarasaurus, particularly with it short tail, but it has also been suggested it was more likely a member of the titanosaur family. If it was a camarasaur, it is strong evidence that sauropods other than titanosuars lived in the late Cretaceous.

## Links

http://web.archive.org/web/20040214155047fw\_/http://www.jpinstitute.com/dinopedia/dinocards/dc\_opist.html#

## References

1. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `borsuk-bialynicka_1977`
2. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `schwarz_07`