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Elasmosaurus (/ɪˌlæzməˈsɔːrəs, -m-/;[1] from Greek ελασμος elasmos 'thin plate' (referring to thin plates in its pelvic girdle) + σαυρος sauros 'lizard') is a genus of plesiosaur with an extremely long neck that lived in the Late Cretaceous period (Campanian stage), 80.5 million years ago.

Elasmosaurus
Temporal range: Late Cretaceous, 80.5 Ma
Elasomosaurus Face Clean.png
Reconstructed skeleton in the Rocky Mountain Dinosaur Resource Center
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: Sauropterygia
Order: Plesiosauria
Family: Elasmosauridae
Genus: Elasmosaurus
Cope, 1868
Species: E. platyurus
Binomial name
Elasmosaurus platyurus
Cope, 1868

Contents

DescriptionEdit

 
Restoration with size comparison

Elasmosaurus was a long plesiosaur at 10.3 metres (34 ft) in length.[2] Though the only known specimen of Elasmosaurus (specimen ANSP 10081) is fragmentary and missing many elements, related elasmosaurids show it would have had a compact, streamlined body, long, paddle-like limbs, a short tail, a proportionately small head, and an extremely long neck. Elasmosaurus and its relative Albertonectes were some of the longest-necked animals to have ever lived, with the largest amount of neck vertebrae of any known vertebrate animals.[3][4]

Like other elasmosaurids, Elasmosaurus would have had a slender, triangular skull. The snout was rounded and almost formed a semi-circle when viewed from above, and the premaxillae (which form the front of the upper jaw) bore a low keel at the midline. It is uncertain how many teeth Elasmosaurus had, due to the fragmentary fossils. It probably had six teeth in each premaxilla, and the teeth preserved there were formed like large fangs. The number of premaxillary teeth distinguishes Elasmosaurus from primitive plesiosauroids and most other elasmosaurids, which usually have less. The two teeth at the front were smaller than the succeeding ones, and were located between the first two teeth in the dentaries of the lower jaws. The known teeth of the front part of the lower jaw are large fangs, and the teeth at the back of the jaws appear to have been smaller. The dentition of elasmosaurids was generally heterodont (irregular throughout the jaws), with the teeth becoming progressively smaller form front to back. The maxillae (largest tooth bearing bone of the upper jaw) of elasmosaurids usually contained 14 teeth, whereas the dentaries (the main part fo the lower jaws) usually contained 17 to 19. The teeth interlocked, and their tooth crowns were slender and rounded in cross-section. The mandibular symphysis (where the two halves of the lower jaw connected) was well-ossified, with no visible suture.[3][5]

The pectoral and pelvic girdles of the holotype specimen were noted as missing by 1906, but observations about these elements were since been made based on the original descriptions and figures from the late 19th century. The shoulder blades (scapulae) were fused and met at the midline, bearing no trace of a median bar. The upper processes of the shoulder blades were very broad, and the "necks" of the shoulder blades were long. The pectoral girdle had a long bar, a supposedly advanced feature thought to be absent from juvenile plesiosaurs. The ischia, a pair of bones that formed part of the pelvis, were joined at the middle, so that a medial bar was present along the length of the pelvis, a feature usually not found in plesiosaurs.[5] Like other elasmosaurids (and plesiosaurs in general), Elasmosaurus would have had large, paddle-like limbs with very long digits. The paddles at the front (the pectoral paddles) were longer than those at the back (the pelvic paddles).[3]

VertebraeEdit

 
Reconstructed skull and neck, North American Museum of Ancient Life

Elasmosaurus differed from all other plesiosaurs by having 72 neck (or cervical) vertebrae, though more may have been present, but since lost to erosion or after excavation. Only Albertonectes had more neck vertebrae, 75, and the two are the only plesiosaurs with a count higher than 70; more than 60 vertebrae is derived (or "advanced") for plesiosaurs.[4][5] The atlas and axis bone-complex, consisting of the first two neck vertebrae that articulated with the back of the skull, was long, low, and horizontally rectangular in side-view. The centra or "bodies" of these vertebrae were co-ossified in the holotype specimen, which indicates it was an adult. The neural arches of these vertebrae were very thin and rather high, which gives the neural canal (the opening through the middle of the vertebrae) a triangular outline when seen from the back. The lower part of the neural canal was narrow towards the back by the axis, where it was half the breadth of the centrum. It became broader towards the front, where it was almost the same breadth as the centrum of the atlas. The neural arches were also more robust there than in the axis, and the neural canal was hihgher. The neural spine was low and directed upwards upwards and back. The centra of the atlas and axis were of equal length, and had a quadratic shape in side view. The surface (or facet) were the axis articulated with the next vertebra had an oval outline, and an excavation for the neural canal in the middle of its upper edge. A distinct keel ran along the lower middle of the atlas and axis vertebrae.[5]

 
Vertebra from the front part of the neck of the holotype specimen

Most of the neck vertebrae were compressed sideways, especially at the middle of the neck. A crest (also termed ridge or keel) ran longitudinally along the side of the neck vertebrae (a feature typical of elasmosaurids), visible from the third to the fifty-fifth vertebrae, at the hind part of the neck. This crest was positioned at the middle of the centrum in the front vertebrae, and at the upper half of the centrum from the 19th vertebra and onwards. The crest would have served to anchor the musculature of the neck. The centra differed in shape depending on the position of the vertebrae in the neck; that of the third vertebra was about as long as it was broad, but the centra became longer than broad form the fourth vertebra and onwards. The centra became very elongated at the middle of the neck, but became shorter again at the back of the neck, with the length and breadth being about equal at the 61th vertebra, and those of the hindmost vertbrae being broader than long. The articular surfaces of the vertebrae in the front of the neck were broad oval, and moderately deepened, with rounded, thickened edges, with an excavation (or cavity) at the upper and lower sides. Further back in the front part of the neck, around the 25th, the lower edge of the articular facets became more concave, and the facet shaped like a quadrate with rounded edges. By the 63th vertebra, the articular facet was also quadratic in shape with rounded edges, whereas the centra of the hindmost vertebrae had a broad oval outline.[3][4][5]

The neural arches of the neck vertebrae were well-fused to the centra, leaving no visible sutures, and the neural canal was narrow in the front vertebrae, becoming more prominently developed in the hind vertebrae, where it was as broad as high, and almost circular. The pre and post zygapophyses of the neck vertebrae, processes that articulate adjacent vertebrae so they fit together, were of equal length; the former reached entirely over the level of the centrum, whereas the latter reached only with their back half. The neural spines of the neck vertebrae appear to have been low, and almost semi-circular by the 20th vertebra. The facets where the neck ribs articulated with the neck vertebrae were placed on the lower sides of the centra, but were only placed higher in the last three vertebrae, reaching around the middle of the sides. The neck ribs were semicircular to quadratic in side view, and were directed rather straight down. The bottom of each neck vertebrae had pairs of nutritive foramina (openings) at the middle, separated by a ridge, which became progressively more prominent and thickened towards the back of the neck.[5]

 
Back vertebrae from the pectoral region of the holotype specimen

The vertebrae that transitioned between the neck and back (or dorsal) vertebrae in the pectoral region of plesiosaurs, close to the front margin of the forelimb girdle, are often termed pectoral vertebrae. Elasmosaurus had three pectoral vertebrae, which is a common number for elasmosaurids. The rib facets of the pectoral vertebrae were triangular in shape and situated on transverse processes, and the centra bore pairs of nutritive foramina in the middle of the lower sides. The back vertebrae had rib facets level with the neural canal, and the front and back part of the transverse processes here had distinct ridges on their margins. Here the rib facets where placed higher than the transverse processes, separating the two, and were oval to rectangular in outline. The pre-zygapophyses here were shorter than those in the neck and pectoral vertebrae, and only reached above the level of the centrum with the front third of their length. The post-zygapophyses reached over the level of the centrum with the back half of their length. Back vertebrae are not useful for distinguishing between elasmosaurids, since they are not diagnostic at the genus level.[4][5]

Elasmosaurus had four sacral vertebrae (the fused vertebrae that form the sacrum connected to the pelvis), a number typical of elasmosaurids. The transverse processes here were very short, and the rib facets increased in size from the first to the fourth sacral vertebra. A ridge ran along the top of these vertebrae, and the lower sides of the centra were rounded, and bore pairs of nutritive foramina, separated by low ridges. The first tail (or caudal) vertebra could be distinguished by the preceding sacral vertebra by having smaller rib facets, and by being positioned in the lower half of the centrum. These vertebrae were almost circular in shape, and the first two bore a narrow keel in the middle of the upper side. The rib facets of the tail vertebrae were located on the lower side of the centra, and their oval shape became larger and broader from the third vertebra and onwards, but became smaller from the 14th vertebra. Here, the pre zygapophyses also reached over the level of the centra for most of their length, while the post zygapophyses reached over this level by half their length. The lower part of the centra were rounded from the first to the third tail vertebrae, but concave from the fourth to the 18th. The usual number of trail vertebrae in elasmosaurids is 30.[5] Since the last tail-vertebrae of elasmosaurids were fused into a structure similar to the pygostyle of birds, it is possible this supported a tail-fin, but the shape it would have had is unknown.[3]

History of discoveryEdit

 
Officers at Fort Wallace, Kansas, in 1867. Turner, who discovered Elasmosaurus in the area the same year, is second from the left

In the spring of 1867, the American army surgeon Theophilus Hunt Turner and the army scout William Comstock explored the rocks around Fort Wallace, Kansas where they were stationed during the construction of the Union Pacific Railway. Approximately 23 km northeast of Fort Wallace, near McAllaster, Turner discovered the bones of a large fossil reptile in a ravine, and though he had no paleontological experience, he recognized the remains as belonging to an "extinct monster". In June, Turner gave three fossil vertebrae to the American scientist John LeConte, a member of the railway survey, to take back east to be identified. In December, LeConte delivered some of the vertebrae to the American paleontologist Edward Drinker Cope at the Academy of Natural Sciences of Philadelphia (known as Academy of Natural Sciences of Drexel University since 2011). Recognizing them as the remains of a plesiosaur, larger than any he had seen in Europe, Cope wrote to Turner asking him to deliver the rest of the specimen, on the ANSP's expense.[5][6][7]

In December 1867, Turner and others from Fort Wallace returned to the site, and recovered much of the vertebral column, as well as concretions that contained other bones; the material had a combined weight of 800 pounds. The fossils were dug or pried out of the relatively soft shale with picks and shovels, and loaded on a horse drawn wagon and transported back to Fort Wallace. Cope sent instructions on how to pack the bones, which were thereafter sent in hay-padded crates on a military wagon east to the rail road, which had not yet reached the fort. The specimen arrived in Philadelphia by rail in March 1868, where after Cope examined it hurriedly; he reported on it at the March ANSP meeting, during which he named it Elasmosaurus platyurus. The generic name Elasmosaurus means "thin-plate reptile", in reference to the "plate" bones of the sternal and pelvic regions, and the specific name platyurus means "flat-tailed", in reference to the compressed "tail" and laminae of the vertebrae there.[6][8][9][10][11]

 
Cope's 1869 reconstruction of Elasmosaurus above, with head on the wrong end and no hind-limbs, and holotype elements of E. platyurus (1-9) and E. orientalis (10) below

Cope requested that Turner searched for more parts of the Elasmosaurus specimen, and was sent more fossils during August or September, 1868. The ANSP thanked Turner for his "very valuable gift" at their meeting in December 1868, and Turner visited the museum during spring, at a time when Cope was absent. Turner died unexpectedly at Fort Wallace on July 27, 1869, without seeing the completion of the work he began, but Cope continued to write him, unaware of his death until 1870. The circumstances around Turner's discovery of the type specimen were not covered in Cope's report, and remained unknown until Turner's letters were published in 1987. Elasmosaurus was the first major fossil discovery in Kansas (and the largest from there at the time), and marked the beginning of a fossil collecting rush that sent thousands of fossils from Kansas to prominent museums on the American east coast.[6] Elasmosaurus was one of few plesiosaurs known from the New World at the time, and the first known member of the long-necked family of plesiosaurs, the Elasmosauridae.[5]

 
Cope's corrected 1870 reconstruction of Elasmosaurus above

In 1869, Cope scientifically described and figured Elasmosaurus, and the preprint version of the manuscript contained a reconstruction of the skeleton which he had formerly presented during his report at an ANSP meeting in September 1868. At an ANSP meeting a year and a half later, in March 1870, the American paleontologist Joseph Leidy (Cope's mentor) noted that Cope's reconstruction of Elasmosaurus showed the skull at the wrong end of the vertebral column, at the end of the tail instead of the neck (he was also unsure whether it had hind limbs). The reconstruction therefore showed the animal with a sort neck and a long tail, unlike other plesiosaurs. To hide his mistake, Cope attempted to recall all copies of the preprint article, and printed a corrected version with a new skeletal reconstruction that placed the head on the neck and different wording in 1870. Though Cope had tried to destroy the preprint, one copy came to the attention of the American paleontologist Othniel Charles Marsh, who made fun of the mistake. This led to antagonism between Cope, who was embarrassed by by the mistake, and Marsh, who brought up the mistake repeatedly for decades, and returned to the issue during their controversy in the New York Herald in the 1890s (Marsh claimed he had pointed out the error to Cope immediately). The argument was part of the "Bone Wars" rivalry between the two, and is well known in the history of paleontology.[6][12][11][13]

 
Neck and back vertebrae of Elasmosaurus and Cimoliasaurus, as figured by Cope in 1870

Even in his revised reconstruction of the skeleton, Cope had reversed the orientation of individual vertebrae, and had named a new order of reptiles based on this condition, "Streptosauria".[14][15][16][17][18][19]

 
Outdated restoration of two individuals with curled, snake-like necks, by Charles R. Knight, 1897

Because of Cope's reputation as a brilliant paleontologist, it has been questioned why he would make such an obvious anatomical error. It has been suggested that being a unique specimen in 1868, the original Elasmosaurus may have been hard to interpret based on the knowledge available at the time, and Cope initially thought it consisted of two specimens of different animals. In an 1868 letter to LeConte, Cope had referred to the supposed "smaller specimen" as Discosaurus carinatus, placing it in an existing genus named by Leidy. Cope was also only in his late twenties and not formally trained in paleontology, and may have been influenced by an earlier mistake by Leidy, who had reversed the vertebral column of the plesiosaur Cimoliasaurus in its description. In 2002, the American art historian Jane P. Davidson noted that the fact that other scientists pointed out the error early on argues against this explanation, and that Cope was not convinced he had made a mistake. Plesiosaur anatomy was known well enough at the time that Cope should not have been able to make the mistake, according to Davidson.[12] Cope did little work on the specimen since his 1870 description, and it was kept in storage for nearly 30 years.[6] It was only redescribed in detail in 2005 by the German paleontologist Sven Sachs.[5]

Known and possible fossil elementsEdit

 
Reconstructed skeleton, Centennial Centre for Interdisciplinary Science

Today, the incomplete holotype specimen, catalogued as ANSP 10081, is the only definite specimen of Elasmosaurus. It was long on exhibit, but is now stored in a cabinet with other assigned fragments. The specimen consists of the premaxillae, part of the hind-section of the right maxilla, two maxilla fragments with teeth, front part of the dentaries, three more jaw fragments, two cranial fragments of indeterminable identity, 72 neck vertebrae, including the atlas and axis, 3 pectoral vertebrae, 6 back vertebrae, 4 sacral vertebrae, 18 tail vertebrae, as well as rib fragments.[4][5] In 2013, a "lost" incomplete neck vertebra centrum of the holotype that had been mentioned by Cope was rediscovered in storage by Sachs, and the count of neck vertebrae was revised from 71 to 72.[4] In 1986, a three dimensional reconstruction of the holotype skeleton was completed and is now displayed at the ANSP. This cast was later copied by the Triebold Paleontology Incorporated, and copies were provided to other museums.[6]

 
Cope's figures of the pectoral and pelvic girdles of Elasmosaurus, these elements were lost by 1906

Though Cope described and figured the pectoral and pelvic girdles of Elasmosaurus in 1869 and 1875, these elements were noted as missing from the collection by the American paleontologist Samuel Wendell Williston in 1906. Cope had loaned these elements to the English sculptor Benjamin Waterhouse Hawkins to help prepare them out of their surrounding concretions. At the time, Hawkins was working on a "Paleozoic Museum" in New York's Central Park, where a reconstruction of Elasmosaurus was to appear, an American equivalent to his life-sized Crystal Palace Dinosaurs in London. In May 1871, many of the exhibits in Hawkin's workshop were destroyed by vandals and their fragments buried; it is possible that the girdle elements of Elasmosaurus were at the workshop and were likewise destroyed. Nothing was subsequently mentioned about their loss by Hawkins or Cope. Fossils that may have belonged to the holotype were found by the American geologist Benjamin Franklin Mudge in 1871, but have probably been lost since.[5][6][20]

Additional plesiosaur fossils were recovered near the original locality in 1954, 1991, 1994, and 1998, including back vertebrae, ribs, gastralia (belly ribs), and gastroliths (stomach stones). As none of these elements overlap with those of the holotype specimen, in 2005, the American paleontologist Michael J. Everhart concluded they belonged to the same individual, and that the parts had been separated before burial of the carcass. He also noted that a small stone wedged in the neural canal of one of the tail vertebrae of the holotype may be a gastrolith, based on its polished appearance.[21] In 2007, Leslie Noè and Marcela Gómez-Pérez expressed doubt that the additional elements belonged to the type specimen, or even to Elasmosaurus, due to lack of evidence. They explained that elements missing from the holotype may have been lost to weathering or simply not collected, and that parts may have been lost or damaged during transportation or preparation. Gastroliths may also not have been recognised as such during collection, since such stones were not reported from a plesiosaur until ten years after.[22]

Other referred speciesEdit

Over subsequent years, a number of other Elasmosaurus species were described by Cope, Wiliston, and other authors. However, none of these are still definitely referable to the genus Elasmosaurus today, and most of them either have been moved to genera of their own or are recognized as dubious, nomina dubia - that is, of questionable validity.[5][23][24]

 
Cope's outdated 1869 restoration of fossil reptiles from New Jersey, including an Elasmosaurus confronting a Dryptosaurus

Accompanying his 1869 description of E. platyurus, Cope named another species of Elasmosaurus, E. orientalis, based on two dorsal vertebrae from New Jersey.[25] He distinguished it from E. platyurus by the more strongly-developed processes known as parapophyses on the vertebrae, in which he considered it to approach closer to Cimoliasaurus; however, he still assigned it to Elasmosaurus on account of its large size and angled sides. The first of these vertebrae was used as a doorstop in a tailor's shop, whereas the other was found in a pit by Samuel Lockwood, the superintendent of the schools of Monmouth County. Cope gave the name orientalis to the new species, on account of it possibly having a more easterly distribution than E. platyurus.[11] The same year, Cope also wrote an article about the fossil reptiles of New Jersey, wherein he described E. orientalis as an animal with a "long neck". Yet in an accompanying illustration, Cope showed a short-necked Elasmosaurus confronting a Dryptosaurus (then Laelaps), with a plesiosaur-like Mosasaurus and other animals in the background. It is unknown which species of Elasmosaurus is depicted, but if E. orientalis, the short neck contradicts Cope's own text, and if E. platyurus, he showed the animal with a short neck after acknowledging this was incorrect. Davidson has suggested that even though Leidy had pointed out Cope's error in 1868, Cope may not have accepted this.[12][26] Leidy subsequently moved E. orientalis to the (now dubious) genus Discosaurus in the following year.[27] In 1952, Samuel Welles considered the species a nomen dubium, given how fragmentary it is.[28]

In the same publication, Cope assigned an additional species, E. constrictus,[11] based on a partial centrum from a cervical vertebra found in the Turonian-aged clay deposits at Steyning, Sussex in the United Kingdom. It was described by Richard Owen as Plesiosaurus constrictus in 1850; Owen named the species after the extremely narrow breadth of the vertebra between the pleurapophyses, or the processes that articulate between the ribs. He considered this to be partially an artifact of preservation, but could not understand how the compression only affected the central portion and not the articular ends of the centrum.[29] Cope recognized this as a natural condition, and considered constrictus to be "a species of Elasmosaurus or an ally".[11] In 1962, Welles considered Plesiosaurus constrictus to be a nomen dubium, given its fragmentary nature.[30][31] Per Ove Persson retained it as valid in 1963, noting the longitudinal ridge on the sides of the centra as an elasmosaurid trait.[32] In 1995, Nathalie Bardet and Pascal Godefroit also recognized it as an elasmosaurid, albeit indeterminate.[33]

 
Scapula of E. snowii (Styxosaurus snowii), as figured by Williston

Cope discovered another elasmosaurid skeleton in 1876. He named it as a new species, E. serpentinus, in 1877, and differentiated it by the lack of compression in the rear cervicals, the presence of few sessile ribs among the first few dorsals, and the presence of "weak angles" below the front caudals. Cope had also discovered another large skeleton that bore great resemblance to the known remains of E. orientalis from the black shale of the "Cretaceous bed No. 4"; he excavated it with the help of George B. Cledenning and Capt. Nicholas Buesen.[34] Williston published a figure of another E. serpentinus specimen in 1914;[35] Elmer Riggs formally described it in 1939.[36] In 1943, Welles removed E. serpentinus from Elasmosaurus, and placed it in a new genus Hydralmosaurus.[37] Subsequently, all Hydralmosaurus specimens were moved to Styxosaurus in 2016, rending the former a nomen dubium.[38]

Subsequently, a series of 19 cervical and dorsal vertebrae from the Big Bend region of the Missouri - which is part of the Pierre Shale - were found by John H. Charles. Cope, upon receiving the bones at the Academy of Natural Sciences, considered them yet another species of Elasmosaurus. The vertebrae were, according to Cope, the shortest among members of the genus (approaching Cimoliasaurus in this condition), but he still considered them as belonging to Elasmosaurus due to their compressed form. He named it E. intermedius in 1894.[39] However, in his 1906 revision of North American plesiosaurs, Williston regarded the vertebrae as "all more or less mutilated", and found no distinct differences between the remains of E. intermedius and E. platyurus.[20] In 1952, Welles opined that, if E. intermedius were valid, "it must be referred to a pliosaurian genus";[28] however, he proceeded to label it a nomen dubium in 1962.[30] Three shorter vertebrae found alongside E. intermedius, assigned by Cope to the new genus and species Embaphias circulosus,[39] were also considered by Welles to be a nomen dubium in 1962.[30]

 
Remains of E. ischiadicus (Styxosaurus snowii), as figured by Williston

Williston named a number of other new Elasmosaurus species in his 1906 revision.[40] In 1874, he and Mudge discovered a specimen in Plum Creek, Kansas; it was "the first specimen of plesiosaur" Williston ever saw.[20] While he initially assigned it in 1890 to a new species of Cimoliasaurus, C. snowii,[41] he subsequently recognized the elasmosaurid nature of its humerus and widely-separated coracoids. Thus, he renamed the species E. snowii, distinguishing it from other Elasmosaurus species by the concave front and outer rims of its pubis. A second specimen, discovered by Elias West in 1890, was also assigned by him to E. snowii.[20] In 1943, Welles moved E. snowii to its own genus, Styxosaurus,[37] where the species has since remained. However, the West specimen was assigned to Thalassiosaurus ischiadicus (see below) by Welles in 1952;[28] Kenneth Carpenter returned it to S. snowii in 1999.[23][40]

He also reassigned the species E. ischiadicus from the genus Polycotylus, where he had initially placed it when he named it in 1903 (albeit "gravely doubting" the correctness of this assignment). The type remains were discovered by him in the same 1874 expedition with Mudge. Williston also referred another specimen discovered by Mudge and H.A. Brous in 1876, notwithstanding differences in the vertebrae between the specimens. He refrained from separating them until a better identification of the vertebrae could be made. Compared to other Elasmosaurus species, E. ischiadicus appeared to have a more primitive arrangement of bones in the lower arm.[20] In 1943, both specimens were referred to the new genus Thalassiosaurus by Welles,[37] who then assigned the latter to the new genus and species Alzadasaurus kansasensis in 1952.[28] Glenn Storrs considered both to be indeterminate elasmosaurids in 1999;[42] in the same year, Carpenter assigned both to Styxosaurus snowii.[23][40]

An elasmosaurid specimen was found by Handel Martin in Logan County, Kansas in 1889. Williston named this as a new species, E. (?) marshii. He bore reservations about its referral to the genus due to the presence of an additional articular facet on the humerus and an additional bone in the wrist, which he considered to be potentially sufficient for separation at the generic level.[20] In 1943, Welles moved E. (?) marshii to a genus of its own, Thalassonomosaurus;[37] however, Carpenter sunk T. marshii into Styxosaurus snowii in 1999.[23] Another species, E. nobilis, was named by Williston from very large remains discovered by Mudge in 1874 in Jewell County, Kansas.[20] Welles named E. nobilis as another species of Thalassonomosaurus, T. nobilis, in 1943,[37] but it too was considered to be an individual of S. snowii by Carpenter.[23] Finally, two exceptionally large dorsal vertebrae collected by Charles Sternberg in 1895 were named E. sternbergii by Williston, but were considered to be indeterminate by Storrs.[40][42]

 
Remains of E. nobilis (Styxosaurus snowii), as figured by Williston

Williston mentioned three additional Elasmosaurus species, which he would figure and describe at a later date.[20] He again made reference to a new species of Elasmosaurus, from Kansas, in 1908.[43]

Several Russian species, based on poorly-preserved vertebral remains, were referred to Elasmosaurus by N.N. Bogolubov in 1911. One was E. helmerseni, which was first described by W. Kiprijanoff in 1882 from Maloje Serdoba, Saratov as Plesiosaurus helmerseni. Some material from Scania, Sweden was referred to helmerseni in 1885 by H. Schröder.[44] Vertebral and limb remains[45] from Kursk initially referred by Kiprijanoff to P. helmerseni were also moved by Bogolubov to the new species E. kurskensis, which he considered to be "identical with Elasmosaurus or related to it". He also named E. orskensis, based on "very large" cervical and caudal remains from Konopljanka, Orenburg; and E. serdobensis, based on a single cervical from Maloje Serdoba.[46] However, the validity of all of these species has been questioned. Welles considered E. kurskensis as an indeterminate plesiosaur in 1962.[30] Persson noted in a 1959 review of the Swedish "E." helmerseni material that, while the species was probably closely related to Elasmosaurus proper, it is too fragmentary for this hypothesis to be assessed;[44] he later remarked in 1963 that, regarding the latter three species, "their generic and specific definition is questionable", although he declined to specifically label them as invalid on account of not having seen the fossil material.[32] Similarly, in 1999, Evgeniy Pervushov, Maxim Arkhangelsky, and A.V. Ivanov considered E. helmerseni to be an indeterminate elasmosaurid.[47] In 2000, Storrs, Archangelsky, and Vladimir Efimov concurred with Welles on E. kurskensis, and labelled E. orskensis and E. serdobensis as indeterminate elasmosaurids.[48]

Two additional Russian species were described by subsequent authors. A.N. Riabinin described a single phalanx from a flipper in 1915 as E. (?) sachalinensis; the species was named after the island of Sakhalin, where N.N. Tikhonovich found it in 1919.[49] However, this specimen cannot be identified more specifically than an indeterminate elasmosaurid, which was followed by Persson[32] and Pervushov and colleagues.[47] Storrs, Arkhangelsky, and Efimov were less specific, labelling it as an indeterminate plesiosaur;[48] this classification was followed by Alexander Averianov and V.K. Popov in 2005.[49] Then, in 1916, P.A. Pravoslavlev named E. amalitskii from the Don River region, based on a specimen containing vertebrae, limb girdles, and limb bones. Persson considered it a valid species, and a relatively large member of the elasmosaurids;[32] however, like E. (?) sachalinensis, Pervushov and colleagues considered E. amalitskii an indeterminate elasmosaurid.[47]

 
A flipper referred to E. chilensis (Plesiosaurus chilensis)

In a 1918 review of the geographic distribution and evolution of Elasmosaurus, Pravoslavlev provisionally referred three other previously-named species to Elasmosaurus;[45] his taxonomic opinions have not been widely followed. One of these was E. chilensis, based on the Chilean Plesiosaurus chilensis named from a single caudal vertebra by Claude Gay in 1848.[50] Wilhelm Deecke moved chilensis to Pliosaurus in 1895,[51] a classification which was acknowledged by Pravoslavlev. Edwin Colbert later assigned the type vertebra in 1949 to a pliosauroid, and also assigned other referred remains to indeterminate elasmosauroids;[52][53] the type vertebra was recognized as potentially belonging to Aristonectes parvidens by José O'Gorman, Zulma Gasparini, and Leonardo Salgado in 2013.[54] Another was E. haasti, originally Mauisaurus haasti, named by James Hector in 1874 based on remains found in New Zealand. Although its validity was supported for a considerable time, M. haasti is regarded as a nomen dubium as of 2017.[55] Pravoslavlev recognized another species from New Zealand, E. hoodii, named by Owen in 1870 as Plesiosaurus hoodii based on a cervical vertebra.[56] Welles recognized it as a nomen dubium in 1962;[30] Joan Wiffen and William Moisley concurred in a 1986 review of New Zealand plesiosaurs.[57]

 
Remains of E. haasti (Mauisaurus haasti)

In 1949, Welles named a new species of Elasmosaurus, E. morgani. It was named from a well-preserved skeleton found in Dallas County, Texas.[58] However, part of the specimen was accidentally thrown out during the relocation of the Southern Methodist University's paleontological collections.[59] Welles recognized E. morgani's similarity to E. platyurus in its shoulder girdle, but maintained it as a separate species due to its shorter neck and more robust rear cervicals.[58] In 1997, Carpenter reconsidered the differences between the two species, and found them sufficient to place E. morgani in its own genus, which he named Libonectes.[60] Despite its reassignment and the loss of its material, L. morgani is often considered an archetypal elasmosaurid. Data based on these lost elements were unquestionably accepted in subsequent phylogenetic analyses, until a redescription of the surviving elements was published by Sven Sachs and Benjamin Kear in 2015.[59]

Persson referred another species to Elasmosaurus alongside his 1959 description of "E." helmerseni remains from Sweden, namely E. (?) gigas. It was based on Schröder's Pliosaurus (?) gigas, named in 1885 from two dorsals; one was found in Prussia, the other in Scania. While they were incomplete, Persson recognized that their proportions and the shape of their articular ends differed greatly from pliosauroids, and agreed well instead with elasmosaurids. Given that, at the time of Persson's writing, "there [was] nothing to contradict that they are nearest akin to Elasmosaurus", he referred them to Elasmosaurus "with hesitation". Theodor Wagner had previously referred gigas to Plesiosaurus in 1914.[44] As of 2013, this questionable attribution remains unchanged.[61] Another species from Russia, E. antiquus, was named by Dubeikovskii and Ochev in 1967[48] from the Kamsko-Vyatsky phosphorite quarry, but Pervushov and colleagues in 1999, followed by Storrs and colleagues in 2000, reinterpreted it as an indeterminate elasmosaurid.[47][48]

ClassificationEdit

 
Reconstructed skeleton in side view, Milwaukee Public Museum
 
Back vertebrae of the holotype, 1870

By the early part of the Late Cretaceous, there were two distinct groups of plesiosaurs left.[62] Elasmosaurus is the type genus for one of these groups, the elasmosaurids which had extremely long necks with relatively short heads, in contrast to the polycotylids which had shorter necks and relatively larger heads. Late Cretaceous elasmosaurids from the Western Interior of North America have few features that separate them and are morphologically primitive.[23] However, as noted above, Elasmosaurus and some others have some derived features. It has been suggested that Elasmosaurus was closely related to Hydralmosaurus and Styxosaurus due to these advanced features.[5]

The following cladogram shows the placement of Elasmosaurus within Elasmosauridae following an analysis by Rodrigo A. Otero in 2016:[38]

Elasmosauridae

Eromangasaurus




Callawayasaurus




Libonectes





Tuarangisaurus



Thalassomedon





Mauisaurus




Hydrotherosaurus



Futabasaurus





Aristonectinae

Kaiwhekea




Alexandronectes




Morturneria




Aristonectes



Aristonectes






Elasmosaurinae

Terminonatator





Elasmosaurus



Albertonectes





Styxosaurus










PaleobiologyEdit

 
Restoration showing two individuals swimming with straight necks

The unusual body structure of elasmosaurids would have limited the speed in which they could swim and their paddles may have moved in manner similar to birds wings but could not twist and were thus held rigidly.[63] A 2015 study concluded that locomotion was mostly done by the fore-flippers while the hind-flippers function in maneuverability and stability,[64] while a 2017 study concluded that the hind-flippers of plesiosaurs produced 60% more thrust and had 40% more efficiency when moving along in harmony with the fore-flippers.[65] The paddles were so rigid and specialized for swimming that they could not have come on land to lay eggs like sea turtles. Thus they most likely gave live birth to their young like sea snakes.[66] Evidence for viviparity (live-birth) in plesiosaurs came in the form of a fossil of an adult Polycotylus with a single fetus inside.[67]

Neck movement and functionEdit

Like other plesiosaurs, and in stark contrast to many common media depictions, Elasmosaurus was incapable of raising anything more than its head above the water. The weight of its long neck placed the center of gravity behind the front flippers. Thus Elasmosaurus could only have raised its head and neck above the water if in shallow water, where it could rest its body on the bottom. The weight of the neck, the limited musculature, and the limited movement between the vertebrae would have prevented Elasmosaurus from raising its head and neck very high as well. The head and shoulders of the Elasmosaurus most likely acted as a rudder. If the animal moved the anterior part of the body in a certain direction, it would cause the rest of the body to move in that direction. Thus, Elasmosaurus could not have swum in one direction while moving its head and neck either horizontally or vertically in a different direction.[68]

One study found that the necks of elasmosaurids were capable of 75–177˚ of ventral movement, 87–155° of dorsal movement, and 94–176° of lateral movement, depending on the amount of tissue between the vertebrae, which likely increased in rigidness towards the back of the neck. The researchers concluded that lateral and vertical arches and shallow S-shaped curves were feasible in contrast to the "swan-like" S-shape neck postures which required more than 360° of vertical flexion.[69]

It has also been suggested that the long necks of plesiosaurs served as a snorkel and allowed then to breathe air while the body remained underwater. This is disputed as there would be large hydrostatic pressure differences, particularly for the extremely long-necked elasmosaurids. The neck anatomy of elasmosaurids was capable of making a gentle slop to allow them to breathe at the surface but would have required them to make energy expensive swimming at the sub-surface. In addition, the longer neck would also have increased dead space and the animals may have required larger lungs. The neck could have had other vulnerabilities; it could have been targeted by predators.[70]

FeedingEdit

 
Gastroliths and bones of an undetermined plesiosaur from Kansas

The flexion ranges of Elasmosaurus would have allowed them to employ a number of hunting methods including "benthic grazing" which would have involved swimming close to the bottom and using the head and neck to dig for prey on the sea floor. Elasmosaurids may also have been active hunters in the pelagic zone and retracted their necks back to launch a strike or used side-swip motions to stun or kill prey with their laterally projected teeth (like sawsharks).[69]

It is possible that Elasmosaurus and its kin stalked schools of fish, concealing themselves below and moving the head slowly up as they approached. The eyes of the animal were located at the top of the head and allowed them to see directly upward. This stereoscopic vision would help it find small prey. Hunting from below would also have helped by silhouetting the prey in the sunlight while concealing Elasmosaurus in the dark waters below. Elasmosaurids probably ate small bony fish and marine invertebrates, as their small, non-kinetic skulls would have limited the prey they could eat. The long, slender teeth were adapted for seizing prey and not tearing, and hence it was swallowed whole.[63]

Although elasmosaurids are commonly found with several stomach stones, Elamosaurus has only been found uncontroversially with a pebble lodged in the neural arch one of its distal caudals.[22] A specimen of Elasmosaurus relative Styxosaurus contained fragmented fish bones and stones in the abdominal region behind the pectoral girdle. The fish remains were identified as Enchodus and other clupeomorph fish. The stones match rock from 600 km (370 mi) away from were the specimen was found. Plesiosaurs likely used these stones to help in the digestion of food.[71] Elasmosaurids themselves also appear to have been fed on. A humerus of an unidentified subadult elasmosaurids was found with bite marks matching the teeth of the shark Cretoxyrhina,[72] while a crushed Woolungasaurus skull has tooth-marks matched to the pliosaur Kronosaurus.[73]

PaleoecologyEdit

 
Western Interior Seaway during the mid-Cretaceous, about 100 million years ago

Elasmosaurus fossils have been found in the Campanian-age Upper Cretaceous Pierre Shale of western Kansas. The Pierre Shale represents a period of marine deposition from the Western Interior Seaway, a shallow continental sea that submerged much of central North America during the Cretaceous.[74] At its largest, the Western Interior Seaway stretched from the Rockies east to the Appalachians, some 1,000 km (620 mi) wide. At its deepest, it may have been only 800 or 900 metres (2,600 or 3,000 ft) deep, shallow in terms of seas. Two great continental watersheds drained into it from east and west, diluting its waters and bringing resources in eroded silt that formed shifting delta systems along its low-lying coasts. There was little sedimentation on the eastern shores of the Seaway; the western boundary, however, consisted of a thick clastic wedge eroded eastward from the Sevier orogenic belt.[75][76] The western shore was thus highly variable, depending on variations in sea level and sediment supply.[75]

While the soft, muddy sea floor likely received very little sunlight, it teemed with life due to steady rains of organic debris from plankton and other organisms further up the water column. The bottom was dominated by large Inoceramus clams, which were covered with oysters; there was little biodiversity. Clam shells would have accumulated over the centuries in layers under the sea floor's surface, and would have provided shelter for small fish. Other invertebrates known to have lived the sea include cephalopods (such as squids and ammonites), Baculites, and the crinoid Uintacrinus.[77]

Large fish known to have inhabited the sea include the bony fishes Pachyrhizodus, Enchodus, Cimolichthys, Saurocephalus, Saurodon, Gillicus, Ichthyodectes, Xiphactinus, Protosphyraena and Martinichthys;[78] and the sharks Cretoxyrhina, Cretolamna, Scapanorhynchus, Pseudocorax and Squalicorax.[79] In addition to Elasmosaurus, other marine reptiles present include fellow plesiosaurs Libonectes, Styxosaurus, Thalassomedon, Terminonatator, Polycotylus, Brachauchenius, Dolichorhynchops and Trinacromerum;[80] the mosasaurs Mosasaurus, Halisaurus, Prognathodon, Tylosaurus, Ectenosaurus, Globidens, Clidastes, Platecarpus and Plioplatecarpus;[81] and the sea turtles Archelon, Protostega, Porthochelys and Toxochelys.[82] The flightless aquatic bird Hesperornis also made its home there.[83] In addition, the pterosaurs Pteranodon and Nyctosaurus[84], and the bird Ichthyornis,[83] are also known far from land.[85]

See alsoEdit

ReferencesEdit

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BibliographyEdit

  • Everhart, M. J. (2005). Oceans of Kansas - A Natural History of the Western Interior Sea. Indiana University. ISBN 0-253-34547-2. 

External linksEdit