Editing Elasmosaurus

{{short description|Genus of reptiles (fossil)}}
{{Use mdy dates|date=April 2021}}
{{Speciesbox
| fossil_range     = [[Late Cretaceous]] ([[Campanian]]), {{fossilrange|80.64|77}}
| image            = Elasomosaurus Face Clean.png
| image_caption    = Reconstructed skeleton in the [[Rocky Mountain Dinosaur Resource Center]]
| image_alt        = Photo of a mounted skeleton on a transparent background
| genus            = Elasmosaurus
| display_parents  = 2
| species          = platyurus
| parent_authority = [[Edward Drinker Cope|Cope]], 1868
| authority        = Cope, 1868
| synonyms = * ''[[Discosaurus]] carinatus'' <small>Cope, 1868</small>
}}

'''''Elasmosaurus''''' ({{IPAc-en|ɪ|ˌ|l|æ|z|m|ə|ˈ|s|ɔːr|ə|s|,_|-|m|oʊ|-}}{{refn|{{Cite dictionary |url=http://www.lexico.com/definition/Elasmosaurus |archive-url=https://web.archive.org/web/20200728103919/https://www.lexico.com/definition/elasmosaur?s=t |url-status=dead |archive-date=2020-07-28 |title=Elasmosaurus |dictionary=[[Lexico]] UK English Dictionary |publisher=[[Oxford University Press]]}} }}) is a [[genus]] of [[plesiosaur]] that lived in North America during the [[Campanian]] stage of the [[Late Cretaceous]] period, at about 80.6 to 77{{nbsp}}million years ago. The first specimen was discovered in 1867 near [[Fort Wallace]], Kansas, US, and was sent to the American paleontologist [[Edward Drinker Cope]], who named it '''''E.{{nbsp}}platyurus''''' in 1868. The [[Generic name (biology)|generic name]] means "thin-plate reptile", and the [[specific name (zoology)|specific name]] means "flat-tailed". Cope originally reconstructed the skeleton of ''Elasmosaurus'' with the skull at the end of the tail, an error which was made light of by the paleontologist [[Othniel Charles Marsh]], and became part of their "[[Bone Wars]]" rivalry. Only one incomplete ''Elasmosaurus'' skeleton is definitely known, consisting of a fragmentary skull, the spine, and now lost [[pectoral girdle|pectoral]] and [[pelvic girdle]]s, and a single species is recognized today; other species are now considered invalid or have been moved to other genera. A fragmentary specimen from Germany may belong to this genus.

Measuring {{convert|10.3|m|sp=us}} in length, ''Elasmosaurus'' would have had a streamlined body with paddle-like limbs, a short tail, a small head, and an extremely long neck. The neck alone was around {{convert|7.1|m|sp=us}} long. Along with its relative ''[[Albertonectes]]'', it was one of the longest-necked animals to have lived, with the second largest number of neck [[vertebrae]] known, 72, 4 fewer than ''Albertonectes''. The skull would have been slender and triangular, with large, fang-like teeth at the front, and smaller teeth towards the back. It had six teeth in each [[premaxilla]] of the upper jaw, and may have had 14 teeth in the [[maxilla]] and 19 in the [[dentary]] of the lower jaw. Most of the neck vertebrae were compressed sideways, and bore a longitudinal crest or keel along the sides.

The family [[Elasmosauridae]] was based on the genus ''Elasmosaurus'', the first recognized member of this group of long-necked plesiosaurs. Elasmosaurids were well adapted for aquatic life, and used their flippers for swimming. Contrary to earlier depictions, their necks were not very flexible, and could not be held high above the water surface. It is unknown what their long necks were used for, but they may have had a function in feeding. Elasmosaurids probably ate small fish and marine [[invertebrates]], seizing them with their long teeth, and may have used [[gastroliths]] (stomach stones) to help digest their food. ''Elasmosaurus'' is known from the [[Pierre Shale]] formation, which represents marine deposits from the [[Western Interior Seaway]].

==History of study==
[[File:Officers at Fort Wallace.jpg|thumb|alt=Black and white photo of a group of men in uniforms in front of a wooden building|Officers at [[Fort Wallace]], [[Kansas]], in 1867. Theophilus H. Turner, who the same year discovered ''Elasmosaurus'' in the area, is second from left.]]
In early 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 Railroad]]. Approximately {{convert|23|km|mi|sp=us}} northeast of Fort Wallace, near [[McAllaster, Kansas|McAllaster]], Turner discovered the bones of a large fossil reptile in a ravine in the [[Pierre Shale]] formation, 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]] (ANSP, known since 2011 as the Academy of Natural Sciences of Drexel University). 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, at the ANSP's expense.<ref name=Sachs2005/><ref name="TurnerDiscovery">{{cite journal|last1=Everhart|first1=M. J.|s2cid=89988230|title=Captain Theophilus H. Turner and the Unlikely Discovery of ''Elasmosaurus platyurus''|journal=Transactions of the Kansas Academy of Science|date=2017|volume=120|issue=3–4|pages=233–246|doi=10.1660/062.120.0414}}</ref>{{sfn|Everhart|2005a|pages=121–123}}

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 {{convert|800|lbs|kg|sp=us|order=flip}}. The fossils were dug or pried out of the relatively soft [[shale]] with picks and shovels, 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 railroad, which had not yet reached the fort. The specimen arrived in Philadelphia by rail in March 1868, whereafter Cope examined it hurriedly; he reported on it at the March ANSP meeting, during which he named it ''Elasmosaurus platyurus''. The [[Generic name (biology)|generic name]] ''Elasmosaurus'' is a [[portmanteau]] of the [[Ancient Greek]] {{lang|grc-x-classic|ἔλᾰσμᾰ}} (''élăsmă'', "metal plate") prefixed onto {{lang|grc-x-classic|σαῦρος}} (''saûros'', "lizard"), in reference to the "plate" bones of the sternal and pelvic regions. However, this name was translated as "thin-plate reptile" in many later works. The [[specific name (zoology)|specific name]] also comes from the Ancient Greek {{lang|grc-x-classic|πλατύς}} (''platýs'', "flat") and {{lang|grc-x-classic|ουρά}} (''ourá'', "tail"), all meaning "flat-tailed", in reference to the compressed "tail" (actually the neck) and laminae of the vertebrae there.<ref name="TurnerDiscovery"/>{{sfn|Everhart|2005a|pages=121–123}}<ref name="ScatteredShattered">{{cite journal|last1=Davidson|first1=J. P.|last2=Everhart|first2=M. J.|title=Scattered and Shattered: A Brief History of the Early Methods of Digging, Preserving and Transporting Kansas Fossils|journal=Transactions of the Kansas Academy of Science|date=2017|volume=120|issue=3–4|pages=247–258|doi=10.1660/062.120.0416|s2cid=90362192|url=https://www.academia.edu/110911820}}</ref><ref name="Cope 1868">{{cite journal|last1=Cope |first1=E. D. |year=1868 |title=Remarks on a new enaliosaurian, ''Elasmosaurus platyurus'' |journal=Proceedings of the Academy of Natural Sciences of Philadelphia |volume=20 |pages=92–93 |url=https://archive.org/details/cbarchive_50660_copeed1868remarksonanewenalios1841}}</ref><ref name="Synopsis"/><ref name="Meaning">{{cite web|title=Ben Creisler's Plesiosaur Pronunciation Guide|year=2012|last=Creisler|first=B.|website=Oceans of Kansas|url=http://oceansofkansas.com/Creisler_Plesiosaurs2.html|access-date = 26 June 2021}}</ref>

[[File:Dorsal vertebrae of Elasmosaurus.jpg|thumb|left|alt=Black and white drawing of some connected vertebrae|[[Back vertebrae]] of the [[holotype]], 1869]]
Cope requested that Turner search 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{{nbsp}}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.<ref name="TurnerDiscovery"/> ''Elasmosaurus'' was one of few plesiosaurs known from the [[New World]] at the time, and the first recognized member of the long-necked family of plesiosaurs, the [[Elasmosauridae]].<ref name=Sachs2005/>

In 1869 Cope [[scientifically described]] and figured ''Elasmosaurus'', and the [[Manuscript (publishing)#Preprint|preprint]] version of the manuscript contained a reconstruction of the skeleton which he had earlier presented during his report at an ANSP meeting in September 1868. The reconstruction showed ''Elasmosaurus'' with a short neck and a long tail, unlike other plesiosaurs, and Cope was also unsure whether it had hind limbs. 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. Cope had apparently concluded that the tail vertebrae belonged to the neck, since the jaws had been found at that end of the skeleton, even though the opposite end terminated in the axis and atlas bones that are found in the neck. Leidy also concluded that ''Elasmosaurus'' was identical to ''[[Discosaurus]]'', a plesiosaur he had named in 1851.<ref name="Synopsis">{{cite journal|last1=Cope|first1=E. D.|title=Synopsis of the extinct Batrachia, Reptilia and Aves of North America, Part I|journal=Transactions of the American Philosophical Society|date=1869|volume=14|pages=44–55|doi=10.5962/bhl.title.60482|url=https://www.biodiversitylibrary.org/page/39852057#page/48/mode/1up|hdl=2027/nyp.33433090912423|access-date=November 8, 2017|archive-url=https://web.archive.org/web/20171108153137/https://www.biodiversitylibrary.org/page/39852057#page/48/mode/1up|archive-date=November 8, 2017|url-status=live}}</ref><ref name="Boneheaded"/><ref>{{cite journal | last = Leidy | first = J. | title = Remarks on Elasmosaurus platyurus | journal = Proceedings of the Academy of Natural Sciences of Philadelphia | volume = 22 | url = http://oceansofkansas.com/Leidy1870.html | pages = 9–10 | year = 1870 | access-date = November 21, 2017 | archive-url = https://web.archive.org/web/20180528074807/http://oceansofkansas.com/Leidy1870.html | archive-date = May 28, 2018 | url-status = live }}</ref><ref name="TurnerDiscovery"/>

{{multiple image
| direction = vertical
| width = 250
|align = right
| header = 
| image1 = Elasmosaurus Cope.jpg
| alt1 = Drawings of a skeleton with an inaccurate long tail and short neck, and various bones
| caption1 = [[Edward Drinker Cope]]'s original 1869 reconstruction of ''Elasmosaurus'', above, with the head on the wrong end and no hind-limbs, and holotype elements of ''E.{{nbsp}}platyurus'' (1–9) and ''E.{{nbsp}}orientalis'' (10), below
| image2 = Elasmosaurus corrected.jpg
| alt2= Drawings of a skeleton with a long neck and various bones
| caption2 = Cope's corrected 1870 reconstruction of ''Elasmosaurus'', above
}}

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 (though it reversed the orientation of the individual vertebrae) and different wording in 1870. In a reply to Leidy, Cope claimed that he had been misled by the fact that Leidy had arranged the vertebrae of ''[[Cimoliasaurus]]'' in the reverse order in his 1851 description of that genus, and pointed out that his reconstruction had been corrected. Cope also rejected the idea that ''Elasmosaurus'' and ''Discosaurus'' were identical, and noted that the latter and ''Cimoliasaurus'' did not have any distinguishing features. Though Cope had tried to destroy the preprints, one copy came to the attention of the American paleontologist [[Othniel Charles Marsh]], who made light of the mistake. This led to antagonism between Cope, who was embarrassed by the mistake, and Marsh, who brought up the mistake repeatedly for decades. Marsh 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), when their dispute gained widespread public attention. The argument was part of the "[[Bone Wars]]" rivalry between the two, and is well known in the history of paleontology.<ref name="TurnerDiscovery"/><ref name="Boneheaded">{{cite journal |last1=Davidson |first1=J. P. |title=Bonehead mistakes: The background in scientific literature and illustrations for Edward Drinker Cope's first restoration of ''Elasmosaurus platyurus'' |journal=Proceedings of the Academy of Natural Sciences of Philadelphia |year=2002 |volume=152 |issue=1 |pages=215–240 |doi=10.1635/0097-3157(2002)152[0215:HPOVBM]2.0.CO;2|s2cid=146688988|url={{google books|plainurl=yes|id=1vNykbteN30C|page=215}}}}</ref><ref>{{cite news |last=Marsh |first=O. C. |title=Wrong End Foremost |work=[[New York Herald]] |date=1890 |url=http://www.oceansofkansas.com/NYHerald.html |access-date=February 2, 2009 |archive-url=https://web.archive.org/web/20190413215048/http://oceansofkansas.com/NYHerald.html |archive-date=April 13, 2019 |url-status=live }}</ref><ref>{{cite journal | last = Cope | first = E. D. | title = On ''Elasmosaurus platyurus'' Cope | journal = American Journal of Science | series = 2 | pages = 140–141 | url = http://oceansofkansas.com/Cope1870a.html | year = 1870 | volume = 50 | issue = 148 | access-date = September 26, 2017 | archive-url = https://web.archive.org/web/20170828102446/http://oceansofkansas.com/Cope1870a.html | archive-date = August 28, 2017 | url-status = live }}</ref><ref>{{cite journal|last1=Cope|first1=E. D.|title=Synopsis of the extinct Batrachia, Reptilia and Aves of North America, Part I|journal=Transactions of the American Philosophical Society|date=1870|volume=14|pages=44–55|doi=10.5962/bhl.title.60499|url=https://www.biodiversitylibrary.org/item/123170#page/52/mode/1up|hdl=2027/nyp.33433090912423|access-date=November 11, 2017|archive-url=https://web.archive.org/web/20171112074142/https://www.biodiversitylibrary.org/item/123170#page/52/mode/1up|archive-date=November 12, 2017|url-status=live|doi-access=free}}</ref><ref name="Page"/>

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, as a unique specimen in 1868, the original ''Elasmosaurus'' may have been hard to interpret based on the knowledge available at the time. Also, Cope initially thought it consisted of two specimens of different animals{{snd}}in an 1868 letter to LeConte, Cope had referred to the supposed "smaller specimen" as ''Discosaurus carinatus''. Cope was only in his late twenties and not formally trained in paleontology, and may have been influenced by Leidy's mistake of reversing the vertebral column of ''Cimoliasaurus''. In 2002 the American art historian Jane P. Davidson noted that the fact that other scientists early on had pointed out Leidy's error argues against this explanation, adding that Cope was not convinced he had made a mistake. Plesiosaur anatomy was sufficiently well known at the time that Cope should not have made the mistake, according to Davidson.<ref name="Boneheaded"/> Cope did little work on the specimen since his 1870 description, and it was kept in storage for nearly 30 years.<ref name="TurnerDiscovery"/> It was only redescribed in detail in 2005 by the German paleontologist Sven Sachs.<ref name=Sachs2005/>

===Known and possible fossil elements===
Today, the incomplete [[holotype specimen]], cataloged as ANSP 10081, is the only definite specimen of ''Elasmosaurus''. It was long exhibited, 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, the front part of the dentaries, three more jaw fragments, two cranial fragments of indeterminable identity, 72 [[cervical vertebrae]] of the neck, including the atlas and axis, 3{{nbsp}}pectoral vertebrae, 6{{nbsp}}[[back vertebrae]], 4{{nbsp}}sacral vertebrae, 18 tail vertebrae, as well as rib fragments.<ref name="Count"/><ref name=Sachs2005/> In 2013 an incomplete neck vertebra centrum of the holotype that had been mentioned by Cope but thought to have been lost was rediscovered in storage by Sachs, and the neck vertebra count was revised from 71 to 72.<ref name="Count"/> The neck vertebrae have been [[taphonomically]] distorted (changes occurring during decay and [[fossilization]]), with some parts being unnaturally compressed or displaced.<ref name="Elongation">{{cite journal |last1=O’Gorman |first1=José P.|title=How elongated? The pattern of elongation of cervical centra of ''Elasmosaurus platyurus'' with comments on cervical elongation patterns among plesiosauromorphs |journal=Diversity |year=2024 |volume=16 |issue=2 |at=106 |doi=10.3390/d16020106 |s2cid=267558631 |doi-access=free }}</ref> 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 company [[Triebold Paleontology Incorporated]], and replicas were provided to other museums. The replica at the Fort Wallace Museum measures about {{convert|12.8|m|sp=us}} in length.<ref name="TurnerDiscovery"/>

[[File:Elasmosaurus girdle elements.jpg|thumb|left|alt=Illustrations of plated bones with holes|Cope's 1869 figures of the now lost pectoral (left) and pelvic (right) girdles of the holotype; the [[generic name (biology)|generic name]] refers to these "plate" bones]]
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 much of the exhibit material in Hawkins' workshop was destroyed by vandals for unclear reasons 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.<ref name=Sachs2005/><ref name="TurnerDiscovery"/><ref name="williston1906">{{cite journal
 | last1         = Williston
 | first1        = S. W.
 | title         = North American plesiosaurs ''Elasmosaurus'', ''Cimoliasaurus'', and ''Polycotylus''
 | journal       = American Journal of Science
 | date          = 1906
 | series        = 4
 | volume        = 21
 | issue         = 123
 | pages         = 221–236
 | doi           = 10.2475/ajs.s4-21.123.221
 | url           = https://zenodo.org/record/1450146
 | bibcode       = 1906AmJS...21..221W
 | access-date   = 2022-12-09
 | archive-url   = 
 | archive-date  = 
 | url-status    = 
}}</ref><ref name="Mystery"/><ref name="Hilton">{{cite journal |last1=Coules |first1=Victoria |last2=Benton |first2=Michael J. |title=The curious case of Central Park's dinosaurs: The destruction of Benjamin Waterhouse Hawkins' Paleozoic Museum revisited |journal=Proceedings of the Geologists' Association |date=2023 |volume=134 |issue=3 |pages=344–360 |doi=10.1016/j.pgeola.2023.04.004|bibcode=2023PrGA..134..344C |doi-access=free }}</ref> In 2018, Davidson and Everhart documented the events leading up to the disappearance of these fossils, and suggested that a photo and drawing of Waterhouse's workshop from 1869 appear to show concretions on the floor that may have been the unprepared girdles of ''Elasmosaurus''. They also noted that conceptual sketches of the Palaeozoic Museum show that the model ''Elasmosaurus'' was originally envisioned with a long "tail", though later updated with a long neck. Davidson and Everhart concluded that the girdle fossils were most likely destroyed in Hawkins' workshop.<ref name="Mystery">{{cite journal
 | last1         = Davidson
 | first1        = J. P.
 | last2         = Everhart
 | first2        = M. J.
 | s2cid         = 91379054
 | title         = The Mystery of ''Elasmosaurus platyurus'' Cope 1868 – Where is the rest of the type specimen?
 | journal       = Transactions of the Kansas Academy of Science
 | date          = 2018
 | volume        = 121
 | issue         = 3–4
 | pages         = 335–345
 | doi           = 10.1660/062.121.0403
}}</ref>

[[File:Central Park studio.jpg|thumb|1869 drawing of [[Benjamin Waterhouse Hawkins]]' workshop in [[Central Park]]; the rounded shapes in the lower center left may be concretions that contained the now missing girdle fossils of the holotype]]
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.<ref name="TurnerDiscovery"/> Additional plesiosaur fossils were recovered near the [[Type (biology)#type locality|original locality]] in 1954, 1991, 1994, and 1998, including back vertebrae, ribs, [[gastralia]] (belly ribs), and [[gastroliths]]. 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.<ref>{{cite journal
 | last          = Everhart
 | first         = M. J.
 | title         = Elasmosaurid remains from the Pierre Shale (Upper Cretaceous) of western Kansas. Possible missing elements of the type specimen of ''Elasmosaurus platyurus'' Cope 1868?
 | journal       = PalArch's Journal of Vertebrate Palaeontology
 | volume        = 4
 | issue         = 3
 | date          = 2005
 | issn          = 1567-2158
 | url           = https://www.researchgate.net/publication/242762375
}}</ref> In 2007 the Colombian paleontologists 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 recognized as such during collection, since such stones were not reported from a plesiosaur until ten years after.<ref name=missing>{{cite journal
 | last1         = Noè
 | first1        = L. F.
 | last2         = Gómez–Pérez
 | first2        = M.
 | title         = Postscript to Everhart, M.J. 2005. "Elasmosaurid remains from the Pierre Shale (Upper Cretaceous) of western Kansas. Possible missing elements of the type specimen of ''Elasmosaurus platyurus'' Cope 1868?" – PalArch's Journal of Vertebrate Palaeontology 4, 3: 19–32
 | journal       = PalArch's Journal of Vertebrate Palaeontology
 | date          = 2007
 | volume        = 2
 | issue         = 1
 | url           = http://www.palarch.nl/2007/04/noe-lf-m-gomez-perez-2007-postscript-to-everhart-mj-2005-%E2%80%9Celasmosaurid-remains-from-the-pierre-shale-upper-cretaceous-of-western-kansas-possible-missing-elements-of-the-type-spec/
 | access-date   = 2017-11-13
 | archive-url   = https://web.archive.org/web/20171114041109/http://www.palarch.nl/2007/04/noe-lf-m-gomez-perez-2007-postscript-to-everhart-mj-2005-%E2%80%9Celasmosaurid-remains-from-the-pierre-shale-upper-cretaceous-of-western-kansas-possible-missing-elements-of-the-type-spec/
 | archive-date  = 2017-11-14
 | url-status    = dead
}}</ref>

In 2017 Sachs and Joachim Ladwig suggested that a fragmentary elasmosaurid skeleton from the upper Campanian of [[Kronsmoor]] in [[Schleswig-Holstein]], Germany, and housed in the [[Naturkunde-Museum Bielefeld]], may have belonged to ''Elasmosaurus''. Additional parts of the same skeleton are housed at the Institute for Geology of the [[University of Hamburg]], as well as in private collections. Combined, the specimen consists of neck, back and tail vertebrae, [[phalanges]], a tooth, limb elements, 110 gastroliths, and unidentifiable fragments.<ref>{{cite journal
 | last1         = Sachs
 | first1        = S.
 | last2         = Ladwig
 | first2        = J.
 | title         = Reste eines Elasmosauriers aus der Oberkreide von Schleswig-Holstein in der Sammlung des Naturkunde-Museums Bielefeld
 | trans-title   = Remains of an elasmosaur from the Upper Cretaceous of Schleswig-Holstein in the collection of the Natural History Museum in Bielefeld
 | journal       = Berichte des Naturwissenschaftlichen Vereins für Bielefeld und Umgegend
 | date          = 2017
 | volume        = 55
 | pages         = 28–36
 | url           = https://www.researchgate.net/publication/320979464
 | language      = de
}}</ref>

==Description==
[[File:Elasmosaurus Size.svg|thumb|left|alt=Diagram of a green plesiosaur next to a diver|Size compared to a human]]
Though the only known specimen of ''Elasmosaurus'' (holotype specimen ANSP{{nbsp}}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.<ref name="Focus"/><ref name="Sachs2005"/> The neck of ''Elasmosaurus'' is estimated at {{convert|7.1|m|sp=us}} in length;<ref name="Neck">{{cite journal|last1=Taylor|first1=M. P.|last2=Wedel|first2=M. J.|title=Why sauropods had long necks; and why giraffes have short necks|journal=PeerJ|date=2013|volume=1|pages=e36|doi=10.7717/peerj.36|pmid=23638372|pmc=3628838 |doi-access=free }}</ref> thus, ''Elasmosaurus'' and its relative ''[[Albertonectes]]'' were some of the longest-necked animals ever to have lived, with the largest number of neck vertebrae of any known [[vertebrate]] animals.<ref name="Focus">{{cite web|last1=Sachs|first1=S.|last2=Kear|first2=B. P.|title=Fossil Focus: Elasmosaurs|publisher=Palaeontology Online|date=2015|volume=5|pages=1–8|url=http://www.palaeontologyonline.com/articles/2015/fossil-focus-elasmosaurs/|access-date=January 18, 2018|work=www.palaeontologyonline.com|archive-url=https://web.archive.org/web/20180201203232/https://www.palaeontologyonline.com/articles/2015/fossil-focus-elasmosaurs/|archive-date=February 1, 2018|url-status=live}}</ref><ref name="Count"/> In spite of their many neck vertebrae, the necks of elasmosaurids were less than half as long as those of the longest-necked [[sauropod]] [[dinosaurs]].<ref name="Neck"/> Initially, in his 1869 description of ''Elasmosaurus'', Cope estimated the length of the animal by summing up vertebral lengths and estimations of missing parts, resulting in a total length of {{convert|13.1|m|sp=us}}; he believed that the living animal would have been slightly larger due to [[cartilage]] present between the vertebral bodies, and was estimated at roughly {{convert|13.7|m|ft|sp=us}}.<ref name="Synopsis"/> However, in 1952, the American paleontologist [[Samuel Paul Welles|Samuel Welles]] estimated the body length to have been {{convert|10.3|m|sp=us}},<ref name="welles1952"/> a number that was repeated by José Patricio O'Gorman in 2016.<ref name="gorman2016">{{cite journal|last1=O'Gorman |first1=J. P. |s2cid=133139689 |year=2016 |title=A Small Body Sized Non-Aristonectine Elasmosaurid (Sauropterygia, Plesiosauria) from the Late Cretaceous of Patagonia with Comments on the Relationships of the Patagonian and Antarctic Elasmosaurids |journal=Ameghiniana |volume=53 |issue=3 |pages=245–268 |doi=10.5710/AMGH.29.11.2015.2928|hdl=10915/108247|hdl-access=free}}</ref>

[[File:Elasmosaurus platyurus.png|thumb|[[paleoart|Life restoration]]]]
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 state of the fossils. It probably had six teeth in each premaxilla, and the teeth preserved there were formed like large fangs. The number of premaxillary teeth distinguished ''Elasmosaurus'' from primitive plesiosauroids and most other elasmosaurids, which usually had fewer. 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 were 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 from front to back. The [[maxillae]] (largest tooth bearing bone of the upper jaw) of elasmosaurids usually contained 14{{nbsp}}teeth, whereas the dentaries (the main part of 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 (anatomy)|suture]].<ref name="Focus"/><ref name=Sachs2005>{{cite journal |last=Sachs |first=S. |year=2005 |title=Redescription of ''Elasmosaurus platyurus'' Cope, 1868 (Plesiosauria: Elasmosauridae) from the Upper Cretaceous (lower Campanian) of Kansas, U.S.A |url=https://www.researchgate.net/publication/260421820 |journal=Paludicola |volume=5 |issue=3 |pages=92–106}}</ref>

The pectoral and pelvic girdles of the holotype specimen were noted as missing by 1906, but observations about these elements were since made based on the original descriptions and figures from the late 19th{{nbsp}}century. The [[shoulder blades]] (scapulae) were fused and met at the midline, bearing no trace of a median bar. The upper [[Process (anatomy)|processes]] of the shoulder blades were very broad, and the [[Scapula#Angles|"necks"]] of the shoulder blades were long. The pectoral girdle had a long bar at the middle, a supposedly advanced feature thought to be absent from juvenile plesiosaurs. The [[ischium|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.<ref name=Sachs2005/> Like other elasmosaurids (and plesiosaurs in general), ''Elasmosaurus'' would have had large, paddle-like limbs with very long [[Digit (anatomy)|digits]]. The paddles at the front (the pectoral paddles) were longer than those at the back (the pelvic paddles).<ref name="Focus"/>

===Vertebrae===

[[File:Elasmosaurusskull.jpg|thumb|alt=Small skull on a long neck of a mounted, gray skeleton, on a blue background|Reconstructed skull and neck, [[North American Museum of Ancient Life]]]]
Unlike those of many other long-necked animals, the individual neck vertebrae were not particularly elongated; rather, the extreme neck length was achieved by a much increased number of vertebrae.<ref name="Neck"/> ''Elasmosaurus'' differed from all other plesiosaurs by having 72 neck vertebrae; more may have been present but were later lost to erosion or after excavation. Only ''Albertonectes'' had more neck vertebrae, 76, and the two are the only plesiosaurs with a count higher than 70; more than 60 vertebrae is very [[derived feature|derived]] (or "advanced") for plesiosaurs.<ref name="Count">{{cite journal |last1=Sachs |first1=S. |last2=Kear |first2=B. P. |last3=Everhart |first3=M. |year=2013 |title=Revised vertebral count in the "longest-necked vertebrate" ''Elasmosaurus platyurus'' Cope 1868, and clarification of the cervical-dorsal transition in Plesiosauria |journal=PLOS ONE |volume=8 |issue=8 |page=e70877 |doi=10.1371/journal.pone.0070877 |bibcode=2013PLoSO...870877S|pmc=3733804 |pmid=23940656|doi-access=free }}</ref><ref name=Sachs2005/>

The [[atlas bone|atlas]] and [[axis bone]] complex, consisting of the first two neck vertebrae and 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 gave 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 higher. The neural spine was low and directed 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) where 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.<ref name=Sachs2005/>

[[File:Cervical vertebrae of Elasmosaurus.png|thumb|left|alt=Gray vertebrae seen from different angles|[[Cervical vertebrae]] from different areas of the holotype's neck shown from the left, behind, and below]]
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 from the fourth vertebra and onwards. The centra became more 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 61st vertebra, and those of the hindmost vertebrae 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 vertebra, the lower edge of the articular facets became more concave, and the facet shaped like a quadrate with rounded edges. By the 63rd vertebra, the articular facet was also quadratic in shape with rounded edges, whereas the centra of the hindmost vertebrae had a broad oval outline.<ref name="Focus"/><ref name="Count"/><ref name=Sachs2005/>

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 articulated 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 [[Cervical rib|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.<ref name=Sachs2005/>

[[File:Pectoral vertebrae of Elasmosaurus.png|thumb|alt=Three connected gray vertebrae on a white background|Vertebrae from the [[pectoral region]] of the holotype specimen]]
The vertebrae that transitioned between the neck and back 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.<ref name="Count"/><ref name=Sachs2005/>

''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 tail vertebrae in elasmosaurids is 30.<ref name=Sachs2005/> 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.<ref name="Focus"/>

==Formerly assigned species==
[[File:Laelaps-cope.jpg|thumb|right|alt=Inaccurate drawing of various prehistoric creatures, two of which are confronting each other in the foreground|Cope's outdated 1869 restoration of fossil reptiles from [[New Jersey]], including a short-necked ''E.{{nbsp}}orientalis'' confronting a ''[[Dryptosaurus]]'']]
Following the description of the [[type species]], ''E. platyurus'', a number of other ''Elasmosaurus'' species were described by Cope, Williston, and other authors. However, none of these species are still definitely referable to the genus ''Elasmosaurus'' today, and most of them either have been moved to genera of their own or are considered dubious names, ''[[nomina dubia]]''{{snd}}that is, with no distinguishing features, and therefore of questionable validity.<ref name=Sachs2005/><ref name=Carpenter1999>{{cite journal |last=Carpenter |first=K. |year=1999 |title=Revision of North American elasmosaurs from the Cretaceous of the western interior |journal=Paludicola |volume=2 |issue=2 |pages=148–173|url=https://www.researchgate.net/publication/40662805}}</ref><ref name=Brown1993>{{cite journal |last=Brown |first=D. S. |year=1993 |title=A taxonomic reappraisal of the families Elasmosauridae and Cryptoclididae (Reptilia: Plesiosauroidea) |journal=Revue de Paléobiologie |volume=7 |pages=9–16}}</ref>

Accompanying his 1869 description of ''E. platyurus'', Cope named another species of ''Elasmosaurus'', ''E.{{nbsp}}orientalis'', based on two back vertebrae from New Jersey.<ref>{{cite journal|last1=Cope|first1=E. D.|title=On the reptilian orders Pythonomorpha and Streptosauria|journal=Proceedings of the Boston Society of Natural History|date=1869|volume=12|pages=265–268|url=https://www.biodiversitylibrary.org/page/9493892#page/273/mode/1up|access-date=November 12, 2017|archive-url=https://web.archive.org/web/20171112074217/https://www.biodiversitylibrary.org/page/9493892#page/273/mode/1up|archive-date=November 12, 2017|url-status=live}}</ref> He distinguished ''E.{{nbsp}}orientalis'' from ''E.{{nbsp}}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, a [[superintendent (education)|superintendent]]. Cope gave the name ''orientalis'' to the new species, on account of it possibly having a more easterly distribution than ''E.{{nbsp}}platyurus''.<ref name="Synopsis"/> Leidy subsequently moved ''E.{{nbsp}}orientalis'' to the now dubious genus ''Discosaurus'' in the following year.<ref name="leidy1870">{{cite journal | last1 = Leidy | first1 = J. | title = April 5th | journal = Proceedings of the Academy of Natural Sciences of Philadelphia | volume = 22 | issue = 1 | date = 1870 | pages = 18–22 | jstor = 4624074 }}</ref> In 1952 Welles considered the species a ''nomen dubium'', given how fragmentary it was.<ref name="welles1952"/>

In 1869 Cope also published an article about the fossil reptiles of New Jersey, wherein he described ''E.{{nbsp}}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. According to Davidson, it is uncertain which species of ''Elasmosaurus'' is depicted, but if it is ''E.{{nbsp}}orientalis'', the short neck contradicts Cope's own text, and if ''E.{{nbsp}}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.<ref name="Boneheaded"/><ref>{{cite journal|last1=Cope|first1=E. D.|title=The Fossil Reptiles of New Jersey (Continued)|journal=The American Naturalist|date=1869|volume=3|issue=2|pages=84–91|doi=10.1086/270371|jstor=2447100|doi-access=|s2cid=85021016 }}</ref> In an 1870 reply to Leidy, Cope himself stated that the generic placement of ''E.{{nbsp}}orientalis'' was in doubt, and that he had illustrated it with a short neck due to believing this was the condition of ''Cimoliasaurus''. If more remains showed ''E.{{nbsp}}orientalis'' to have had a long neck like ''Elasmosaurus'', he stated the image may instead represent ''Cimoliasaurus'' better.<ref name="Additionalnote">{{cite journal | last = Cope | first = E. D. | title = Additional note on ''Elasmosaurus'' | journal = American Journal of Science | series = 2 | pages = 268–269 | url = http://oceansofkansas.com/cope1870b.html | year = 1870 | volume = 50 | access-date = November 21, 2017 | archive-url = https://web.archive.org/web/20180528074812/http://oceansofkansas.com/cope1870b.html | archive-date = May 28, 2018 | url-status = live }}</ref>

[[File:Plesiosaurus constrictus.jpg|thumb|left|alt=Drawing of a vertebra from two angles|Vertebra of ''[[Plesiosaurus]] constrictus'', which Cope assigned to ''Elasmosaurus'']]
In the same 1869 publication wherein he named ''E. platyurus'' and ''E.{{nbsp}}orientalis'', Cope assigned an additional species, ''E.{{nbsp}}constrictus'',<ref name="Synopsis"/> based on a partial centrum from a neck vertebra found in the [[Turonian]]-aged [[clay]] deposits at [[Steyning]], [[Sussex]], in the United Kingdom. It was described by the British paleontologist [[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 affected only the central portion and not the articular ends of the centrum.<ref name="owen1850">{{cite book | last1 = Owen | first1 = R. | chapter = Order – Enaliosauria | title = A History of British Fossil Reptiles | volume = 1 | publisher = Cassell & Company Ltd | location = London | pages = 215–217 | chapter-url = https://books.google.com/books?id=NTJYAAAAYAAJ&pg=PA215 | date = 1850}}</ref> Cope recognized this as a natural condition, and considered ''constrictus'' to be "a species of ''Elasmosaurus'' or an ally".<ref name="Synopsis"/> In 1962 Welles considered ''P.{{nbsp}}constrictus'' to be a ''nomen dubium'', given its fragmentary nature.<ref name="welles1962">{{cite journal | last = Welles | first = S.P. | title = A new species of elasmosaur from the Aptian of Colombia and a review of the Cretaceous plesiosaurs | year = 1962 | journal = University of California Publications in Geological Sciences | volume = 44 | issue = 1 | pages = 1–96 | isbn = 978-0-598-20148-5 | oclc = 5734397 | url = http://redciencia.cu/geobiblio/paper/1962_Welles_%20rvw%20of%20K%20plesiosaurs.pdf | archive-url = https://web.archive.org/web/20220104165630/http://redciencia.cu/geobiblio/paper/1962_Welles_%20rvw%20of%20K%20plesiosaurs.pdf | archive-date = 2022-01-04}}</ref><ref name="sachsea2016">{{cite journal | last1 = Sachs | first1 = S. | last2 = Wilmsen | first2 = M. | last3 = Knüppe | first3 = J. | last4 = Hornung | first4 = J.J. | last5 = Kear | first5 = B.P. | title = Cenomanian–Turonian marine amniote remains from the Saxonian Cretaceous Basin of Germany | journal = Geological Magazine | date = 2017 | pages = 237–246 | doi = 10.1017/S0016756815001004 | volume = 154 | issue = 2| bibcode = 2017GeoM..154..237S | s2cid = 131854749 }}</ref> Per Ove Persson retained it as valid in 1963, noting the longitudinal ridge on the sides of the centra as an elasmosaurid trait.<ref name="persson1963">{{cite journal | last1 = Persson | first1 = P.O. | title = A revision of the classification of the Plesiosauria with a synopsis of the stratigraphical and geographical distribution of the group | date = 1963 | journal = Lunds Universitets Arsskrift | volume = 59 | issue = 1 | pages = 1–59 | url = http://paleoarchive.com/literature/Persson1963-RevisionClassificationPlesiosauria.pdf | access-date = November 17, 2017 | archive-url = https://web.archive.org/web/20171118222539/http://paleoarchive.com/literature/Persson1963-RevisionClassificationPlesiosauria.pdf | archive-date = November 18, 2017 | url-status = live }}</ref> In 1995 Nathalie Bardet and [[Pascal Godefroit]] also recognized it as an elasmosaurid, albeit indeterminate.<ref name="bardet1995">{{cite journal | last1 = Bardet | first1 = N. | last2 = Godefroit | first2 = P. | title = ''Plesiosaurus houzeaui'' <small>Dollo, 1909</small> from the Upper Campanian of Ciply (Belgium) and a review of the Upper Cretaceous plesiosaurs from Europe | date = 1995 | journal = Bulletin de l'Institut Royal des Sciences Naturelles de Belgique | volume = 65 | pages = 179–186 | url = http://biblio.naturalsciences.be/rbins-publications/bulletin-of-the-royal-belgian-institute-of-natural-sciences-earth-sciences/bulletin-of-the-royal-belgian-institute-of-natural-sciences-earth-sciences/bardet_plesiosaurus_1995 | access-date = November 17, 2017 | archive-url = https://web.archive.org/web/20171118221844/http://biblio.naturalsciences.be/rbins-publications/bulletin-of-the-royal-belgian-institute-of-natural-sciences-earth-sciences/bulletin-of-the-royal-belgian-institute-of-natural-sciences-earth-sciences/bardet_plesiosaurus_1995 | archive-date = November 18, 2017 | url-status = live }}</ref>

[[File:The Osteology of the Reptiles p139.png|thumb|right|alt=Drawing of two bony flippers attached to plated bones of the pectoral girdle|Pectoral girdle and front paddles of a juvenile [[elasmosaurid]] originally assigned to ''E.{{nbsp}}serpentinus'']]
Cope discovered another elasmosaurid skeleton in 1876. He named it as a new species, ''E.{{nbsp}}serpentinus'', in 1877, and differentiated it by the lack of compression in the rear neck vertebrae, the presence of few sessile ribs among the first few dorsals, and the presence of "weak angles" below the front tail vertebrae. Cope had also discovered another large skeleton that bore great resemblance to the known remains of ''E.{{nbsp}}orientalis'' from the black shale of the "Cretaceous bed No.{{nbsp}}4"; he excavated it with the help of George B. Cledenning and Capt. Nicholas Buesen.<ref name="cope1877">{{cite journal | last1 = Cope | first1 = E.D. | title = Report on the Geology of the Region of the Judith River, Montana: And on Vertebrate Fossils Obtained on Or Near the Missouri River | date = 1877 | volume = 3 | issue = 3 | journal = Bulletin of the United States Geological and Geographical Survey of the Territories | url = https://www.biodiversitylibrary.org/item/98702#page/643/mode/1up | pages = 565–598 | access-date = November 25, 2017 | archive-url = https://web.archive.org/web/20171201035534/https://www.biodiversitylibrary.org/item/98702#page/643/mode/1up | archive-date = December 1, 2017 | url-status = live }}</ref> In 1943 Welles removed ''E.{{nbsp}}serpentinus'' from ''Elasmosaurus'', and placed it in a new genus, ''Hydralmosaurus''.<ref name="welles1943">{{cite journal | last = Welles | first = S.P. | title = Elasmosaurid plesiosaurs with description of new material from California and Colorado | journal = Memoir of the University of California | volume = 13 | pages = 125–254 | date = 1943}}</ref> Subsequently, all ''Hydralmosaurus'' specimens were moved to ''[[Styxosaurus]]'' in 2016, rendering the former a ''nomen dubium''.<ref name="otero2016">{{cite journal | last1 = Otero | first1 = R.A. | date = 2016 | title = Taxonomic reassessment of ''Hydralmosaurus'' as ''Styxosaurus'': new insights on the elasmosaurid neck evolution throughout the Cretaceous | journal = PeerJ | volume = 4 | page = e1777 | doi = 10.7717/peerj.1777 | pmid = 27019781 | pmc = 4806632 | doi-access = free }}</ref> Williston published a figure of another ''E. serpentinus'' specimen in 1914;<ref name="williston1914">{{cite book | last1 = Williston | first1 = S.W. | title = Water Reptiles of the Past and Present | url = https://archive.org/details/waterreptilesofp00will | chapter = Sauropterygia | chapter-url = https://archive.org/stream/waterreptilesofp00will#page/86/mode/2up | page = [https://archive.org/details/waterreptilesofp00will/page/86 86] | date = 1914 | location = Chicago | publisher = University of Chicago Press | access-date = November 17, 2017 | archive-url = https://web.archive.org/web/20150418002133/https://archive.org/details/waterreptilesofp00will | archive-date = April 18, 2015 | url-status = live }}</ref> [[Elmer S. Riggs|Elmer Riggs]] formally described it in 1939.<ref name="riggs1939">{{cite journal|doi=10.5962/bhl.title.5289|title=A specimen of ''Elasmosaurus serpentinus''|date=1939|last1=Riggs|first1=E.S.|journal=Geological Series of the Field Museum of Natural History|series=Publication. Field Museum of Natural History |volume=6|issue=25|url=https://www.biodiversitylibrary.org/item/25222#page/5/mode/1up|pages=385–391|doi-access=free|access-date=November 17, 2017|archive-url=https://web.archive.org/web/20171201031202/https://www.biodiversitylibrary.org/item/25222#page/5/mode/1up|archive-date=December 1, 2017|url-status=live}}</ref> Welles moved this specimen to the new genus and species ''Alzadasaurus riggsi'' in 1943.<ref name="welles1943"/> [[Kenneth Carpenter]] reassigned it to ''[[Thalassomedon haningtoni]]'' in 1999;<ref name=Carpenter1999/> Sachs, Johan Lindgren, and Benjamin Kear noted that the remains represented a juvenile and were significantly distorted, and preferred to retain it as a ''nomen dubium'' in 2016.<ref name="sachs2016">{{cite conference | last1 = Sachs | first1 = S. | last2 = Lindgren | first2 = J. | last3 = Kear | first3 = B.P. | title = Re-description of ''Thalassomedon haningtoni'' – an elasmosaurid from the Cenomanian of North America | conference = 5th Triennial Mosasaur Meeting – A Global Perspective on Mesozoic Marine Amniotes | location = Uppsala | url = https://www.researchgate.net/publication/303446624 | publisher = Museum of Evolution, Uppsala University | book-title = Abstracts and Programs | date = 2016}}</ref>

Subsequently, a series of 19 neck and back vertebrae from the [[Big Bend Dam|Big Bend]] region of the Missouri{{snd}}part of the Pierre Shale formation{{snd}}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.{{nbsp}}intermedius'' in 1894.<ref name="cope1894">{{cite journal | last1 = Cope | first1 = E.D. | title = On the Structure of the Skull in the Plesiosaurian Reptilia, and on Two New Species from the Upper Cretaceous | journal = Proceedings of the American Philosophical Society | volume = 33 | issue = 144 | pages = 109–113 | date = 1894 | jstor = 983364 | url = https://www.biodiversitylibrary.org/item/94449#page/137/mode/1up}}</ref> 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.{{nbsp}}intermedius'' and ''E.{{nbsp}}platyurus''.<ref name="williston1906"/> In 1952 Welles opined that, if ''E.{{nbsp}}intermedius'' was valid, "it must be referred to a pliosaurian genus";<ref name="welles1952">{{cite journal | ref = welles-1952 | last = Welles | first = S. P. | title = A review of North American Cretaceous elasmosaurs | journal = University of California Publications in the Geological Sciences | volume = 29 | issue = 3 | pages = 47–144 | date = 1952 }}</ref> however, he proceeded to label it a ''nomen dubium'' in 1962.<ref name="welles1962"/> Three shorter vertebrae found alongside ''E.{{nbsp}}intermedius'', assigned by Cope to the new genus and species ''Embaphias circulosus'',<ref name="cope1894"/> were also considered by Welles to be a ''nomen dubium'' in 1962.<ref name="welles1962"/>

Williston named a number of other new ''Elasmosaurus'' species in his 1906 revision.<ref name="everhart2006">{{cite journal | last1 = Everhart | first1 = M.J. | title = The occurrence of elasmosaurids (Reptilia: Plesiosauria) in the Niobrara Chalk of Western Kansas | journal = Paludicola | date = 2006 | volume = 5 | issue = 4 | pages = 170–183 | url = https://www.researchgate.net/publication/40662835}}</ref> In 1874 he and Mudge discovered a specimen in Plum Creek, Kansas.<ref name="williston1906"/> While he initially assigned it in 1890 to a new species of ''Cimoliasaurus'', ''C.{{nbsp}}snowii'',<ref name="williston1890">{{cite journal | last1 = Williston | first1 = S.W. | title = Structure of the Plesiosaurian Skull | date = 1890 | journal = Science | volume = 16 | issue = 405 | pages = 262 | doi = 10.1126/science.ns-16.405.262 | pmid = 17829759 | bibcode = 1890Sci....16Q.262B | s2cid = 42251402 | url = https://zenodo.org/record/1448321 }}</ref> he subsequently recognized the elasmosaurid nature of its [[humerus]] and [[coracoid]]s. Thus, he renamed the species ''E.{{nbsp}}snowii''. A second specimen, discovered by Elias West in 1890, was also assigned by him to ''E.{{nbsp}}snowii''.<ref name="williston1906"/> In 1943 Welles moved ''E.{{nbsp}}snowii'' to its own genus, ''Styxosaurus'',<ref name="welles1943"/> where the species has remained. However, the West specimen was assigned to ''Thalassiosaurus ischiadicus'' (see below) by Welles in 1952;<ref name="welles1952"/> Carpenter returned it to ''S.{{nbsp}}snowii'' in 1999.<ref name=Carpenter1999/><ref name="everhart2006"/> Williston also reassigned the species ''E.{{nbsp}}ischiadicus'' from the genus ''[[Polycotylus]]'', where he had initially placed it when he named it in 1903. The type remains were discovered by him in the same 1874 expedition with Mudge. Williston assigned another specimen discovered by Mudge and H.{{nbsp}}A. Brous in 1876.<ref name="williston1906"/> In 1943 both specimens were assigned to the new genus ''Thalassiosaurus'' by Welles,<ref name="welles1943"/> who then assigned the latter to the new genus and species ''Alzadasaurus kansasensis'' in 1952.<ref name="welles1952"/> Glenn Storrs considered both to be indeterminate elasmosaurids in 1999;<ref name="storrs1999">{{cite journal | last1 = Storrs | first1 = G.W. | title = An examination of Plesiosauria (Diapsida: Sauropterygia) from the Niobrara Chalk (Upper Cretaceous) of central North America | journal = University of Kansas Paleontological Contributions | volume = 11 | date = 1999 | pages = 1–15 | url = https://www.academia.edu/3012858}}</ref> in the same year, Carpenter assigned both to ''Styxosaurus snowii''.<ref name=Carpenter1999/><ref name="everhart2006"/>

[[File:North American Plesiosaurs Elasmosaurus, Cimoliasaurus, and Polycotylus Plate 4.png|thumb|alt=Black and white photo of various bones|left|Remains of ''E. nobilis'' (now ''[[Styxosaurus snowii]]'')]]
An elasmosaurid specimen was found by Handel Martin in [[Logan County, Kansas]] in 1889. Williston named this as a new species, ''E.{{nbsp}}(?){{nbsp}}marshii''. He bore reservations about its referral to the genus, and he recognized that it possibly pertained to another genus.<ref name="williston1906"/> In 1943 Welles moved ''E.{{nbsp}}(?){{nbsp}}marshii'' to a genus of its own, ''Thalassonomosaurus'';<ref name="welles1943"/> however, Carpenter sunk ''T.{{nbsp}}marshii'' into ''Styxosaurus snowii'' in 1999.<ref name=Carpenter1999/> Another species, ''E.{{nbsp}}nobilis'', was named by Williston from very large remains discovered by Mudge in 1874 in [[Jewell County, Kansas]].<ref name="williston1906"/> Welles named ''E.{{nbsp}}nobilis'' as a species of ''Thalassonomosaurus'', ''T.{{nbsp}}nobilis'', in 1943,<ref name="welles1943"/> but it too was considered to be part of ''S.{{nbsp}}snowii'' by Carpenter.<ref name=Carpenter1999/> Finally, two exceptionally large dorsal vertebrae collected by [[Charles Hazelius Sternberg|Charles Sternberg]] in 1895 were named ''E.{{nbsp}}sternbergii'' by Williston, but were considered indeterminate by Storrs.<ref name="everhart2006"/><ref name="storrs1999"/> Williston mentioned three additional ''Elasmosaurus'' species, which he would figure and describe at a later date.<ref name="williston1906"/> He again made reference to a new species of ''Elasmosaurus'', from Kansas, in 1908.<ref name="willistion1908">{{cite journal | last1 = Williston | first1 = S.W. | title = North American Plesiosaurs: ''Trinacromerum'' | journal = Journal of Geology | volume = 16 | issue = 8 | date = 1908 | pages = 715–736 | jstor = 30068152 | doi = 10.1086/621573 | bibcode = 1908JG.....16..715W | url = https://zenodo.org/record/1431449 | doi-access = free | access-date = June 26, 2019 | archive-url = https://web.archive.org/web/20200412185908/https://zenodo.org/record/1431449 | archive-date = April 12, 2020 | url-status = live }}</ref>

Several [[Russia]]n species, based on poorly preserved vertebral remains, were assigned to ''Elasmosaurus'' by Nikolay N. Bogolubov in 1911. One was ''E.{{nbsp}}helmerseni'', which was first described by W.{{nbsp}}Kiprijanoff in 1882 from Maloje Serdoba, [[Saratov]], as ''Plesiosaurus helmerseni''. Some material from [[Scania]], Sweden, was assigned to ''P. helmerseni'' in 1885 by H.{{nbsp}}Schröder.<ref name="persson1959">{{cite journal | last1 = Persson | first1 = P.O. | title = Reptiles from the Senonian (U. Cret.) of Scania (S. Sweden) | journal = Arkiv för Mineralogi och Geologi | volume = 2 | issue = 35 | date = 1959 | pages = 431–519 | url = http://paleoarchive.com/literature/Persson1959-ReptilesSenonianScania.pdf | access-date = November 19, 2017 | archive-url = https://web.archive.org/web/20171201030943/http://paleoarchive.com/literature/Persson1959-ReptilesSenonianScania.pdf | archive-date = December 1, 2017 | url-status = live }}</ref> Vertebral and limb remains<ref name="pravoslavlev1918">{{cite journal | last1 = Pravoslavlev | first1 = P.A. | date = 1918 | title = Геологическое распространенiе эласмозавровъ | trans-title = Geological distribution of ''Elasmosaurus'' | journal = Bulletin of the Russian Academy of Sciences | volume = 12 | series = VI | issue = 17 | pages = 1955–1978 | url = http://www.mathnet.ru/php/archive.phtml?wshow=paper&jrnid=im&paperid=6011&option_lang=rus | language = ru | access-date = November 18, 2017 | archive-url = https://web.archive.org/web/20171201033259/http://www.mathnet.ru/php/archive.phtml?wshow=paper&jrnid=im&paperid=6011&option_lang=rus | archive-date = December 1, 2017 | url-status = live }}</ref> from [[Kursk]] initially assigned by Kiprijanoff to ''P.{{nbsp}}helmerseni'' were also moved by Bogolubov to the new species ''E.{{nbsp}}kurskensis'', which he considered to be "identical with ''Elasmosaurus'' or related to it". He also named ''E.{{nbsp}}orskensis'', based on "very large" neck and tail vertebra remains from Konopljanka, [[Orenburg]]; and ''E.{{nbsp}}serdobensis'', based on a single neck vertebra from Maloje Serdoba.<ref name="bogolubov1912">{{cite journal | last1 = Bogolubov | first1 = N.N. | date = 1912 | trans-title = The occurrence of ''Elasmosaurus'' and ''Polycotylus'' in Russian deposits | title = Sur la présence de l{{'}}''Elasmosaurus'' et du ''Polycotylus'' dans les dépots de la Russie | language = French | journal = Annuaire Géologique et Minéralogique de la Russie | volume = 14 | pages = 174–176 | translator-last1 = Wist | translator-first1 = W | url = http://paleoglot.org/files/Bogolubow%201912.pdf | access-date = November 18, 2017 | archive-url = https://web.archive.org/web/20181005180343/https://paleoglot.org/files/Bogolubow%201912.pdf | archive-date = October 5, 2018 | url-status = live }}</ref> However, the validity of all these species has been questioned. Welles considered ''E.{{nbsp}}kurskensis'' as an indeterminate plesiosaur in 1962.<ref name="welles1962"/> Persson noted in a 1959 review of the Swedish ''"E."{{nbsp}}helmerseni'' material that, while the species was probably closely related to ''Elasmosaurus'' proper, it was too fragmentary for this hypothesis to be assessed;<ref name="persson1959"/> 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.<ref name="persson1963"/> Similarly, in 1999, Evgeniy Pervushov, Maxim Arkhangelsky, and Alexander Ivanov considered ''E.{{nbsp}}helmerseni'' to be an indeterminate elasmosaurid.<ref name="pervushov1999">{{cite book | last1 = Pervushov | first1 = E. | last2 = Arkhangelsky | first2 = M.S. | last3 = Ivanov | first3 = A.V. | trans-title = Catalog of the locations of the remains of sea reptiles in the Jurassic and Cretaceous of the Lower Volga Region | doi = 10.13140/RG.2.1.5178.3760 | trans-chapter = Sauropterygia | pages = 28–34 | chapter = Завроптеригии | title = Каталог местонахождений остатков морских рептилий в юр ских и меловых отложениях Нижнего Поволжья | date = 1999 | publisher = Saratov State University Press | location = Saratov | chapter-url = https://www.researchgate.net/publication/288991642}}</ref> In 2000 Storrs, Archangelsky, and Vladimir Efimov concurred with Welles on ''E.{{nbsp}}kurskensis'', and labelled ''E.{{nbsp}}orskensis'' and ''E.{{nbsp}}serdobensis'' as indeterminate elasmosaurids.<ref name="storrs2000"/>

Two additional Russian species were described by subsequent authors. [[Anatoly Riabinin]] described a single phalanx from a flipper in 1915 as ''E.{{nbsp}}(?){{nbsp}}sachalinensis''; the species was named after the island of [[Sakhalin]], where N.{{nbsp}}N. Tikhonovich found it in 1909.<ref name="averianov2005"/> However, this specimen cannot be identified more specifically than an indeterminate elasmosaurid, which was followed by Persson<ref name="persson1963"/> and Pervushov and colleagues.<ref name="pervushov1999"/> Storrs, Arkhangelsky, and Efimov were less specific, labelling it as an indeterminate plesiosaur;<ref name="storrs2000">{{cite book | last1 = Storrs | first1 = G.W. | last2 = Arkhangelsky | first2 = M.S. | last3 = Efimov | first3 = V.M. | chapter = Mesozoic marine reptiles of Russia and other former Soviet republics | title = The Age of Dinosaurs in Russia and Mongolia | editor1-last = Benton | editor1-first = M.J. | editor2-last = Shishkin | editor2-first = M.A. | editor3-last = Unwin | editor3-first = D.M. | editor4-last = Kurochkin | editor4-first = E.N. | chapter-url = https://books.google.com/books?id=NzVGpo3M998C&pg=PA187 | isbn = 978-0-521-55476-3 | date = 2000 | publisher = Cambridge University Press | location = Cambridge | url = https://books.google.com/books?id=NzVGpo3M998C | pages = 187–210}}</ref> this classification was followed by Alexander Averianov and Vasilii Popov in 2005.<ref name="averianov2005">{{cite journal | last1 = Averianov | first1 = A. O. | last2 = Popov | first2 = V. K. | s2cid = 44328986 | title = The first plesiosaur from the Primorye krai | journal = Doklady Biological Sciences | date = 2005 | volume = 401 | issue = 1 | pages = 133–135 | doi = 10.1007/s10630-005-0056-3 | pmid = 16003869 | url = http://doc.rero.ch/record/16185/files/PAL_E3392.pdf }}</ref> Then, in 1916, [[Pavel Pravoslavlev|Pavel A. Pravoslavlev]] named ''E.{{nbsp}}amalitskii'' from the [[Don River (Russia)|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;<ref name="persson1963"/> however, like ''E.{{nbsp}}(?){{nbsp}}sachalinensis'', Pervushov and colleagues considered ''E.{{nbsp}}amalitskii'' an indeterminate elasmosaurid.<ref name="pervushov1999"/>

[[File:Mauisaurus.jpg|thumb|alt=Drawing of various bones|right|Remains of ''E. haasti'' (now ''[[Mauisaurus|Mauisaurus haasti]]'')]]
In a 1918 review of the geographic distribution and evolution of ''Elasmosaurus'', Pravoslavlev provisionally assigned three other previously named species to ''Elasmosaurus'';<ref name="pravoslavlev1918"/> his taxonomic opinions have not been widely followed. One of these was ''E.{{nbsp}}chilensis'', based on the [[Chile]]an ''Plesiosaurus chilensis'' named from a single tail vertebra by [[Claude Gay]] in 1848.<ref name="gay1848">{{cite book | last1 = Gay | first1 = C. | series = Historia Física y Política de Chile [Physical and Political History of Chile] | title = Zoologia, Vol. 2 | pages = 130–136 | chapter-url = https://www.biodiversitylibrary.org/item/130101#page/130/mode/1up | publisher = Imprenta Maulde y Renou | location = Paris |date = 1848 | trans-title = Zoology, Vol. 2 | chapter = Reptiles Fosiles | volume = 2 | trans-chapter = Fossil Reptiles| language =es}}</ref> In a work published in 1889, [[Richard Lydekker]] assigned this species to ''Cimoliasaurus''.<ref name="lydekker1889">{{cite book | last1 = Lydekker | first1 = R. | title = Catalogue of the fossil Reptilia and Amphibia in the British Museum. Part II | publisher = Printed by order of the Trustees | place = London | page = 222 | date = 1889 | url = https://www.biodiversitylibrary.org/item/125714#page/252/mode/1up }}</ref> Wilhelm Deecke moved ''chilensis'' to ''[[Pliosaurus]]'' in 1895,<ref name="deecke1895">{{cite journal | last1 = Deecke | first1 = W. | title = Ueber Saurierreste aus den Quiriquina−Schichten | trans-title = Concerning Dinosaur Remains from the Quiriquina Strata | date = 1895 | journal = Beiträge zur Geologie und Palae<!--JCW-CleanerBot-->ontologie von Südamerika | volume = 14 | pages = 32–63 |url = https://books.google.com/books?id=AgBGAQAAMAAJ&pg=PA32 | language = de}}</ref> a classification which was acknowledged by Pravoslavlev. [[Edwin H. Colbert|Edwin Colbert]] later assigned the type vertebra in 1949 to a [[pliosauroidea|pliosauroid]], and also assigned other assigned remains to indeterminate elasmosauroids;<ref name="colbert1949">{{cite journal | title = A new Cretaceous plesiosaur from Venezuela | last1 = Colbert | first1 = E.H. | journal = American Museum Novitates | issue = 1420 | pages = 1–22 | date = 1949| citeseerx = 10.1.1.1033.3285 }}</ref><ref name="otero2010">{{cite journal | last1 = Otero | first1 = R.A. | last2 = Soto-Acuña | first2 = S. | last3 = Rubilar-Rogers | first3 = D. | title = Presence of ''Mauisaurus'' in the Maastrichtian (Late Cretaceous) of central Chile | journal = Acta Palaeontologica Polonica | date = 2010 | volume = 55 | issue = 2 | pages = 361–364 | doi = 10.4202/app.2009.0065 | doi-access = free }}</ref> the type vertebra was recognized as potentially belonging to ''[[Aristonectes|Aristonectes parvidens]]'' by José O'Gorman and colleagues in 2013.<ref name="ogorman2013">{{cite journal | first1 = J.P. | last1 = O'Gorman | first2 = Z. | last2 = Gasparini | first3 = L. | last3 = Salgado | title = Postcranial morphology of ''Aristonectes'' (Plesiosauria, Elasmosauridae) from the Upper Cretaceous of Patagonia and Antarctica | journal = Antarctic Science | volume = 25 | issue = 1 | date = 2013 | doi = 10.1017/S0954102012000673 | pages = 71–82 | bibcode = 2013AntSc..25...71O | hdl = 11336/11188 | s2cid = 128417881 | hdl-access = free }}</ref> Another was ''E.{{nbsp}}haasti'', originally ''[[Mauisaurus|Mauisaurus haasti]]'', named by [[James Hector]] in 1874 based on remains found in [[New Zealand]].<ref name="hector1874">{{cite journal|last1=Hector|first1=J.|title=On the fossil Reptilia of New Zealand|journal=Transactions and Proceedings of the Royal Society of New Zealand|date=1874|volume=6|pages=333–358|url=https://www.biodiversitylibrary.org/item/108479#page/407/mode/1up}}</ref> Although its validity was supported for a considerable time, ''M.{{nbsp}}haasti'' is regarded as a ''nomen dubium'' as of 2017.<ref name="hiller2017">{{cite journal | last1 = Hiller | first1 = N. | last2 = O'Gorman | first2 = J.P. | last3 = Otero | first3 = R.A. | last4 = Mannering | first4 = A.A. | title = A reappraisal of the Late Cretaceous Weddellian plesiosaur genus ''Mauisaurus'' <small>Hector, 1874</small> | journal = New Zealand Journal of Geology and Geophysics | date = 2017 | volume = 60 | issue = 2 | doi = 10.1080/00288306.2017.1281317 | pages = 112–128 | bibcode = 2017NZJGG..60..112H | s2cid = 132037930 }}</ref> Pravoslavlev recognized another species from New Zealand, ''E.{{nbsp}}hoodii'', named by Owen in 1870 as ''Plesiosaurus hoodii'' based on a neck vertebra.<ref name="owen1870">{{cite journal | last1 = Owen | first1 = R. | title = Notice of some Saurian Fossils discovered by J. H. Hood, Esq., at Waipara, Middle Island, New Zealand | journal = Geological Magazine | volume = 7 | issue = 68 | date = 1870 | pages = 49–53 | doi = 10.1017/S0016756800209205 | bibcode = 1870GeoM....7...49O | url = https://zenodo.org/record/1810670 }}</ref> Welles recognized it as a ''nomen dubium'' in 1962;<ref name="welles1962"/> [[Joan Wiffen]] and William Moisley concurred in a 1986 review of New Zealand plesiosaurs.<ref name="wiffen1986">{{cite journal | last1 = Wiffen | first1 = J. | last2 = Moisley | first2 = W.L. | title = Late Cretaceous reptiles (Families Elasmosauridae and Pliosauridae) from the Mangahouanga Stream, North Island, New Zealand | journal = New Zealand Journal of Geology and Geophysics | volume = 29 | issue = 2 | date = 1986 | doi = 10.1080/00288306.1986.10427535 | pages = 205–252| bibcode = 1986NZJGG..29..205W }}</ref>

In 1949 Welles named a new species of ''Elasmosaurus'', ''E.{{nbsp}}morgani''. It was named from a well-preserved skeleton found in [[Dallas County, Texas]].<ref name="welles1949">{{cite journal | last1 = Welles | first1 = S.P. | title = A new elasmosaur from the Eagle Ford Shale of Texas | journal = Fondren Science Series | volume = 1 | date = 1949 | pages = 1–40 | url = https://sites.smu.edu/shulermuseum/publication_pdfs/fondren_sci/v1-Welles1949.pdf | access-date = November 19, 2017 | archive-url = https://web.archive.org/web/20171201043438/https://sites.smu.edu/shulermuseum/publication_pdfs/fondren_sci/v1-Welles1949.pdf | archive-date = December 1, 2017 | url-status = live }}</ref> However, part of the specimen was accidentally thrown out during the relocation of the [[Southern Methodist University]]'s paleontological collections.<ref name="sachs2015"/> Welles recognized ''E.{{nbsp}}morgani''{{'}}s similarity to ''E.{{nbsp}}platyurus'' in its shoulder girdle, but maintained it as a separate species due to its shorter neck and more robust rear neck vertebrae.<ref name="welles1949"/> In 1997 Carpenter reconsidered the differences between the two species, and found them sufficient to place ''E.{{nbsp}}morgani'' in its own genus, which he named ''[[Libonectes]]''.<ref name="carpenter1997">{{cite book | last1 = Carpenter | first1 = K. | date = 1997 | chapter = Comparative cranial anatomy of two North American plesiosaurs | title = Ancient Marine Reptiles | url = https://archive.org/details/ancientmarinerep00call | url-access = limited | editor1-first = J.M. | editor1-last = Callaway | editor2-first = E.L. | editor2-last = Nicholls | pages = [https://archive.org/details/ancientmarinerep00call/page/n237 191]–216 | location = San Diego | publisher = Academic Press | doi = 10.1016/B978-012155210-7/50011-9| isbn = 9780121552107 }}</ref> Despite its reassignment and the loss of its material, ''L.{{nbsp}}morgani'' is often considered an archetypal elasmosaurid. Data based on these lost elements were unquestionably accepted in subsequent [[phylogeny|phylogenetic analyses]], until a redescription of the surviving elements was published by Sachs and Benjamin Kear in 2015.<ref name="sachs2015">{{cite journal | last1 = Sachs | first1 = S. | last2 = Kear | first2 = B.P. | title = Postcranium of the paradigm elasmosaurid plesiosaurian ''Libonectes morgani'' <small>(Welles, 1949)</small> | journal = Geological Magazine | volume = 152 | issue = 4 | date = 2015 | pages = 694–710 | doi = 10.1017/S0016756814000636 | bibcode = 2015GeoM..152..694S | s2cid = 83740713 }}</ref>

Persson assigned another species to ''Elasmosaurus'' alongside his 1959 description of ''"E."{{nbsp}}helmerseni'' remains from Sweden, namely ''E.{{nbsp}}(?){{nbsp}}gigas''. It was based on Schröder's ''Pliosaurus{{nbsp}}(?) gigas'', named in 1885 from two dorsals; one was found in [[Prussia (region)|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 instead agreed well with elasmosaurids. Given that, at the time of Persson's writing, "there [was] nothing to contradict that they are nearest akin to ''Elasmosaurus''", he assigned them to ''Elasmosaurus'' "with hesitation". Theodor Wagner had previously assigned ''gigas'' to ''Plesiosaurus'' in 1914.<ref name="persson1959"/> As of 2013, this questionable attribution remains unchanged.<ref name="sorenson2013">{{cite journal | last1 = Sørensen | first1 = A.M. | last2 = Surlyk | first2 = F. | last3 = Lindgren | first3 = J. | title = Food resources and habitat selection of a diverse vertebrate fauna from the upper lower Campanian of the Kristianstad Basin, southern Sweden | date = 2013 | volume = 42 | journal = Cretaceous Research | pages = 85–92 | doi = 10.1016/j.cretres.2013.02.002 | bibcode = 2013CrRes..42...85S | url = https://www.academia.edu/5004617}}</ref> Another species from Russia, ''E.{{nbsp}}antiquus'', was named by Dubeikovskii and Ochev in 1967<ref name="storrs2000"/> 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.<ref name="pervushov1999"/><ref name="storrs2000"/>

==Classification==
[[File:Vertebrae of Elasmosaurus and Cimoliosaurus.jpg|thumb|left|Neck and back vertebrae of ''[[Cimoliasaurus]]'', above, and ''Elasmosaurus'', figured by Cope, 1869|alt=Drawing of vertebrate on a white background]]
Though Cope had originally recognized ''Elasmosaurus'' as a plesiosaur, in an 1869 paper he placed it, with ''Cimoliasaurus'' and ''[[Crymocetus]]'', in a new order of [[sauropterygian]] reptiles. He named the group Streptosauria, or "reversed lizards", due to the orientation of their individual vertebrae supposedly being reversed compared to what is seen in other vertebrate animals.<ref name="Page">{{cite journal|last1=Storrs|first1=G. W.|title=''Elasmosaurus platyurus'' and a page from the Cope-Marsh war|url=https://www.researchgate.net/publication/236201644|journal=Discovery|date=1984|volume=17|issue=2|pages=25–27}}</ref><ref>{{cite journal | last = Cope | first = E. D. | title = On the reptilian orders, Pythonomorpha and Streptosauria | journal = Proceedings of the Boston Society of Natural History | volume = 12 | pages = 250–266 | year = 1869 | url = http://oceansofkansas.com/cope1869a.html | access-date = November 23, 2017 | archive-url = https://web.archive.org/web/20150115042557/http://oceansofkansas.com/Cope1869a.html | archive-date = January 15, 2015 | url-status = live }}</ref> He subsequently abandoned this idea in his 1869 description of ''Elasmosaurus'', where he stated he had based it on Leidy's erroneous interpretation of ''Cimoliasaurus''. In this paper, he also named the new family Elasmosauridae, containing ''Elasmosaurus'' and ''Cimoliasaurus'', without comment. Within this family, he considered the former to be distinguished by a longer neck with compressed vertebrae, and the latter by a shorter neck with square, depressed vertebrae.<ref name="Synopsis"/>
[[File:The Snake-necked Elasmosaurus.jpg|thumb|Outdated restoration of two individuals with curled, snake-like necks, by [[Charles R. Knight]], 1897|alt=Old paleoart of two elasmosaurs with inaccurately curled necks. One is in the foreground with a fish in its mouth and the another chasing fish in background.]]
In subsequent years, Elasmosauridae came to be one of three groups in which plesiosaurs were classified, the others being the [[Pliosauridae]] and [[Plesiosauridae]] (sometimes merged into one group).<ref name="okeefe2001">{{cite journal | first1 = F.R. | last1 = O'Keefe | title = A cladistic analysis and taxonomic revision of the Plesiosauria (Reptilia: Sauropterygia) | journal = Acta Zoologica Fennica | volume = 213 | isbn = 978-951-9481-58-6 | issn = 0001-7299 | s2cid = 82936031 | pages = 1–63 | date = 2001 | url = http://mds.marshall.edu/cgi/viewcontent.cgi?article=1051&context=bio_sciences_faculty | access-date = November 26, 2017 | archive-url = https://web.archive.org/web/20171201033431/http://mds.marshall.edu/cgi/viewcontent.cgi?article=1051&context=bio_sciences_faculty | archive-date = December 1, 2017 | url-status = live }}</ref> [[Charles William Andrews|Charles Andrews]] elaborated on differences between elasmosaurids and pliosaurids in 1910 and 1913. He characterized elasmosaurids by their long necks and small heads, as well as by their rigid and well-developed scapulae (but atrophied or absent clavicles and interclavicles) for forelimb-driven locomotion. Meanwhile, pliosaurids had short necks but large heads, and used hindlimb-driven locomotion.<ref name="andrews1910">{{cite book | last1 = Andrews | first1 = C.W. | title = A Descriptive Catalogue of the Marine Reptiles of the Oxford Clay | date = 1910 | publisher = British Museum (Natural History) |lccn=11013249| location = London | chapter-url = https://archive.org/details/descriptivecatal01brit | chapter = Introduction | pages =v–xvii}}</ref><ref name="andrews1913">{{cite book | last1 = Andrews | first1 = C.W. | title = A Descriptive Catalogue of the Marine Reptiles of the Oxford Clay | date = 1913 | publisher = British Museum (Natural History) | location = London | chapter-url = https://archive.org/details/descriptivecatal02brit | chapter = Introduction | pages =v–xvi}}</ref> Although the placement of ''Elasmosaurus'' in the Elasmosauridae remained uncontroversial, opinions on the relationships of the family became variable over subsequent decades. Williston created a revised taxonomy of plesiosaurs in 1925.<ref name="williston1925">{{cite book | last1 = Williston | first1 = S.W. | editor1-last = Gregory | editor1-first = W.K. | title = The Osteology of the Reptiles | date = 1925 | location = Cambridge | publisher = Harvard University Press |lccn=25019418| chapter = The Subclass Synaptosauria | pages = 246–252 | isbn = 9780353315594 | chapter-url = https://archive.org/details/osteologyofrepti00will}}</ref>

In 1940 Theodore White published a hypothesis on the interrelationships between different plesiosaurian families. He considered Elasmosauridae to be closest to the Pliosauridae, noting their relatively narrow coracoids as well as their lack of interclavicles or clavicles. His diagnosis of the Elasmosauridae also noted the moderate length of the skull (i.e., a mesocephalic skull); the neck ribs having one or two heads; the scapula and coracoid contacting at the midline; the blunted rear outer angle of the coracoid; and the pair of openings (fenestrae) in the scapula–coracoid complex being separated by a narrower bar of bone compared to pliosaurids. The cited variability in the number of heads on the neck ribs arises from his inclusion of ''[[Simolestes]]'' to the Elasmosauridae, since the characteristics of "both the skull and shoulder girdle compare more favorably with ''Elasmosaurus'' than with ''Pliosaurus'' or ''[[Peloneustes]]''." He considered ''Simolestes'' a possible ancestor of ''Elasmosaurus''.<ref name="white1940">{{cite journal | first1 = T.E. | last1 = White | date = 1940 | journal = Journal of Paleontology | title = Holotype of ''Plesiosaurus longirostris'' Blake and Classification of the Plesiosaurs | volume = 14 | issue = 5 | pages = 451–467 | jstor = 1298550}}</ref> [[Oskar Kuhn]] adopted a similar classification in 1961.<ref name="persson1963"/>
[[File:Elasmosaurus at Centennial Centre for Interdisciplinary Science.jpg|left|thumb|alt=Gray skeleton with a long neck hanging form a ceiling|Reconstructed skeleton, Centennial Centre for Interdisciplinary Science]]
Welles took issue with White's classification in his 1943 revision of plesiosaurs, noting that White's characteristics are influenced by both preservation and [[ontogeny]]. He divided plesiosaurs into two superfamilies, the [[Plesiosauroidea]] and Pliosauroidea, based on neck length, head size, ischium length, and the slenderness of the humerus and femur (the propodialia). Each superfamily was further subdivided by the number of heads on the ribs, and the proportions of the epipodialia. Thus, elasmosaurids had long necks, small heads, short ischia, stocky propodialia, single-headed ribs, and short epipodialia.<ref name="welles1943"/> Pierre de{{nbsp}}Saint-Seine in 1955 and [[Alfred Romer]] in 1956 both adopted Welles' classification.<ref name="persson1963"/> In 1962 Welles further subdivided elasmosaurids based on whether they possessed pelvic bars formed from the fusion of the ischia, with ''Elasmosaurus'' and ''[[Brancasaurus]]'' being united in the subfamily Elasmosaurinae by their sharing of completely closed pelvic bars.<ref name="welles1962"/>

Carpenter's 1997 phylogenetic analysis of plesiosaurs challenged the traditional subdivision of plesiosaurs based on neck length. While polycotylids had previously been part of the Pliosauroidea, Carpenter moved polycotylids to become the [[sister group]] of the elasmosaurids based on cranial similarities, thus implying that polycotylids and pliosauroids evolved their short necks independently.<ref name="carpenter1997"/> The content of Elasmosauridae also received greater scrutiny. Since its initial assignment to the Elasmosauridae, the relationships of ''Brancasaurus'' had been considered well supported, and an elasmosaurid position was recovered by O'Keefe's 2004 analysis<ref name="okeefe2004">{{cite journal | first1 = F.R. | last1 = O'Keefe | title = Preliminary description and phylogenetic position of a new plesiosaur (Reptilia: Sauropterygia) from the Toarcian of Holzmaden, Germany | journal = Journal of Paleontology | volume = 78 | issue = 5 | pages = 973–988 | doi = 10.1666/0022-3360(2004)078<0973:PDAPPO>2.0.CO;2 | year = 2004 | bibcode = 2004JPal...78..973O | s2cid = 53590349 | url = http://doc.rero.ch/record/14995/files/PAL_E2146.pdf }}</ref> and Franziska Großmann's 2007 analysis.<ref name="grossman2007">{{cite journal | first1 = F. | last1 = Großman | title = The taxonomic and phylogenetic position of the Plesiosauroidea from the Lower Jurassic Posidonia Shale of south-west Germany | journal = Palaeontology | volume = 50 | issue = 3 | date = 2007 | doi = 10.1111/j.1475-4983.2007.00654.x | pages = 545–564 | bibcode = 2007Palgy..50..545G | doi-access = free }}</ref> However, Ketchum and Benson's analysis instead included it in the Leptocleidia,<ref name="ketchum2010">{{cite journal | last1= Ketchum | first1 = H.F. |last2 = Benson | first2 = R.B.J. |year=2010 |title = Global interrelationships of Plesiosauria (Reptilia, Sauropterygia) and the pivotal role of taxon sampling in determining the outcome of phylogenetic analyses | journal = Biological Reviews | volume = 85 | issue = 2 | pages = 361–392 | doi = 10.1111/j.1469-185X.2009.00107.x | pmid = 20002391 | s2cid = 12193439 }}</ref> and its inclusion in that group has remained consistent in subsequent analyses.<ref name="benson2013">{{cite journal | last1 = Benson | first1 = R.B.J. | last2 = Ketchum | first2 = H.F. | last3 = Naish | first3 = D. | last4 = Turner | first4 = L.E. | s2cid = 18562271 | title = A new leptocleidid (Sauropterygia, Plesiosauria) from the Vectis Formation (Early Barremian–early Aptian; Early Cretaceous) of the Isle of Wight and the evolution of Leptocleididae, a controversial clade | journal = Journal of Systematic Palaeontology | volume = 11 | issue = 2 | date = 2013 | doi = 10.1080/14772019.2011.634444 | pages = 233–250| bibcode = 2013JSPal..11..233B }}</ref><ref name="benson2014">{{cite journal | last1 = Benson | first1 = R.B.J. | last2 = Druckenmiller | first2 = P.S. | title = Faunal turnover of marine tetrapods during the Jurassic–Cretaceous transition | volume = 89 | issue = 1 | journal = Biological Reviews | pages = 1–23 | date = 2014 | doi = 10.1111/brv.12038| pmid = 23581455 | s2cid = 19710180 }}</ref><ref name="otero2016"/> Their analysis also moved ''Muraenosaurus'' to the Cryptoclididae, and ''[[Microcleidus]]'' and ''Occitanosaurus'' to the Plesiosauridae;<ref name="ketchum2010"/> Benson and Druckenmiller isolated the latter two in the group [[Microcleididae]] in 2014, and considered ''Occitanosaurus'' a species of ''Microcleidus''.<ref name="benson2014"/> These genera had all previously been considered to be elasmosaurids by Carpenter, Großmann, and other researchers.<ref name=Carpenter1999/><ref name="grossman2007"/><ref name="bardet1999">{{cite journal | last1 = Bardet | first1 = N. | last2 = Godefroit | first2 = P. | last3 = Sciau | first3 = J. | title = A new elasmosaurid plesiosaur from the Lower Jurassic of southern France | journal = Palaeontology | volume = 42 | issue = 5 | date = 1999 | doi = 10.1111/1475-4983.00103 | pages = 927–952 | bibcode = 1999Palgy..42..927B | s2cid = 129719346 | url = http://doc.rero.ch/record/14772/files/PAL_E1900.pdf }}</ref><ref name="gasparini2003">{{cite journal | last1 = Gasparini | first1 = Z. | last2 = Bardet | first2 = N. | last3 = Martin | first3 = J.E. | last4 = Fernandez | first4 = M.S. | title = The elasmosaurid plesiosaur ''Aristonectes'' Cabreta from the Latest Cretaceous of South America and Antarctica | journal = Journal of Vertebrate Paleontology | volume = 23 | issue = 1 | pages = 104–115 | doi = 10.1671/0272-4634(2003)23[104:TEPACF]2.0.CO;2 | year = 2003 | s2cid = 85897767 }}</ref>

Within the Elasmosauridae, ''Elasmosaurus'' itself has been considered a "wildcard taxon" with highly variable relationships.<ref name="serratos2017"/> Carpenter's 1999 analysis suggested that ''Elasmosaurus'' was more [[basal (phylogenetics)|basal]] (i.e. less specialized) than other elasmosaurids with the exception of ''Libonectes''.<ref name=Carpenter1999/> In 2005 Sachs suggested that ''Elasmosaurus'' was closely related to ''Styxosaurus'',<ref name=Sachs2005/> and in 2008 Druckenmiller and Russell placed it as part of a [[polytomy]] with two groups, one containing ''Libonectes'' and ''[[Terminonatator]]'', the other containing ''[[Callawayasaurus]]'' and ''[[Hydrotherosaurus]]''.<ref name="druckenmiller2008">{{cite book |last1=Druckenmiller |first1=P.S. |last2=Russell |first2=A.P. |year=2007 |title=A phylogeny of Plesiosauria (Sauropterygia) and its bearing on the systematic status of ''Leptocleidus'' Andrews, 1922 |url=http://mapress.com/zootaxa/2008/f/z01863p120f.pdf |publisher=Zootaxa |volume=1863 |pages=1–120 |isbn=978-1-86977-262-8 |issn=1175-5334 |doi=10.11646/zootaxa.1863.1.1 |access-date=December 9, 2017 |archive-url=https://web.archive.org/web/20190724232752/https://www.mapress.com/zootaxa/2008/f/z01863p120f.pdf |archive-date=July 24, 2019 |url-status=live }}</ref> Ketchum and Benson's 2010 analysis included ''Elasmosaurus'' in the former group.<ref name="ketchum2010"/> Benson and Druckenmiller's 2013 analysis (below, left) further removed ''Terminonatator'' from this group and placed it as one step more derived.<ref name="benson2013"/> In Rodrigo Otero's 2016 analysis based on a modification of the same dataset (below, right), ''Elamosaurus'' was the closest relative of ''Albertonectes'', forming the [[Styxosaurinae]] with ''Styxosaurus'' and ''Terminonatator''.<ref name="otero2016"/> Danielle Serratos, Druckenmiller, and Benson could not resolve the position of ''Elasmosaurus'' in 2017, but they noted that Styxosaurinae would be a [[synonym (taxonomy)|synonym]] of Elasmosaurinae if ''Elasmosaurus'' did fall within the group.<ref name="serratos2017">{{cite journal|doi=10.1080/02724634.2017.1278608 | last1=Serratos |first1=D.J. |last2=Druckenmiller |first2=P. |last3=Benson |first3=R.B.J. | s2cid=132717607 |title=A new elasmosaurid (Sauropterygia, Plesiosauria) from the Bearpaw Shale (Late Cretaceous, Maastrichtian) of Montana demonstrates multiple evolutionary reductions of neck length within Elasmosauridae | journal=Journal of Vertebrate Paleontology |date=2017 |page=e1278608 |volume=37 |issue=2 | bibcode=2017JVPal..37E8608S | url=https://ora.ox.ac.uk/objects/uuid:75843f95-d662-4d23-adca-ce9786fe0195 }}</ref> In 2020, O'Gorman formally synonymized Styxosaurinae with Elasmosaurinae based on the inclusion of ''Elasmosaurus'' within the group, and also provided a list of diagnostic characteristics for the clade.<ref name="ogorman2020">{{cite journal |last=O'Gorman |first=J.P. |s2cid=215756238 |year=2020 |title=Elasmosaurid phylogeny and paleobiogeography, with a reappraisal of ''Aphrosaurus furlongi'' from the Maastrichtian of the Moreno Formation |journal=Journal of Vertebrate Paleontology |volume=39 |issue=5 |pages=e1692025 |doi=10.1080/02724634.2019.1692025|bibcode=2019JVPal..39E2025O |url=https://figshare.com/articles/dataset/Elasmosaurid_phylogeny_and_paleobiogeography_with_a_reappraisal_of_i_Aphrosaurus_furlongi_i_from_the_Maastrichtian_of_the_Moreno_Formation/11980359 |url-access=subscription }}</ref>
[[File:Lost Elasmosaurus vertebra.png|thumb|"Lost" incomplete vertebra, which, when it was rediscovered in 2013, increased the neck vertebra count to 72, a distinct feature of this genus|alt=Photo of vertebrate on a white background]]
{{col-begin|width=75%}}
{{col-break}}

'''Topology A:''' Benson ''et al.'' (2013)<ref name="benson2013"/>
{{clade| style=font-size:90%; line-height:90%;
|1={{clade
|label2=[[Elasmosauridae]]
|1={{clade
|label2=[[Leptocleidia]]
|1=[[Cryptoclididae]]
|2={{clade
|1=[[Leptocleididae]]
|2=[[Polycotylidae]]}}}}
|2={{clade
|1=''[[Thalassomedon]]''
|2={{clade
|1={{clade
|1=''[[Libonectes]]''
|2='''''Elasmosaurus'''''}}
|2={{clade
|1=''[[Terminonatator]]''
|2={{clade
|1={{clade
|1=''[[Styxosaurus]]''
|2=''[[Hydrotherosaurus]]'' }}
|2={{clade
|1=''[[Callawayasaurus]]''
|2={{clade
|1=''[[Eromangasaurus]]''
|2={{clade
|1=''[[Kaiwhekea]]''
|2=''[[Aristonectes]]'' }} }} }} }} }} }} }} }} }}
{{col-break}}

'''Topology B:''' Otero (2016),<ref name="otero2016"/> with clade names following O'Gorman (2020)<ref name="ogorman2020"/>
{{clade| style=font-size:90%; line-height:90%;
|label1=[[Cryptoclidia]]
|1={{clade
|label2=[[Cryptoclidia|Xenopsaria]]
|1=[[Cryptoclididae]]
|2={{clade
|label1=[[Leptocleidia]]
|label2=[[Elasmosauridae]]
|1={{clade
|1=[[Leptocleididae]]
|2=[[Polycotylidae]]}}
|2={{clade
|1=''[[Eromangasaurus]]''
|2={{clade
|1=''[[Callawayasaurus]]''
|2={{clade
|1=''[[Libonectes]]''
|2={{clade
|1={{clade
|1=''[[Tuarangisaurus]]''
|2=''[[Thalassomedon]]'' }}
|2={{clade
|1=Specimen CM Zfr 115
|2={{clade
|1=''[[Hydrotherosaurus]]''
|2=''[[Futabasaurus]]'' }} }}
|3={{clade
|label1=[[Aristonectinae]]
|1={{clade
|1=''[[Kaiwhekea]]''
|2={{clade
|1=''[[Alexandronectes]]''
|2={{clade
|1=''[[Morturneria]]''
|2=''[[Aristonectes]]''}} }} }}
|label2=[[Elasmosaurinae]]
|2={{clade
|1=''[[Terminonatator]]''
|2={{clade
|1={{clade
|1='''''Elasmosaurus'''''
|2=''[[Albertonectes]]'' }}
|2=''[[Styxosaurus]]'' }} }} }} }} }} }} }} }} }} }}
{{col-end}}

==Paleobiology==
[[File:Elasmosaurus NT.jpg|thumb|Restoration showing two individuals swimming with straight necks|alt=Drawing of two elasmosaurs underwater.]]
Elasmosaurids were fully adapted to life in the ocean, with streamlined bodies and long paddles that indicate they were active swimmers.<ref name="Focus"/> The unusual body structure of elasmosaurids would have limited the speed at which they could swim, and their paddles may have moved in a manner similar to the movement of oars rowing, and due to this, could not twist and were thus held rigidly.{{sfn|Everhart|2005a|pages=133–135}} Plesiosaurs were even believed to have been able to maintain a constant and high body temperature ([[homeothermy]]), allowing for sustained swimming.<ref>{{Cite journal| doi = 10.1111/j.1095-8312.2012.02002.x| issn = 0024-4066| volume = 108| issue = 1| pages = 3–21| last = Houssaye| first = A.| title = Bone histology of aquatic reptiles: what does it tell us about secondary adaptation to an aquatic life?| journal = Biological Journal of the Linnean Society| date = January 1, 2013| doi-access = | s2cid = 82741198}}</ref>

A 2015 study concluded that locomotion was mostly done by the fore-flippers while the hind-flippers functioned in maneuverability and stability;<ref>{{cite journal|author1=Liu, S. |author2=Smith, A. S. |author3=Gu, Y. |author4=Tan, J. |author5=Liu, K. |author6=Turk, G. |year=2015|title=Computer Simulations Imply Forelimb-Dominated Underwater Flight in Plesiosaurs|journal=PLOS Computational Biology|volume=11|issue=12|page=e1004605|doi=10.1371/journal.pcbi.1004605|pmid=26683221|pmc=4684205|bibcode=2015PLSCB..11E4605L |doi-access=free }}</ref> a 2017 study concluded that the hind-flippers of plesiosaurs produced 60% more thrust and had 40% more efficiency when moving in harmony with the fore-flippers.<ref>{{cite journal|author1=Muscutt, L. E. |author2=Dyke, G. |author3=Weymouth, G. D. |author4=Naish, D. |author5=Palmer, C. |author6=Ganapathisubramani, B. |year=2017|title=The four-flipper swimming method of plesiosaurs enabled efficient and effective locomotion|journal=Proceedings of the Royal Society B: Biological Sciences|volume=284|issue=1861|page=20170951|doi=10.1098/rspb.2017.0951|pmid=28855360|pmc=5577481}}</ref> The paddles of plesiosaurs were so rigid and specialized for swimming that they could not have come on land to lay eggs like [[sea turtle]]s. Therefore, they probably gave live-birth ([[viviparity]]) to their young like some species of [[sea snake]]s.{{sfn|Everhart|2005a|page=140}} Evidence for live-birth in plesiosaurs is provided by the fossil of an adult ''Polycotylus'' with a single fetus inside.<ref name="Okeefe2011">{{cite journal|author=O'Keefe, F. R. |author2=Chiappe, L. M.|s2cid=36165835|year=2011|title=Viviparity and K-selected life history in a Mesozoic marine plesiosaur (Reptilia, Sauropterygia)|journal=Science|volume=333|issue=6044|pages=870–873|doi=10.1126/science.1205689|pmid=21836013|bibcode=2011Sci...333..870O|url=https://www.researchgate.net/publication/51566068}}</ref>

Elasmosaurid remains provide some evidence they were preyed upon. A humerus of an unidentified subadult elasmosaurid was found with bite marks matching the teeth of the shark ''[[Cretoxyrhina]]'',<ref>{{cite journal|author=Everhart, M.|year=2005|title=Bite marks on an elasmosaur (Sauropterygia; Plesiosauria) paddle from the Niobrara Chalk (Upper Cretaceous) as probable evidence of feeding by the lamniform shark, ''Cretoxyrhina mantelli''|journal=Journal of Vertebrate Paleontology|volume=2|issue=2|pages=14–22|url=https://www.researchgate.net/publication/237263120}}</ref> while a crushed ''[[Eromangasaurus]]'' skull<ref>{{cite journal|year=2005|author=Kear, B. P.|title=A new elasmosaurid plesiosaur from the Lower Cretaceous of Queensland, Australia|journal=Journal of Vertebrate Paleontology|volume=25|issue=4|pages=792–805|doi=10.1671/0272-4634(2005)025[0792:ANEPFT]2.0.CO;2|s2cid=86297695|url=https://www.researchgate.net/publication/228657659}}</ref> has tooth-marks matched to the pliosaur ''[[Kronosaurus]]''.<ref>{{cite journal|author1=Thulborn, T. |author2=Turner, S. |year=1993|title=An elasmosaur bitten by a pliosaur|journal=Modern Geology|volume=18|pages=489–501|url=https://www.researchgate.net/publication/291825992}}</ref>

===Neck movement and function===
[[File:Image from page 90 of "Water reptiles of the past and present" (1914) (14749987246).jpg|thumb|left|Outdated restoration of ''Elasmosaurus'' showing its neck raised above water, by [[Samuel Wendell Williston]], 1914|alt=Old inaccurate drawing of elasmosaur raising its neck from ocean with pterosaurs, mosasaurs, swimming birds, and other elasmosaurs and a cliff in foreground and background]]
Cope, in 1869, compared the build and habits of ''Elasmosaurus'' with those of a snake. Although he suggested that the vertebral column of the trunk did not allow for much vertical movement due to the elongated neural spines which nearly form a continuous line with little space between adjacent vertebrae, he envisaged the neck and tail to have been much more flexible: "The snake-like head was raised high in the air, or depressed at the will of the animal, now arched [[swan]]-like preparatory to a plunge after a fish, now stretched in repose on the water or deflexed in exploring the depths below".<ref name="Synopsis"/>

Although followed by many common media depictions, more recent research showed that ''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 have raised its head and neck above the water only when in shallow water, where it could rest its body on the bottom. Also, 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. The head and shoulders of the ''Elasmosaurus'' probably 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'' would have been unable to swim in one direction while moving its head and neck either horizontally or vertically in a different direction.{{sfn|Everhart|2005a|pages=132–133}}

[[File:Elasmosaurid Neck Flexibility.svg|thumb|right|Chart showing several hypotheses on the neck flexibility of elasmosaurids, using ''Elasmosaurus'' as a model. A – Swan-like neck held upright, B – Ramrod straight neck held out directly in front, C – Downward curve for feeding on benthic prey items, D – Wide horizontal curve, E – Serpentine undulating position. Neck positions B–E would have been within the estimated neck flexibility ranges.<ref name="Zammit2008"/>|alt=Five gray silhouettes of elasmosaurs in different neck positions on a white background]]
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 probably 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 that required more than 360° of vertical flexion.<ref name="Zammit2008"/>

The exact function of the neck of elasmosaurids is unknown,<ref name="Focus"/> though it may have been important for hunting.{{sfn|Everhart|2005a|pages=133–135}} It has also been suggested that the long necks of plesiosaurs served as a snorkel and allowed them 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 slope to allow them to breathe at the surface but would have required them to engage in energy-expensive swimming at the sub-surface. In addition, the longer neck would also have increased [[dead space (physiology)|dead space]], and the animals may have required larger lungs. The neck could have had other vulnerabilities, for example being a target for predators.<ref>{{cite journal|author=Noè, L. F.|author2=Taylor, M. A.|author3=Gómez-Pérez, M.|year=2017|title=An integrated approach to understanding the role of the long neck in plesiosaurs|journal=Acta Palaeontologica Polonica|volume=62|issue=1|pages=137–162|doi=10.4202/app.00334.2016|url=https://www.app.pan.pl/archive/published/app62/app003342016.pdf|access-date=July 24, 2017|archive-url=https://web.archive.org/web/20170729190105/https://www.app.pan.pl/archive/published/app62/app003342016.pdf|archive-date=July 29, 2017|url-status=live|doi-access=free}}</ref> 

Simulation of water flow on 3D models showed that more elongated necks, such as those of elasmosaurids, did not increase drag force while swimming compared to shorter necked plesiosaurs. On the other hand, bending the neck sideways did increase drag force, more so in forms with very long necks.<ref name="hydrodynamics">{{cite journal |last1=Troelsen |first1=P. V. |last2=Wilkinson |first2=D. M. |last3=Seddighi |first3=M. |last4=Allanson |first4=D. R. |last5=Falkingham |first5=P. L. |s2cid=181587237 |title=Functional morphology and hydrodynamics of plesiosaur necks: Does size matter? |journal=Journal of Vertebrate Paleontology |volume=39 |issue=2 |date=2019 |pages=e1594850 |doi=10.1080/02724634.2019.1594850 |bibcode=2019JVPal..39E4850T |url=http://researchonline.ljmu.ac.uk/id/eprint/10242/1/Troelsen%20et%20al.pdf }}</ref> Another study found the long necks of elasmosaurs would normally increase drag during forward swimming but this was cancelled out by their large torsos, and hence large body sizes may have facilitated the evolution of longer necks.<ref>{{cite journal|author=Gutarra, S.|author2=Stubbs, T. L.|author3=Moon, B. C.|author4=Palmer, C.|author5=Benton, M. J.|year=2022|title=Large size in aquatic tetrapods compensates for high drag caused by extreme body proportions|journal=Communications Biology|volume=5|issue=1|page=380|doi=10.1038/s42003-022-03322-y|pmid=35484197 |pmc=9051157 }}</ref>

===Feeding===
[[File:Image from page 103 of "Water reptiles of the past and present" (1914) (14772670092).jpg|thumb|left|[[Gastroliths]] and bones (right) of an undetermined [[plesiosaur]] from Kansas|alt=Old picture of bones and stones on a black background]]
In 1869 Cope noted that scales and teeth of six species of fish had been discovered directly beneath the vertebrae of the ''Elasmosaurus'' holotype, and theorized that these fish would have had formed the diet of the animal. From these remains, Cope named a new species of [[barracuda]], ''Sphyraena carinata''.<ref name="Synopsis"/>

The flexion ranges of ''Elasmosaurus'' necks would have allowed the animal 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]], retracting their necks to launch a strike or using side-swipe motions to stun or kill prey with their laterally projected teeth (like [[sawshark]]s).<ref name="Zammit2008">{{cite journal |author=Zammit M. |author2=Daniels, C. B. |author3=Kear, B. P.|year=2008|title=Elasmosaur (Reptilia: Sauropterygia) neck flexibility: Implications for feeding strategies|journal=Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology|volume=150|issue=2|pages=124–130|doi=10.1016/j.cbpa.2007.09.004|pmid=17933571|url=http://doc.rero.ch/record/16518/files/PAL_E3409.pdf }}</ref> It has also been suggested that the predatory abilities of elasmosaurids have been underestimated; their large skulls, big jaw-muscles, strong jaws, and long teeth indicate they could prey on animals between {{convert|30|cm|in|sp=us}} and {{convert|2|m|ft|sp=us}} long, as indicacted by stomach contents including those of sharks, fish, [[mosasaurs]], and [[cephalopods]].<ref name="Naish">{{cite book |last1=Naish |first1=D. |title=Ancient Sea Reptiles: Plesiosaurs, Ichthyosaurs, Mosasaurs, and More |year=2023 |publisher=Smithsonian Books |location=Washington, DC |isbn=978-1588347275 |pages=142–147}}</ref>

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 at the top of the head and allowed them to see directly upward. This [[stereoscopic vision]] would have helped it to find small prey. Hunting from below would also have been possible, with prey silhouetted in the sunlight while concealed in the dark waters below. Elasmosaurids probably ate small [[bony fish]] and [[marine invertebrates]], as their small, non-[[Cranial kinesis|kinetic]] skulls would have limited the size of the prey they could eat. Also, with their long, slender teeth adapted for seizing prey and not tearing, elasmosaurids most certainly swallowed their prey whole.{{sfn|Everhart|2005a|pages=133–135}}<ref name="Zammit2008"/>

Although elasmosaurids are commonly found with several gastroliths, ''Elamosaurus'' has only been found uncontroversially with a pebble lodged in the neural arch of one of its hindmost tail-vertebrae.<ref name=missing/> A specimen of the closely related ''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 {{convert|600|km|mi|sp=us}} away from where the specimen was found.<ref>{{cite journal|author1=Cicimurri, D. J.|author2=Everhart, M.|year=2001|title=An elasmosaur with stomach contents and gastroliths from the Pierre Shale (Late Cretaceous) of Kansas|journal=Transactions of the Kansas Academy of Science|volume=104|issue=3–4|pages=129–143|doi=10.1660/0022-8443(2001)104[0129:AEWSCA]2.0.CO;2|s2cid=86037286 }}</ref> Several different functions have been proposed for gastroliths, including aiding in digestion, mixing food content, mineral supplementation, and storage and buoyancy control.<ref>{{cite journal|author=Wings, O.|year=2007|title=A review of gastrolith function with implications for fossil vertebrates and a revised classification|journal=Acta Palaeontologica Polonica|volume=52|issue=1|pages=1–16|url=http://www.dinosaurhunter.org/files/app-2007-wings-gastrolith_function_classification.pdf|access-date=December 29, 2017|archive-url=https://web.archive.org/web/20160304062304/http://www.dinosaurhunter.org/files/app-2007-wings-gastrolith_function_classification.pdf|archive-date=March 4, 2016|url-status=live}}</ref>

==Paleoecology==
[[File:Map of North America with the Western Interior Seaway during the Campanian (Upper Cretaceous).png|thumb|Map of the [[Western Interior Seaway]] during the [[Late Cretaceous]] about 80{{nbsp}}million years ago|alt=Diagram North America, with blue coloring in the middle of the continent representing an ancient sea.]]
''Elasmosaurus'' is known from the Sharon Springs Member of the [[Campanian]]-age [[Upper Cretaceous]] Pierre Shale formation of western Kansas, which dates to about 80.64 to 77{{nbsp}}million years ago.<ref name="carpenter2008"/> 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.{{sfn|Everhart|2005a|page=6}} At its largest, the Western Interior Seaway stretched from [[the Rockies]] east to the [[Appalachians]], some {{convert|1000|km|sp=us}} wide. At its deepest, it may have been only {{convert|800|or|900|m|sp=us}} deep. Two great continental watersheds drained into it from east and west, diluting its waters and bringing resources in eroded [[silt]] that formed shifting [[river delta]] systems along its low-lying coasts. There was little [[sediment]]ation on the eastern margin of the Seaway; the western margin accumulated a thick pile of sediments eroded from the western land mass.<ref name=stanley/><ref name=monroe>{{cite book|last=Monroe|first=James S. |title=The Changing Earth: Exploring Geology and Evolution |url=https://archive.org/details/changingearthexp00monr|url-access=limited|year=2009 |publisher=Brooks/Cole, Cengage Learning |location=Belmont, CA |isbn=978-0-495-55480-6 |page=[https://archive.org/details/changingearthexp00monr/page/n621 605] |edition=5th |author2=Wicander, Reed}}</ref> The western shore was thus highly variable, depending on variations in [[sea level]] and sediment supply.<ref name=stanley>{{cite book |last=Stanley |first=Steven M. |title=Earth System History |location=New York |publisher=W.H. Freeman and Company |year=1999 |isbn=978-0-7167-2882-5 |pages=487–489}}</ref>

The soft, muddy sea floor probably received very little sunlight, but it teemed with life due to steady rains of organic debris from plankton and other organisms farther up the water column. The bottom was dominated by large ''[[Inoceramus]]'' clams, which were covered with [[oyster]]s; 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 in this sea include various species of [[rudist]]s, [[crinoid]]s and cephalopods (including squids and [[ammonites]]).{{sfn|Everhart|2005a|pages=28–38}}

Large fish known to have inhabited the sea include the bony fishes ''[[Pachyrhizodus]]'', ''Enchodus'', ''[[Cimolichthys]]'', ''[[Saurocephalus]]'', ''[[Saurodon]]'', ''[[Gillicus]]'', ''[[Ichthyodectes]]'', ''[[Xiphactinus]]'', ''[[Protosphyraena]]'' and ''[[Martinichthys]]'';{{sfn|Everhart|2005a|pages=78, 82–84, 88, 93}} and the sharks ''Cretoxyrhina'', ''[[Cretolamna]]'', ''[[Scapanorhynchus]]'', ''[[Pseudocorax]]'' and ''[[Squalicorax]]''.{{sfn|Everhart|2005a|page=58}} In addition to ''Elasmosaurus'', other marine reptiles present include fellow plesiosaurs ''Libonectes'', ''Styxosaurus'', ''Thalassomedon'', ''Terminonatator'', ''Polycotylus'', ''[[Brachauchenius]]'', ''[[Dolichorhynchops]]'' and ''[[Trinacromerum]]'';{{sfn|Everhart|2005a|pages=125, 129, 132–133, 144}} the mosasaurs ''Mosasaurus'', ''[[Halisaurus]]'', ''[[Prognathodon]]'', ''[[Tylosaurus]]'', ''[[Ectenosaurus]]'', ''[[Globidens]]'', ''[[Clidastes]]'', ''[[Platecarpus]]'' and ''[[Plioplatecarpus]]'';{{sfn|Everhart|2005a|pages=160–168}} and the sea turtles ''[[Archelon]]'', ''[[Protostega]]'', ''[[Porthochelys]]'' and ''[[Toxochelys]]''.{{sfn|Everhart|2005a|pages=108–109}} The flightless aquatic bird ''[[Hesperornis]]'' also made its home there.{{sfn|Everhart|2005a|page=221}} The [[pterosaur]]s ''[[Pteranodon]]'' and ''[[Nyctosaurus]]'',{{sfn|Everhart|2005a|page=210}} and the bird ''[[Ichthyornis]]'',{{sfn|Everhart|2005a|page=221}} are also known far from land.<ref name="carpenter2008">{{cite book |last=Carpenter |first=K. |year=2008 |chapter=Vertebrate Biostratigraphy of the Smoky Hill Chalk (Niobrara Formation) and the Sharon Springs Member (Pierre Shale) |editor1-last=Harries |editor1-first=P. J. |title=High-resolution Approaches in Stratigraphic Paleontology |series=Topics in Geobiology |volume=21 |pages=421–437 |publisher=Kluwer Academic Publishers|location=Dordrecht |doi=10.1007/978-1-4020-9053-0_11 |isbn=978-1-4020-9053-0 |s2cid=127932777 |chapter-url=https://www.researchgate.net/publication/226862191}}</ref>

==See also==
{{portal|Paleontology}}
* [[List of plesiosaur genera]]
* [[Timeline of plesiosaur research]]

==References==
{{Reflist}}

===Bibliography===
* {{cite book|last=Everhart|first=M. J.|year=2005a|title=Oceans of Kansas – A Natural History of the Western Interior Sea|title-link=Oceans of Kansas (book)|publisher=[[Indiana University Press]]|location=[[Bloomington, Indiana|Bloomington]]|isbn=978-0-253-34547-9}}

==External links==
{{Commons category|Elasmosaurus|''Elasmosaurus''}}
{{Wikispecies|Elasmosaurus|''Elasmosaurus''}}
* [http://www.oceansofkansas.com/Eplatyurus.html ''Elasmosaurus platyurus'' on Oceans of Kansas]
* [http://www.oceansofkansas.com/Ples-roam.html Where the Elasmosaurs roam: Separating fact from fiction]

{{Plesiosauria|Plesiosauroidea}}
{{Taxonbar|from=Q131106}}

{{Authority control}}
{{featured article}}

[[Category:Late Cretaceous plesiosaurs of North America]]
[[Category:Elasmosauridae]]
[[Category:Taxa named by Edward Drinker Cope]]
[[Category:Fossil taxa described in 1868]]
[[Category:Sauropterygian genera]]