Temporal range: Late Cretaceous, 70 Ma
|Skeleton restoration in the Museum of Evolution of Polish Academy of Sciences, Warsaw|
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 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 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 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 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.
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 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.
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 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 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 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 spread 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 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 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.
Osteoderms have been found with 10 of the over 40 known titanosaur genera, bony plates covering the bodies of these animals. The lack of osteoderms in the nearly complete Opisthocoelicaudia skeleton indicates that this genus indeed lacked osteoderms. Within the Titanosauria, osteoderms have probably evolved independently several times.
Discovery and specimens
The type specimen (ZPAL MgD-Ij48) was discovered by geologist Ryszard Gradzinski in 1965, between the 10th and 23 June, during a joint Polish-Mongolian expedition. 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. 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 very hard sandstone layers, large blocks of stone and bones had to be moved 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.
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 Wojceich Skarzynski, the person who prepared the holotype specimen. 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 a juvenile individual. More recently, Philip Currie and colleagues (2003) mentioned a fragmentary tail (MPD 100/406) that can be attributed to Opisthocoelicaudia. This tail comes from the Nemegt locality within the formation, were the skull of the related titanosaur Nemegtosaurus was also discovered.
Age and paleoenvironment
The Nemegt Formation has been deposited within the Late Cretaceous, although its exact age is unknown as it has never been dated radiometrically. According to different authors, the formation is late Campanian to early Maastrichtian, early Maastrichtian, or middle Maastrichtian in age. The sediments of the Nemegt Formation were deposited in a plain crossed by rivers. The climate was warm and subhumid with seasonal droughts, and the soils were relatively dry. Thus, the Nemegt Formation was somewhat more humid than the underlying (and thus older) Barun Goyot and Djadochta formations, which show a semiarid climate.
The fauna of the Nemegt Formation includes aquatic or amphibious animals such as fish, turtles, and crocodiles as well as birds and the abundant medium to large sized dinosaurs, while smaller terrestrial vertebrates like lizards and mammals are rare. Theropod dinosaurs are very diverse in the Nemegt and include the abundant tyrannosaur Tarbosaurus, which might have preyed upon Opisthocoelicaudia. The only other known sauropod is Nemegtosaurus, which is known from a single skull. Ornithischians are represented by the "duck-billed" hadrosaurids (including the very common Saurolophus), the thick-skulled pachycephalosaurs, and the heavely armored ankylosaurs. Neoceratopsians are absent, despite being present in the older Barun Goyot and Djadochta formations. Other important dinosaur finds from the same locality as Opisthocoelicaudia include the troodontid Borogovia and the ankylosaur Tarchia.
The type specimen was found encased in cross-bedded sandstone deposited by a river. The specimen has been found lying on its back, in contrast to most other nearly complete dinosaur skeletons of the Nemegt Formation, which usually are found lying on their sides. Most of the discovered vertebrae were still connected together, forming a continuous series that consisted of 8 dorsal, 6 sacral and 34 caudal 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. 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.
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. 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 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 being placed outside of 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 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.
Currently, both Opisthocoelicaudia and Nemegtosaurus are classified within the Titanosauria, and Jeffrey Wilson (2005) stated that synonymy cannot 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.
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. Daniela Schwarz and colleagues (2007) 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.
Opisthocoelicaudia may have been able to rear up on its hindlimbs for foraging, using its tail as a third leg. Borsuk-Białynicka (1977) 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 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 within the subfamily Saltasaurinae. Common features of these genera, such as the shortened tail, may have evolved as adaptations to rearing.
Footprints were unknown from the Nemegt Formation until 2003, when Currie and colleagues described several examples from the Nemegt locality. 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, based on the similar morphology to the hind feet of the type specimen. According to Currie and colleagues, Opisthocoelicaudia was probably the only sauropod present in the Nemegt Formation, with Nemegtosaurus constituting a probable synonym, making it unlikely that the tracks were left by another, similar titanosaur. 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 centimetres (25 in) across, so it was probably created by an adult, which would have been larger than the type specimen. Although the lower (ventral) surface 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 (digitigrade). 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 millimetres (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.
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