Opisthocoelicaudia
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''Opisthocoelicaudia'' is a
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of
sauropod Sauropoda (), whose members are known as sauropods (; from '' sauro-'' + '' -pod'', 'lizard-footed'), is a clade of saurischian ('lizard-hipped') dinosaurs. Sauropods had very long necks, long tails, small heads (relative to the rest of their bo ...
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
of the
Late Cretaceous Period The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...
discovered in the
Gobi Desert The Gobi Desert (Chinese: 戈壁 (沙漠), Mongolian: Говь (ᠭᠣᠪᠢ)) () is a large desert or brushland region in East Asia, and is the sixth largest desert in the world. Geography The Gobi measures from southwest to northeast an ...
of
Mongolia Mongolia; Mongolian script: , , ; lit. "Mongol Nation" or "State of Mongolia" () is a landlocked country in East Asia, bordered by Russia to the north and China to the south. It covers an area of , with a population of just 3.3 million, ...
. The type
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
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 Carcass or Carcase (both pronounced ) may refer to: *Dressed carcass, the body of a livestock animal ready for butchery, after removal of skin, visceral organs, head, feet etc. *Carrion, the decaying dead body of an animal or human being *The str ...
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 The human shoulder is made up of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone) as well as associated muscles, ligaments and tendons. The articulations between the bones of the shoulder mak ...
and a fragmentary tail. A relatively small sauropod, ''Opisthocoelicaudia'' measured about 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 Concave or concavity may refer to: Science and technology * Concave lens * Concave mirror Mathematics * Concave function, the negative of a convex function * Concave polygon, a polygon which is not convex * Concave set * The concavity In ca ...
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 Camarasauridae is a family of sauropod dinosaurs. Among sauropods, camarasaurids are small to medium-sized, with relatively short necks. They are visually identifiable by a short skull with large nares, and broad, spatulate teeth filling a thick ...
, but is currently considered a
derived Derive may refer to: * Derive (computer algebra system), a commercial system made by Texas Instruments * ''Dérive'' (magazine), an Austrian science magazine on urbanism *Dérive, a psychogeographical concept See also * *Derivation (disambiguatio ...
member of the
Titanosaur Titanosaurs (or titanosaurians; members of the group Titanosauria) were a diverse group of sauropod dinosaurs, including genera from all seven continents. The titanosaurs were the last surviving group of long-necked sauropods, with taxa still th ...
ia. Its exact relationships within Titanosauria are contentious, but it may have been close to the North American ''
Alamosaurus ''Alamosaurus'' (; meaning "Ojo Alamo lizard") is a genus of opisthocoelicaudiine titanosaurian sauropod dinosaurs, containing a single known species, ''Alamosaurus sanjuanensis'', from the late Cretaceous Period of what is now southern North Am ...
''. All ''Opisthocoelicaudia'' fossils stem from the
Nemegt Formation The Nemegt Formation (also known as Nemegtskaya Svita) is a geological formation in the Gobi Desert of Mongolia, dating to the Late Cretaceous. The formation consists of river channel sediments and contains fossils of fish, turtles, crocodilian ...
. Despite being rich in dinosaur fossils, the only other sauropod from this
rock unit A stratigraphic unit is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic, lithologic or paleontologic features (facies) that characterize it ...
is ''
Nemegtosaurus ''Nemegtosaurus'' (meaning 'Reptile from the Nemegt') was a sauropod dinosaur from the Late Cretaceous Period of what is now Mongolia. It was named after the Nemegt Basin in the Gobi Desert, where the remains — a single skull — were found. Th ...
'', 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

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 Zofia Kielan-Jaworowska (25 April 1925 – 13 March 2015) was a Polish paleobiologist. In the mid-1960s, she led a series of Polish-Mongolian paleontological expeditions to the Gobi Desert. She was the first woman to serve on the executive commit ...
. 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 Ömnögovi ( mn, Өмнөговь ''Ömnögovǐ'', ''South Gobi'') is an aimag (province) of Mongolia, located in the south of the country, in the Gobi Desert. Ömnögovi is Mongolia's largest aimag. The capital is Dalanzadgad. The province is r ...
in southern Mongolia in the '' Altan Uul'' area, which exposes some 100 km2 of
badlands Badlands are a type of dry terrain where softer sedimentary rocks and clay-rich soils have been extensively eroded."Badlands" in ''Chambers's Encyclopædia''. London: George Newnes, 1961, Vol. 2, p. 47. They are characterized by steep slopes, m ...
. The sediments exposed at Altan Uul belong to the
Nemegt Formation The Nemegt Formation (also known as Nemegtskaya Svita) is a geological formation in the Gobi Desert of Mongolia, dating to the Late Cretaceous. The formation consists of river channel sediments and contains fossils of fish, turtles, crocodilian ...
, the youngest of the three
geological formations A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...
of the
Nemegt Basin The Nemegt Basin is a geographical area in the northwestern Gobi Desert, in Ömnögovi Province, southern Mongolia. It is known locally as the "Valley of the Dragons", since it is a source of many fossil finds, including dinosaurs, dinosaur eggs, ...
. ''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 Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family of coelurosaurian theropod dinosaurs that comprises two subfamilies containing up to thirteen genera, including the eponymous ''Tyrannosaurus''. The exact number of genera ...
''
Tarbosaurus ''Tarbosaurus'' ( ; meaning "alarming lizard") is a genus of tyrannosaurid dinosaur that flourished in Asia about 70 million years ago, at the end of the Late Cretaceous Period, considered to contain a single known species, ''Tarbosaurus bataa ...
'' as well as the type specimens of the giant ornithomimosaur ''
Deinocheirus ''Deinocheirus'' ( ) is a genus of large ornithomimosaur that lived during the Late Cretaceous around 70 million years ago. In 1965, a pair of large arms, shoulder girdles, and a few other bones of a new dinosaur were first discovered in the Ne ...
'', the sauropod ''Nemegtosaurus'', and the
pachycephalosaur Pachycephalosauria (; from Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Along with Ceratopsia, it makes up the clade Marginocephalia. With the exception of two species, most pachyc ...
''
Homalocephale ''Homalocephale'' (from Greek ὁμαλός, ''homalos'', "even", and κεφαλή, ''kephalē'', "head") is a genus of pachycephalosaurid dinosaur that lived during the Late Cretaceous period of what is now the Nemegt Formation, Mongolia, about ...
''. On the fifth day of fieldwork, Ryszard Gradziński, the
geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althou ...
of the expedition, found a
concretion A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular ...
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 Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...
layers, several blocks weighed over a ton. On July 9, the packing of the skeleton into 35 crates started for transportation to
Dalanzadgad Dalanzadgad ( mn, Даланзадгад; ) is the capital of Ömnögovi Aimag in Mongolia. It is located south of the national capital Ulaanbaatar. The altitude of the city center is 1,470 meters (4,823 feet). As of 2011, its population is 19,3 ...
; together, the crates weighed about 12 tons. The type specimen belonged to an aged individual. Its
taphonomy Taphonomy is the study of how organisms decay and become fossilized or preserved in the paleontological record. The term ''taphonomy'' (from Greek , 'burial' and , 'law') was introduced to paleontology in 1940 by Soviet scientist Ivan Efremov t ...
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 Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...
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 Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
, ehind, at the back , ollow and
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
ail Ail or AIL may refer to: * Illness, a state of poor health * Ail (''Sailor Moon''), a character in the ''Sailor Moon'' anime series * Acceptance in lieu, an arrangement in the UK for accepting works of art etc. in lieu of tax * Agilus, a Frankis ...
Hentschel, E., & Wagner, G. (1990). ''Zoologisches Wörterbuch. Tiernamen, allgemeinbiologische, anatomische, physiologische Termini und biographische Daten'' (4th edition). Stuttgart: Gustav Fischer. 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 ''Mamenchisaurus'' (or spelling pronunciation ) is a genus of sauropod dinosaur known for their remarkably long necks which made up nearly half the total body length. Numerous species have been assigned to the genus; however, many of these might ...
''. After its discovery, the holotype skeleton became part of the collection of the Institute of Paleobiology in
Warsaw Warsaw ( pl, Warszawa, ), officially the Capital City of Warsaw,, abbreviation: ''m.st. Warszawa'' is the capital and largest city of Poland. The metropolis stands on the River Vistula in east-central Poland, and its population is officia ...
, but later was handed back to its country of origin, now being housed at the Institute of Geology of the
Mongolian Academy of Sciences The Mongolian Academy of Sciences (, ''Mongol ulsyn Shinjlekh ukhaany Akademi'') is Mongolia's first centre of modern sciences. It came into being in 1921 when the government of newly independent Mongolia issued a resolution declaring the establi ...
in
Ulaanbaatar Ulaanbaatar (; mn, Улаанбаатар, , "Red Hero"), previously anglicized as Ulan Bator, is the capital and most populous city of Mongolia. It is the coldest capital city in the world, on average. The municipality is located in north ce ...
under the catalog number MPC-D100/404. Besides the type specimen, Borsuk-Białynicka described a
shoulder blade The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either ...
and
coracoid A coracoid (from Greek κόραξ, ''koraks'', raven) is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is prese ...
(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 In biology, a type is a particular wiktionary:en:specimen, specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally attached. In other words, a type is an example that serves to a ...
was estimated at to from the head to the tip of the tail. The body mass has been estimated at , , , and in separate studies. The skull and neck are not preserved, but the reconstruction of the
nuchal ligament The nuchal ligament is a ligament at the back of the neck that is continuous with the supraspinous ligament. Structure The nuchal ligament extends from the external occipital protuberance on the skull and median nuchal line to the spinous proces ...
indicates the possession of a neck of medium length of roughly . Borsuk-Białynicka, in her 1977 description, noted the presence of eleven dorsal vertebrae.
Gregory Paul Gregory Scott Paul (born December 24, 1954) is an American freelance researcher, author and illustrator who works in paleontology, and more recently has examined sociology and theology. He is best known for his work and research on theropod dino ...
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 articulation The hyposphene-hypantrum articulation is an accessory joint found in the vertebrae of several fossil reptiles of the group Archosauromorpha. It consists of a process on the backside of the vertebrae, the hyposphene, that fits in a depression in the ...
s), 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 joint The ball-and-socket joint (or spheroid joint) is a type of synovial joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. The distal bone is capable of motion around an indefinite number of ...
s. 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 process The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic i ...
es, 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 ''Diplodocus'' (, , or ) was a genus of diplodocid sauropod dinosaurs, whose fossils were first discovered in 1877 by S. W. Williston. The generic name, coined by Othniel Charles Marsh in 1878, is a neo-Latin term derived from Greek διπ ...
'' and ''
Euhelopus ''Euhelopus'' is a genus of sauropod dinosaur that lived between 145 and 133 million years ago during the Berriasian and Valanginian stages of the Early Cretaceous in what is now Shandong Province in China. It was a large quadrupedal herbivore. U ...
'', where it evolved independently. As in the hips of other titanosaurs, the
ischium The ischium () form ...
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 Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by Cell (biology), cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes ...
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 The carpal bones are the eight small bones that make up the wrist (or carpus) that connects the hand to the forearm. The term "carpus" is derived from the Latin carpus and the Greek καρπός (karpós), meaning "wrist". In human anatomy, th ...
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 The talus (; Latin for ankle or ankle bone), talus bone, astragalus (), or ankle bone is one of the group of foot bones known as the tarsus. The tarsus forms the lower part of the ankle joint. It transmits the entire weight of the body from the ...
was strongly reduced as in other titanosaurs, while the
calcaneus In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is the point of the hock. S ...
was probably completely absent in ''Opisthocoelicaudia''. In contrast to the manus, the foot showed well developed digits and claws. The
phalangeal formula The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...
, 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 ''Epachthosaurus'' (meaning "heavy lizard") was a genus of titanosaurian sauropod dinosaur from the Late Cretaceous. It was a basal lithostrotian titanosaur. Its fossils have been found in Central and Northern Patagonia in South America. Discov ...
'' 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 Derive may refer to: * Derive (computer algebra system), a commercial system made by Texas Instruments * ''Dérive'' (magazine), an Austrian science magazine on urbanism *Dérive, a psychogeographical concept See also * *Derivation (disambiguatio ...
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.
Osteoderm Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, temnospondyls (extinct amp ...
s (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 Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
Camarasauridae Camarasauridae is a family of sauropod dinosaurs. Among sauropods, camarasaurids are small to medium-sized, with relatively short necks. They are visually identifiable by a short skull with large nares, and broad, spatulate teeth filling a thick ...
, together with ''
Camarasaurus ''Camarasaurus'' ( ) was a genus of quadrupedal, herbivorous dinosaurs and is the most common North American sauropod fossil. Its fossil remains have been found in the Morrison Formation, dating to the Late Jurassic epoch (Kimmeridgian to Titho ...
'' 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 Ralph E. Molnar is a paleontologist who had been Curator of Mammals at the Queensland Museum and more recently associated with the Museum of Northern Arizona. He is also a research associate at the Texas natural Science Centre. He co-authored descr ...
, on the other hand, considered it a member of the subfamily
Camarasaurinae Camarasauridae is a family of sauropod dinosaurs. Among sauropods, camarasaurids are small to medium-sized, with relatively short necks. They are visually identifiable by a short skull with large nares, and broad, spatulate teeth filling a thick ...
and therefore a close relative of ''Camarasaurus''. Today, both ''Euhelopus'' and ''Opisthocoelicaudia'' are classified outside the Camarasauridae. In 1993, Leonardo Salgado and
Rodolfo Coria Rodolfo Aníbal Coria (born in Neuquén June 1, 1959), is an Argentine paleontologist. He is best known for having directed the field study and co-naming of ''Argentinosaurus'' (possibly the world's largest land animal ever) in 1993, and ''Gigano ...
showed ''Opisthocoelicaudia'' to represent a titanosaur and classified it within the family
Titanosauridae Lithostrotia is a clade of derived titanosaur sauropods that lived during the Early Cretaceous and Late Cretaceous. The group was defined by Upchurch ''et al.'' in 2004 as the most recent common ancestor of ''Malawisaurus'' and ''Saltasaurus'' an ...
. The name Titanosauridae is currently considered
invalid Invalid may refer to: * Patient, a sick person * one who is confined to home or bed because of illness, disability or injury (sometimes considered a politically incorrect term) * .invalid, a top-level Internet domain not intended for real use As t ...
by many scientists; instead, the name
Lithostrotia Lithostrotia is a clade of derived titanosaur sauropods that lived during the Early Cretaceous and Late Cretaceous. The group was defined by Upchurch ''et al.'' in 2004 as the most recent common ancestor of ''Malawisaurus'' and ''Saltasaurus'' an ...
is often used as an equivalent. Within the Lithostrotia, ''Opisthocoelicaudia'' has been found to be closely related to the genera ''Alamosaurus'', ''
Neuquensaurus ''Neuquensaurus'' (meaning "Neuquén lizard") is a genus of saltasaurid sauropod dinosaur that lived in the Late Cretaceous, about 80 million years ago in Argentina and Uruguay in South America. Its fossils were recovered from outcrops of the Ana ...
'', ''
Rocasaurus ''Rocasaurus'' (meaning "General Roca lizard") is a genus of titanosaurian sauropod that lived in South America. ''Rocasaurus'' was discovered in Argentina in 2000, within the Allen Formation which is dated to be middle Campanian to early Maastr ...
'' and ''
Saltasaurus ''Saltasaurus'' (which means "lizard from Salta") is a genus of saltasaurid dinosaur of the Late Cretaceous period of Argentina. Small among sauropods, though still heavy by the standards of modern creatures, ''Saltasaurus'' was characterized by ...
'', together forming the family
Saltasauridae Saltasauridae (named after the Salta region of Argentina where they were first found) is a family of armored herbivorous sauropods from the Upper Cretaceous. They are known from fossils found in South America, Asia, North America, and Europe. Th ...
. Interrelationships of these genera are contested. Many scientists considered ''Opisthocoelicaudia'' to be most closely related to ''Alamosaurus'', with both genera forming a
monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...
group, the
Opisthocoelicaudiinae Opisthocoelicaudiinae is a subfamily of titanosaurian dinosaurs from the Late Cretaceous. It was named by John McIntosh in 1990. Opisthocoelicaudiines are known from Mongolia, Argentina, and the United States. Two genera were assigned to Opisthoc ...
. Other scientists concluded that the Opisthocoelicaudiinae is
paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
(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 ''Pellegrinisaurus'' (meaning ''Lizard from Pelligrini'') is a genus of titanosaurian sauropod dinosaur that lived in South America during the Late Cretaceous period. The holotype was found in the Allen Formation, Argentina. Discovery and namin ...
'', and therefore is not related to ''Opisthocoelicaudia'' at all. This
cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...
, 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 A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are all ...
of ''Nemegtosaurus''. According to the
International Code of Zoological Nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals. It is also informally known as the ICZN Code, for its publisher, the ...
(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 Dicraeosauridae is a family of diplodocoid sauropods who are the sister group to Diplodocidae. Dicraeosaurids are a part of the Flagellicaudata, along with Diplodocidae. Dicraeosauridae includes genera such as ''Amargasaurus'', ''Suuwassea'', ''D ...
, 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 The Morrison Formation is a distinctive sequence of Late Jurassic, Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandsto ...
. 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 The nuchal ligament is a ligament at the back of the neck that is continuous with the supraspinous ligament. Structure The nuchal ligament extends from the external occipital protuberance on the skull and median nuchal line to the spinous proces ...
, 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 acetabulum (), also called the cotyloid cavity, is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint. Structure There are three bones of the ''os coxae'' (hip bone) that c ...
, the flaring ilia, and the fused
pubic symphysis The pubic symphysis is a secondary cartilaginous joint between the left and right superior rami of the pubis of the hip bones. It is in front of and below the urinary bladder. In males, the suspensory ligament of the penis attaches to the pubic ...
, 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

The Nemegt Formation was deposited within the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...
, although its exact age is unknown as it has never been dated radiometrically. According to different authors, the formation is late
Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campanian s ...
to early
Maastrichtian The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the interval from ...
, 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 The Barun Goyot Formation (also known as Baruungoyot Formation or West Goyot Formation) is a geological formation dating to the Late Cretaceous Period. It is located within and is widely represented in the Gobi Desert Basin, in the Ömnögovi Prov ...
and
Djadochta Formation The Djadochta Formation (sometimes transcribed and also known as Djadokhta, Djadokata, or Dzhadokhtskaya) is a highly fossiliferous geological formation situated in Central Asia, Gobi Desert, dating from the Late Cretaceous period, about 75 milli ...
s, which show a
semiarid A semi-arid climate, semi-desert climate, or steppe climate is a dry climate sub-type. It is located on regions that receive precipitation below potential evapotranspiration, but not as low as a desert climate. There are different kinds of semi-ar ...
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 Terrestrial refers to things related to land or the planet Earth. Terrestrial may also refer to: * Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on or near the ground, as opposed to ...
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.
Ornithischia Ornithischia () is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek s ...
ns are represented by the "duck-billed"
hadrosaurids Hadrosaurids (), or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod family, which includ ...
(including the very common ''
Saurolophus ''Saurolophus'' (; meaning "lizard crest") is a genus of large hadrosaurid dinosaur from the Late Cretaceous period of Asia and North America, that lived in what is now the Horseshoe Canyon and Nemegt formations about 70 million to 68 million ...
''), the thick-skulled
pachycephalosaurs Pachycephalosauria (; from Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Along with Ceratopsia, it makes up the clade Marginocephalia. With the exception of two species, most pachycephal ...
, and the heavily armored
ankylosaurs Ankylosauria is a group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms, similar to turtles. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. ...
.
Neoceratopsia Ceratopsia or Ceratopia ( or ; Greek: "horned faces") is a group of herbivorous, beaked dinosaurs that thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. ...
ns 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 Troodontidae is a clade of bird-like theropod dinosaurs. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinosaurian lineages. More recent fossil discov ...
''
Borogovia ''Borogovia'' is a troodontid theropod dinosaur genus which lived during the Late Cretaceous Period, in what is now Mongolia. In 1971, a Polish-Mongolian expedition discovered the remains of a small theropod at the Altan Ula IV site, in the N ...
'' and the
ankylosaur Ankylosauria is a group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms, similar to turtles. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. ...
''
Tarchia ''Tarchia'' (meaning "brainy one") is a genus of herbivorous ankylosauridae, ankylosaurid dinosaur from the late Cretaceous of Mongolia. Discovery and naming In 1970, a Polish-Mongolian expedition discovered an ankylosaurian skull near Khulsan. ...
''.


References


External links


How to Pronounce ''Opisthocoelicaudia''
{{Featured article Saltasaurids Maastrichtian life Late Cretaceous dinosaurs of Asia Cretaceous Mongolia Fossils of Mongolia Nemegt fauna Fossil taxa described in 1977 Controversial dinosaur taxa