Temporal range: Late Cretaceous, 70Ma
|Skeleton restoration, in MEPAN, Poland. The head and neck are not preserved.|
Opisthocoelicaudia (from Greek οπισθή, opisthe [back], κοιλος, koilos [hollow], and Latin cauda [tail], meaning posterior cavity tail) was 12-metre-long (39 feet) sauropod dinosaur of the Late Cretaceous Period discovered in Mongolia in 1965 by Polish and Mongolian scientists in what is now the Gobi Desert. In the Cretaceous, Mongolia had instead lush jungles and murky marshes, which Opisthocoelicaudia might have waded in. The genus was described and named in 1977 by Borsuk-Białynicka as a new type of camarasaur, but studies such as Salgado and Coria (1993) find it to be a saltasaurid titanosaur instead. Its skull and neck were missing and its carcass had apparently been buried before disintegrating. Borsuk-Białynicka suggested tyrannosaurid scavengers had fed on the carcass, eating the head and neck and leaving noticeable tooth marks on the pelvis and femur. Scientists cannot be sure what its head or neck looked like, because neither was ever recovered.
Opisthocoelicaudia was relatively small for a sauropod. The nearly complete skeleton represented an individual that measured over 11 m from the head to the tip of the tail. The body mass was estimated at 8.4, 10.5 and 13 tons in separate studies.
Skull and neck are not preserved, but the reconstruction of the nuchal ligament indicates a neck of medium length that was carried horizontally or slightly downward slanted. As in other titanosaurs, the back was quite flexible due to the lack of accessoary vertebral joints (hyposphene-hypantrum articulations), while the pelvic region was strengthened by an additional sixth hip vertebra. In neutral position, the vertebral column of the trunk was held straight. Traditionally, Opisthocoelicaudia was reconstructed with an horizontally orientated back. However, Daniela Schwarz and colleagues (2007) showed that the back actually dipped towards behind: The shoulder blade was inclined at an angle of 55–60° to the horizontal, much steeper than previously thought, resulting in an elevated shoulder region. In the back vertebrae, the spinous processes were bifurcated, resulting in a double row of bony projections along the top of the spine. This feature is unique to titanosaurs but can be found in several other unrelated sauropods, including Diplodocus and Euhelopus. Another unique feature can be found in the anterior portion of the tail: These vertebrae were opisthocoelous, that 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.
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 2/3 the length of the pubis. Both the ischium and pubis bones were ossified over most of their length, so that the gap normally present between these bones (the thyroid fenestra) is closed, distinguishing Opisthocoelicaudia from other titanosaurs. The limbs were proportionally short, as seen in other titanosaurs. The forelimbs measured 1.87 m in height in the nearly complete specimen, approximately two thirds the length of the hindlimbs, which were reconstructed at 2.64 m height. As in other titanosaurs, the limbs were slightly spreaded outwards rather than standing vertically under the body.
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), is 2-2-2-1-0. The foot skeleton 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 specimens
The type specimen (specimen number ZPAL MgD-I/48) is a nearly complete skeleton lacking only the neck vertebrae and skull. 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. The Opisthocoelicaudia specimen belonged to an old individuum. Most of the discovered vertebrae was still connected together forming a continuous series that consisted of 8 back, 6 hip and 34 tail vertebrae. 3 additional 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.
Besides the type specimen, Borsuk-Białynicka described a shoulder blade and coracoid (ZPAL MgD-I/25c) from the same locality. These bones were not jet fused to each other, indicating an juvenile individual. 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 also the skull of the related titanosaur Nemegtosaurus was discovered.
The type specimen was discovered by geologist Riszard Gradzinski in 1965, between 10 and 23 June, during a joined polnish-mongolian expedition. 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 9 July, packing of the skeleton into 35 crates started in order for transportation to Dalanzadgad, many of the crates weighed over a ton. Today, the skeleton is part of the collection of the Institute of Geology of the Mongolian Academy of Sciences in Ulaanbaatar.
Opisthocoelicaudia skarzynskii was named and described in 1977 by polnish paleontologist Maria Magdalena Borsuk-Białynicka. The genus name (from Greek οπισθή, opisthe [back], κοιλος, koilos [hollow], and Latin cauda [tail]) alludes to the unusual opisthocoel condition of the tail vertebrae, while the species name skarzynskii honors Wojciech Skarżyński, the preparator of the skeleton.
The type specimen presents bite marks, particularly in the pelvis and the thigh bone, showing that carnivores have fed on the carcass. Head and neck are missing, indicating that the carnivores may have carried away these body parts. The specimen was found lying on its back. Notably, 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. The carcass may was transported by a flooding event for a short time and subsequently was covered by sediment, even before the soft tissue has decayed entirely.
Originally, Opisthocoelicaudia has been 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. Borsuk-Białynicka (1977) considered Opisthocoelicaudia closer to Euhelopus than to Camarasaurus, placing it in the subfamily Euhelopodinae. Coombs and Molnar (1981), 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. While the name Titanosauridae is currently considered invalid by many scientists, some use the name Lithostrotia to describe the same group.
Within the Titanosauria and Titanosauridae/Lithostrotia, most phylogenetic analyses show Opisthocoelicaudia to be a close relative of the North American Alamosaurus. Both genera are probably closely related to Saltasaurus and Neuquensaurus, which may form a group called Saltasaurinae. Some scientists consider Alamosaurus and Opisthocoelicaudia to form a monophyletic group, the Opisthocoelicaudiinae, which would be the sister group of the Saltasaurinae. Other scientists came to the conclusion that the grouping Alamosaurus + Opisthocoelicaudia is paraphyletic, with both genera representing outgroups of the Saltasaurinae. Contradicting most other studies, Upchurch and colleagues (2004) argued that Alamosaurus and Opisthocoelicaudia are not related at all. According to these scientists, Alamosaurus is most closely related to Pellegrinisaurus and has to be placed outside the Saltasauridae, while Opisthocoelicaudia has to be considered a member of Saltasauridae.
The following cladograms illustrate two main hypotheses: The classification of Opisthocoelicaudia within a monophyletic Opisthocoelicaudiinae (left, Wilson 2002); and a paraphyletic Opisthocoelicaudiinae (right, Calvo and Gonzáles-Riga 2003).
Synonym of Nemegtosaurus?
Another sauropod of the Nemegt-Formation, Nemegtosaurus, is known only from a skull. In Opisthocoelicaudia, on the other hand, both the skull and neck are missing, precluding a direct comparison and leading to suspicions that both genera may represent one and the same species (synonymy). 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.
Borsuk-Białynicka (1977) 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. Furthermore she argued that different sauropod genera sharing the same habitat is nothing unusual; this is most obvious in the North American Morrison Formation, which contains more than five contemporary sauropod species.
By now, both Opisthocoelicaudia and Nemegtosaurus are classified within the Titanosauria, and Jeffrey Wilson (2005) stated that a synonymy can not be ruled out. Currie and colleagues (2003) argued that a synonymy is very probable and reported a fragmentary tail referable to Opisthocoelicaudia that was discovered in the same locality where the Nemegtosaurus-skull was found. However, these scientists also concluded that only further finds will definitively resolve this question.
Opisthocoelicaudia may was able to rear up on its hindlimbs for foraging, using its tail as a third leg. Borsuk-Białynicka (1977) cited several skeletal features that may were 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, may have allowed the pelvis to withstand the forces that appear during rearing. Heinrich Mallison (2011) argued that Opisthocoelicaudia was 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. Wilson (2005) assumed that rearing was an innovation not only of Opisthocoelicaudia but also of related genera blanketed in the subfamily Saltasaurinae. Common features of these genera, such as the shortened tail, may have evolved as adaptations to rearing.
- Maria Magdalena Borsuk-Białynicka (1977): A new camarasaurid sauropod Opisthocoelicaudia skarzynskii gen. n., sp. n. from the Upper Cretaceous of Mongolia. Palaeontologia Polonica 37, pp. 5–64. (PDF)
- Salgado, L. and Coria, R.A. (1993). 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.
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