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= Opisthocoelicaudia =
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Introduction
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'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 11.4 - 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
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The type specimen was discovered between 10 and 23 June 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 the 9th of July, 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
ὄπισθεν, 'opisthen' [behind, at the back], κοῖλος, 'koilos' [hollow],
and Latin 'cauda' [tail]. 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
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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 11.4 m to 13 m from the head to the tip of the tail. The body mass
has been estimated at 8.4 t, 10.5 t, 22 t, 13 t and 25.4 t 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 5 m. 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 1.87 m in height in the nearly complete specimen,
approximately two thirds the length of the hindlimbs, which were
reconstructed at 2.46 m 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
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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:
{{clade| style=font-size:100%;line-height:80%
|label1=Saltasauridae
|1={{clade
|label1= Opisthocoelicaudiinae
|1=
|label2= Saltasaurinae
|2=}}}}
Relationship to ''Nemegtosaurus''
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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.
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 opisthocoelousa
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.
Posture
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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
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'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
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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 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. 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 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
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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'.
External links
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