Opisthocoelicaudia

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 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. 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 . 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==

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. }} Relationship to Nemegtosaurus , a possible senior synonym, mounted on the Opisthocoelicaudia'' skeletal restoration in Warsaw 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 synonymsif Opisthocoelicaudia would be shown to be a synonym of Nemegtosaurus, the name Nemegtosaurus would remain valid while Opisthocoelicaudia would become invalid. 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 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 . 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, 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==

-aged dinosaur fossil localities of Mongolia. Opisthocoelicaudia was collected in Altan Ula within areaA (left). The Nemegt Formation was 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. Nevertheless, the Nemegt Formation was 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 heavily 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. ==References==