View source for Plesiosaur

{{short description|Order of reptiles (fossil)}}
{{For|the type genus|Plesiosaurus}}
{{Use British English|date=June 2015}}
{{automatic taxobox
| name = Plesiosauria
| fossil_range = [[Late Triassic]] – [[Late Cretaceous]], {{fossilrange|earliest=227|203|66.0|latest=66.0|ref=<ref>{{Cite web|title=PBDB|url=https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=36454&is_real_user=1|access-date=2021-07-11|website=paleobiodb.org}}</ref>}}
| image = Paleo Hall at HMNS plesiosaur.jpg
| image_caption = Restored skeleton of ''[[Plesiosaurus]]''
| image2 = Peloneustes philarchus Tubingen.JPG
| image2_caption = Skeletal mount of ''[[Peloneustes]]''
| taxon = Plesiosauria
| authority = [[Henri Marie Ducrotay de Blainville|Blainville]], 1835
| subdivision_ranks = Subgroups
| subdivision =
*{{extinct}}''[[Alexeyisaurus]]''?
*{{extinct}}''[[Anningasaura]]''
*{{extinct}}''[[Rarosaurus]]''?
*{{extinct}}''[[Termatosaurus]]''?
*{{extinct}}'''Neoplesiosauria'''
**{{extinct}}[[Plesiosauroidea]]
**{{extinct}}[[Pliosauroidea]]
}}

The '''Plesiosauria'''{{efn|{{IPAc-en|ˌ|p|l|iː|s|i|ə|ˈ|s|ɔːr|i|ə|,_|-|z|i|-}} {{respell|PLEE|see|ə|SOR|ee|ə|,_|-|zee|-}};{{refn|{{MerriamWebsterDictionary|Plesiosaur}}}}{{refn|{{Dictionary.com|Plesiosaur}}}} [[Ancient Greek|Greek]]: πλησίος, ''plesios'', meaning "near to" and [[Sauria|''sauros'']], meaning "lizard"}}<ref>{{cite book |last1=Colbert |first1=Edwin H. (Edwin Harris) |last2=Knight |first2=Charles Robert |title=The dinosaur book: the ruling reptiles and their relatives |date=1951 |publisher=McGraw-Hill |location=New York |page=153 |url=https://archive.org/details/bookruli00colb/page/152/mode/2up}}</ref> or '''plesiosaurs''' are an [[Order (biology)|order]] or [[clade]] of extinct [[Mesozoic]] [[marine reptile]]s, belonging to the [[Sauropterygia]].

Plesiosaurs first appeared in the latest [[Triassic]] [[Period (geology)|Period]], possibly in the [[Rhaetian]] stage, about 203&nbsp;million years ago.<ref>{{cite web|title=The Plesiosaur Directory|url=http://www.plesiosauria.com/evolution.php|access-date=20 April 2013|archive-url=https://web.archive.org/web/20160304174253/http://plesiosauria.com/evolution.php|archive-date=4 March 2016}}</ref> They became especially common during the [[Jurassic]] Period, thriving until their disappearance due to the [[Cretaceous–Paleogene extinction event]] at the end of the [[Cretaceous]] Period, about 66&nbsp;million years ago. They had a worldwide oceanic distribution, and some species at least partly inhabited freshwater environments.<ref>{{Cite web |date=27 July 2022 |title=Plesiosaur fossils found in the Sahara suggest they weren't just marine animals |url=https://www.sciencedaily.com/releases/2022/07/220727110711.htm |archive-url=https://web.archive.org/web/20220729171304/https://www.sciencedaily.com/releases/2022/07/220727110711.htm |archive-date=29 July 2022 |access-date=3 August 2022 |website=ScienceDaily |language=en}}</ref>

Plesiosaurs were among the first fossil reptiles discovered. In the beginning of the nineteenth century, scientists realised how distinctive their build was and they were named as a separate order in 1835. The first plesiosaurian genus, the eponymous ''[[Plesiosaurus]]'', was named in 1821. Since then, more than a hundred valid species have been described. In the early twenty-first century, the number of discoveries has increased, leading to an improved understanding of their anatomy, relationships and way of life.

Plesiosaurs had a broad flat body and a short tail. Their limbs had evolved into four long flippers, which were powered by strong muscles attached to wide bony plates formed by the shoulder girdle and the pelvis. The flippers made a flying movement through the water. Plesiosaurs breathed air, and bore live young; there are indications that they were warm-blooded.

Plesiosaurs showed two main [[morphology (biology)|morphological]] types. Some species, with the "plesiosauromorph" build, had (sometimes extremely) long necks and small heads; these were relatively slow and caught small sea animals. Other species, some of them reaching a length of up to seventeen meters, had the "pliosauromorph" build with a short neck and a large head; these were [[apex predator]]s, fast hunters of large prey. The two types are related to the traditional strict division of the Plesiosauria into two suborders, the long-necked [[Plesiosauroidea]] and the short-neck [[Pliosauroidea]]. Modern research, however, indicates that several "long-necked" groups might have had some short-necked members or vice versa. Therefore, the purely descriptive terms "plesiosauromorph" and "pliosauromorph" have been introduced, which do not imply a direct relationship. "Plesiosauroidea" and "Pliosauroidea" today have a more limited meaning. The term "plesiosaur" is properly used to refer to the Plesiosauria as a whole, but informally it is sometimes meant to indicate only the long-necked forms, the old Plesiosauroidea.

Like other ancient marine reptiles, such as those in the clades [[Ichthyosauria]] and [[Mosasauria]], the [[Genus|genera]] in Plesiosauria are not part of the clade [[dinosaur|Dinosauria]].<ref>{{cite web |first1=Sam |last1=Rae |first2=Lisa |last2=Hendry |title=What are dinosaurs? |publisher=[[Natural History Museum, London]] |date=<!--- not stated ---> |url=https://www.nhm.ac.uk/discover/what-are-dinosaurs.html |access-date=2024-11-04}}</ref>

== History of discovery ==
{{main|Timeline of plesiosaur research}}

===Early finds===
[[File:Stukely plesiosaur.jpg|thumb|First published plesiosaur skeleton, 1719 (specimen NHMUK PV R.1330) ]]
Skeletal elements of plesiosaurs are among the first fossils of extinct reptiles recognised as such.<ref name="Evans2010"/> In 1605, [[Richard Verstegen]] of [[Antwerp]] illustrated in his ''A Restitution of Decayed Intelligence'' plesiosaur vertebrae that he referred to fishes and saw as proof that [[Great Britain]] was once connected to the European continent.<ref>Richard Verstegan, 1605, ''A restitution of decayed intelligence or Nationum Origo'', R. Bruney, Antwerpen</ref> The Welshman [[Edward Lhuyd]] in his ''Lithophylacii Brittannici Ichnographia'' from 1699 also included depictions of plesiosaur vertebrae that again were considered fish vertebrae or ''Ichthyospondyli''.<ref>Lhuyd, E., 1699, ''Lithophylacii Brittannici Ichnographia, sive Lapidum aliorumque Fossilium Brittanicorum singulari figurà insignium'', Londen</ref> Other naturalists during the seventeenth century added plesiosaur remains to their collections, such as [[John Woodward (naturalist)|John Woodward]]; these were only much later understood to be of a plesiosaurian nature and are today partly preserved in the [[Sedgwick Museum]].<ref name="Evans2010">{{cite book|last=Evans|first=M.|year=2010|contribution=The roles played by museums, collections, and collectors in the early history of reptile palaeontology|pages=5–31|editor1-first=Richard|editor1-last=Moody|editor2-first=E.|editor2-last=MoodyBuffetaut|editor3-first=D.|editor3-last=MoodyNaish|editor4-first=D. M.|editor4-last=MoodyMartill|title=Dinosaurs and Other Extinct Saurians: A Historical Perspective|publisher= Geological Society of London|contribution-url={{google books|plainurl=yes|id=Ym2ei7Ey0PwC|page=5}}|isbn=978-1-86239-311-0}}</ref>

In 1719, [[William Stukeley]] described a partial skeleton of a plesiosaur, which had been brought to his attention by the great-grandfather of [[Charles Darwin]], [[Robert Darwin of Elston]]. The stone plate came from a quarry at [[Fulbeck]] in [[Lincolnshire]] and had been used, with the fossil at its underside, to reinforce the slope of a watering-hole in [[Elston]] in [[Nottinghamshire]]. After the strange bones it contained had been discovered, it was displayed in the local vicarage as the remains of a sinner drowned in the [[Great Flood]]. Stukely affirmed its "[[diluvial]]" nature but understood it represented some sea creature, perhaps a crocodile or dolphin.<ref>{{cite journal | last= Stukeley | first= W | year = 1719 | title = An account of the impression of the almost entire sceleton of a large animal in a very hard stone, lately presented the Royal Society, from Nottinghamshire | journal = Philosophical Transactions | volume = 30 | issue = 360| pages = 963–968 | doi = 10.1098/rstl.1717.0053 | doi-access = free }}</ref> The specimen is today on display at the [[Natural History Museum, London|Natural History Museum]], and its inventory number is NHMUK PV R.1330 (formerly BMNH R.1330). It is the earliest discovered more or less complete fossil reptile skeleton in a museum collection. It can perhaps be referred to ''[[Plesiosaurus]] dolichodeirus''.<ref name="Evans2010"/>
[[File:Conybeare Plesiosaur 1824.jpg|left|thumb|As this illustration shows, Conybeare by 1824 had gained a basically correct understanding of plesiosaur anatomy.]]
During the eighteenth century, the number of English plesiosaur discoveries rapidly increased, although these were all of a more or less fragmentary nature. Important collectors were the reverends [[William Mounsey (reverend)|William Mounsey]] and [[Baptist Noel Turner]], active in the [[Vale of Belvoir]], whose collections were in 1795 described by [[John Nichols (printer)|John Nicholls]] in the first part of his ''The History and Antiquities of the County of Leicestershire''.<ref>Nicholls, J., 1795, ''The History and Antiquities of the County of Leicestershire. Volume I'', John Nicholls, Londen</ref> One of Turner's partial plesiosaur skeletons is still preserved as specimen NHMUK PV R.45 (formerly BMNH R.45) in the British Museum of Natural History; this is today referred to ''[[Thalassiodracon]]''.<ref name="Evans2010"/>

===Naming of ''Plesiosaurus'' ===
[[File:Mary Anning Plesiosaurus.jpg|thumb|Complete ''Plesiosaurus'' skeleton (specimen NHMUK OR 22656) recovered by the Annings in 1823]]
In the early nineteenth century, plesiosaurs were still poorly known and their special build was not understood. No systematic distinction was made with [[Ichthyosauria|ichthyosaurs]], so the fossils of one group were sometimes combined with those of the other to obtain a more complete specimen. In 1821, a partial skeleton discovered in the collection of Colonel [[Thomas James Birch]],<ref>{{cite journal | last= Conybeare | first= W.D. | year = 1822 | title = Additional notices on the fossil genera ''Ichthyosaurus'' and ''Plesiosaurus'' | url = https://zenodo.org/record/1448575| journal = Transactions of the Geological Society of London | volume = 2 | pages = 103–123 | doi = 10.1144/transgslb.1.1.103 | s2cid = 129545314 }}</ref> was described by [[William Conybeare (geologist)|William Conybeare]] and [[Henry Thomas De la Beche]], and recognised as representing a distinctive group. A new genus was named, ''[[Plesiosaurus]]''. The generic name was derived from the [[Ancient Greek]] πλήσιος, ''plèsios'', "closer to" and the Latinised ''saurus'', in the meaning of "saurian", to express that ''Plesiosaurus'' was in the [[Chain of Being]] more closely positioned to the [[Sauria]], particularly the crocodile, than ''[[Ichthyosaurus]]'', which had the form of a more lowly fish.<ref>{{cite journal | last1 = De la Beche | first1 = H.T. | last2 = Conybeare | first2 = W.D. | year = 1821 | title = Notice of the discovery of a new animal, forming a link between the Ichthyosaurus and crocodile, together with general remarks on the osteology of Ichthyosaurus | journal = Transactions of the Geological Society of London | volume = 5 | pages = 559–594 }}</ref> The name should thus be rather read as "approaching the Sauria" or "near reptile" than as "near lizard".<ref>{{cite web|url=http://oceansofkansas.com/Creisler_Plesiosaurs2.html|title=Plesiosaur_Names|website=oceansofkansas.com}}</ref> Parts of the specimen are still present in the [[Oxford University Museum of Natural History]].<ref name="Evans2010"/>

[[File:Plesiosaurus dolichodeirus NHM.jpg|thumb|upright=.8| Mary Anning's plesiosaur: specimen NHMUK PV OR 22656 of ''[[Plesiosaurus dolichodeirus]]'']]

Soon afterwards, the [[Morphology (biology)|morphology]] became much better known. In 1823, Thomas Clark reported an almost complete skull, probably belonging to ''Thalassiodracon'', which is now preserved by the [[British Geological Survey]] as specimen BGS GSM 26035.<ref name="Evans2010"/> The same year, commercial fossil collector [[Mary Anning]] and her family uncovered an almost complete skeleton at [[Lyme Regis]] in [[Dorset]], England, on what is today called the [[Jurassic Coast]]. It was acquired by the [[Richard Temple-Nugent-Brydges-Chandos-Grenville, 1st Duke of Buckingham and Chandos|Duke of Buckingham]], who made it available to the geologist [[William Buckland]]. He in turn let it be described by Conybeare on 20 February 1824 in a paper read at the [[Geological Society of London]],<ref>{{cite journal | last= Conybeare | first= W.D. | year = 1824 | title = On the discovery of an almost perfect skeleton of the ''Plesiosaurus'' | journal = Transactions of the Geological Society of London | volume = 2 | pages = 382–389 }}</ref> during the same meeting in which for the first time a dinosaur was named, ''[[Megalosaurus]]''. The two finds revealed the unique and bizarre build of the animals, in 1832 by Professor Buckland likened to "a sea serpent run through a turtle". In 1824, Conybeare also provided a [[specific name (zoology)|specific name]] to ''Plesiosaurus'': ''dolichodeirus'', meaning "longneck". In 1848, the skeleton was bought by the British Museum of Natural History and catalogued as specimen NHMUK OR 22656 (formerly BMNH 22656).<ref name="Evans2010"/> When the paper was published in the Transactions of the Geological Society, Conybeare provisionally named a second species: ''Plesiosaurus giganteus''. This was a short-necked form later assigned to the [[Pliosauroidea]].<ref>{{cite journal | last1 = Benson | first1 = R.B.J. | last2 = Evans | first2 = M. | last3 = Smith | first3 = A.S. | last4 = Sassoon | first4 = J. | last5 = Moore-Faye | first5 = S. | last6 = Ketchum | first6 = H.F. | last7 = Forrest | first7 = R. | year = 2013 | title = A giant pliosaurid skull from the Late Jurassic of England | journal = PLOS ONE | volume = 8 | issue = 5| article-number = e65989 | doi=10.1371/journal.pone.0065989 | pmid=23741520 | pmc=3669260| bibcode = 2013PLoSO...865989B | doi-access = free }}</ref>
[[File:Great Sea-Dragons.jpg|left|thumb|Hawkins' demonic plesiosaurs battling other sea-monsters in primordial darkness]]
Plesiosaurs became better known to the general public through two lavishly illustrated publications by the collector [[Thomas Hawkins (geologist)|Thomas Hawkins]]: ''Memoirs of Ichthyosauri and Plesiosauri'' of 1834<ref>{{cite web|last=Hawkins|first=T. H.|year=1834|title=Memoirs on Ichthyosauri and Plesiosauri; Extinct monsters of the ancient Earth|publisher=Relfe and Fletcher|url=http://www.plesiosaur.com/references/hawkins/hawkins.pdf|archive-url=https://web.archive.org/web/20050830021622/http://www.plesiosaur.com/references/hawkins/hawkins.pdf|archive-date=2005-08-30}}</ref> and ''The Book of the Great Sea-Dragons'' of 1840. Hawkins entertained a very idiosyncratic view of the animals,<ref>{{cite journal|url=https://web.stanford.edu/group/journal/cgi-bin/wordpress/wp-content/uploads/2012/09/Peterson_Hum_2012.pdf|first=A.|last= Peterson|title=Terrible lizards and the wrath of God: How 19th century Christianity and Romanticism affected visual representations of dinosaurs and our perceptions of the ancient world|journal=Stanford Undergraduate Research Journal|year=2012}}</ref> seeing them as monstrous creations of the devil, during a [[pre-Adamism|pre-Adamitic]] phase of history.<ref>{{cite book|last=Hawkins|first=T. H.|year=1840|title=The Book of the Great Sea-dragons, Ichthyosauri and Plesiosauri, Gedolim Taninum of Moses. Extinct Monsters of the Ancient Earth|publisher=W. Pickering, London|pages=1–27}}</ref> Hawkins eventually sold his valuable and attractively restored specimens to the British Museum of Natural History.<ref>Christopher McGowan, 2001, ''The Dragon Seekers'', Cambridge, Massachusetts, Perseus Publishing</ref>

During the first half of the nineteenth century, the number of plesiosaur finds steadily increased, especially through discoveries in the sea cliffs of Lyme Regis. Sir [[Richard Owen]] alone named nearly a hundred new species. The majority of their descriptions were, however, based on isolated bones, without sufficient diagnosis to be able to distinguish them from the other species that had previously been described. Many of the new species described at this time have subsequently been [[valid name (zoology)|invalidated]]. The genus ''Plesiosaurus'' is particularly problematic, as the majority of the new species were placed in it so that it became a [[wastebasket taxon]]. Gradually, other genera were named. Hawkins had already created new genera, though these are no longer seen as valid. In 1841, Owen named ''[[Pliosaurus]] brachydeirus''. Its [[etymology]] referred to the earlier ''Plesiosaurus dolichodeirus'' as it is derived from πλεῖος, ''pleios'', "more fully", reflecting that according to Owen it was closer to the Sauria than ''Plesiosaurus''. Its specific name means "with a short neck".<ref>{{cite journal | last= Owen | first= R | year = 1841 | title = Description of some remains of a gigantic crocodilian saurian, probably marine, from the Lower Greensand at Hythe and of teeth from the same formation at Maidstone, referable to the genus ''Polyptychodon'' | url = https://archive.org/details/cbarchive_50181_1841descriptionofsomeremainsof1826 | journal = Proceedings of the Geologists' Association | volume = 3 | pages = 449–452 }}</ref> Later, the [[Pliosauridae]] were recognised as having a morphology fundamentally different from the plesiosaurids. The family [[Plesiosauridae]] had already been coined by [[John Edward Gray]] in 1825.<ref>{{cite journal | last= Edward Gray | first= John | year = 1825 | title = A Synopsis of the Genera of Reptiles and Amphibia, with a Description of some new Species | journal = Annals of Philosophy (British Museum) | volume = 10 | pages = 193–217 }}</ref> In 1835, [[Henri Marie Ducrotay de Blainville]] named the order Plesiosauria itself.<ref>{{cite journal | last= de Blainville | first= H. M. D. | year = 1835 | title = Description de quelques espèces de reptiles de la Californie, précédée de l'analyse d'une système générale d'Erpetologie et d'Amphibiologie | language=fr | journal = Nouvelles Archives du Muséum d'Histoire Naturelle | volume = 4 | pages = 233–296 }}</ref>

===American discoveries===
In the second half of the nineteenth century, important finds were made outside of England. While this included some German discoveries, it mainly involved plesiosaurs found in the sediments of the American Cretaceous [[Western Interior Seaway]], the [[Niobrara Chalk]]. One fossil in particular marked the start of the [[Bone Wars]] between the rival paleontologists [[Edward Drinker Cope]] and [[Othniel Charles Marsh]].
[[File:Cope Elasmosaurus.jpg|upright=1.35|thumb|Cope's ''Elasmosaurus'' with its head on the tail and lacking hindlimbs]]
In 1867, physician Theophilus Turner near [[Fort Wallace]] in [[Kansas]] uncovered a plesiosaur skeleton, which he donated to Cope.<ref>{{cite journal | last= Cope | first= E.D. | year = 1868 | title = [A resolution thanking Dr. Theophilus Turner for his donation of the skeleton of ''Elasmosaurus platyurus'']. | journal = Proc. Acad. Nat. Sci. Phila. | volume = 20 | page = 314 }}</ref> Cope attempted to reconstruct the animal on the assumption that the longer extremity of the vertebral column was the tail, the shorter one the neck. He soon noticed that the skeleton taking shape under his hands had some very special qualities: the neck vertebrae had chevrons and with the tail vertebrae the joint surfaces were orientated back to front.<ref>{{cite journal | last= Cope | first= 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 }}</ref> Excited, Cope concluded to have discovered an entirely new group of reptiles: the [[Streptosauria]] or "Turned Saurians", which would be distinguished by reversed vertebrae and a lack of hindlimbs, the tail providing the main propulsion.<ref>{{cite journal | last= Cope | first= E.D. | year = 1869 | title = On the reptilian orders Pythonomorpha and Streptosauria | journal = Proceedings of the Boston Society of Natural History | volume = XII | pages = 250–266 }}</ref> After having published a description of this animal,<ref>{{cite journal | last= Cope | first= E.D. | year = 1868 | title = On a new large enaliosaur | journal = American Journal of Science Series | volume = 46 | issue = 137| pages = 263–264 }}</ref> followed by an illustration in a textbook about reptiles and amphibians,<ref name=cope1869>{{cite book|last=Cope|first=E. D.|year=1869|title=Synopsis of the Extinct Batrachia and Reptilia of North America, Part I|chapter=Sauropterygia|publisher=Transactions of the American Philosophical Society |series=New Series|volume=14|pages=1–235|chapter-url=https://archive.org/stream/synopsisofextinc00cope#page/34}}</ref> Cope invited Marsh and [[Joseph Leidy]] to admire his new ''[[Elasmosaurus]] platyurus''. Having listened to Cope's interpretation for a while, Marsh suggested that a simpler explanation of the strange build would be that Cope had reversed the vertebral column relative to the body as a whole. When Cope reacted indignantly to this suggestion, Leidy silently took the skull and placed it against the presumed last tail vertebra, to which it fitted perfectly: it was in fact the first neck vertebra, with still a piece of the rear skull attached to it.<ref>{{cite journal | last= Leidy | first= J | year = 1870 | title = On the Elasmosaurus platyurus of Cope | journal = American Journal of Science Series | volume = 49 | issue = 147| page = 392 }}</ref> Mortified, Cope tried to destroy the entire edition of the textbook and, when this failed, immediately published an improved edition with a correct illustration but an identical date of publication.<ref>{{cite journal | last= Cope | first= E.D. | year = 1870 | title = Synopsis of the extinct Batrachia and Reptilia of North America | url =https://babel.hathitrust.org/cgi/imgsrv/download/pdf?id=nyp.33433090912423;orient=0;size=100;seq=7;attachment=0 | journal = Transactions of the American Philosophical Society |series=New Series | volume = 14 | issue = 1| pages = 1–252 | doi=10.2307/1005355| jstor = 1005355 | url-access = subscription }}</ref> He excused his mistake by claiming that he had been misled by Leidy himself, who, describing a specimen of ''[[Cimoliasaurus]]'', had also reversed the vertebral column.<ref>{{cite journal | last= Cope | first= E.D. | year = 1870 | title =On Elasmosaurus platyurus Cope | journal = American Journal of Science Series | volume = 50 | issue = 148| pages = 140–141 }}</ref> Marsh later claimed that the affair was the cause of his rivalry with Cope: "he has since been my bitter enemy". Both Cope and Marsh in their rivalry named many plesiosaur genera and species, most of which are today considered invalid.<ref>Ellis (2003), p. 129</ref>

Around the turn of the century, most plesiosaur research was done by a former student of Marsh, Professor [[Samuel Wendell Williston]]. In 1914, Williston published his ''Water reptiles of the past and present''.<ref>Williston, S.W., 1914, ''Water Reptiles of the Past and Present''. Chicago University Press. Chicago, Illinois. 251 pp</ref> Despite treating sea reptiles in general, it would for many years remain the most extensive general text on plesiosaurs.<ref>{{cite journal|first=J. P.|last=Davidson|year=2015|title=Misunderstood Marine Reptiles: Late Nineteenth-Century Artistic Reconstructions of Prehistoric Marine Life|journal=Transactions of the Kansas Academy of Science|volume=118|issue=1–2|pages=53–67|doi=10.1660/062.118.0107|s2cid=83904449}}</ref> In 2013, a first modern textbook was being prepared by [[Olivier Rieppel]]. During the middle of the twentieth century, the USA remained an important centre of research, mainly through the discoveries of [[Samuel Paul Welles]].

=== Recent discoveries ===
Whereas during the nineteenth and most of the twentieth century, new plesiosaurs were described at a rate of three or four novel genera each decade, the pace suddenly picked up in the 1990s, with seventeen plesiosaurs being discovered in this period. The tempo of discovery accelerated in the early twenty-first century, with about three or four plesiosaurs being named each year.<ref>Smith, A.S., 2003, ''Cladistic analysis of the Plesiosauria (Reptilia: Sauropterygia)''. Masters thesis in palaeobiology, University of Bristol, 91 pp</ref> This implies that about half of the known plesiosaurs are relatively new to science, a result of a far more intense field research. Some of this is taking place away from the traditional areas, e.g. in new sites developed in [[New Zealand]], [[Argentina]], [[Chile]],<ref>{{cite journal |last1=Otero |first1=Rodrigo A. |last2=Suárez |first2=Mario |last3=Le Roux |first3=Jacobus P.|date=2009 |title=First record of Elasmosaurid Plesiosaurs (Sauropterygia: Plesiosauria) in upper levels of the Dorotea Formation, Late Cretaceous (Maastrichtian), Puerto Natales, Chilean Patagonia |url=http://www.redalyc.org/html/1739/173914378008/ |journal=[[Andean Geology]] |volume=36 |issue=2 |pages=342–350 |doi= 10.4067/s0718-71062009000200008|doi-access=free }}</ref> [[Norway]], [[Japan]], [[China]] and [[Morocco]], but the locations of the more original discoveries have proven to be still productive, with important new finds in England and Germany. Some of the new genera are a renaming of already known species, which were deemed sufficiently different to warrant a separate genus name.

In 2002, the "[[Monster of Aramberri]]" was announced to the press. Discovered in 1982 at the village of [[Aramberri]], in the northern Mexican state of [[Nuevo León]], it was originally classified as a [[dinosaur]]. The specimen is actually a very large plesiosaur, possibly reaching {{cvt|15|m}} in length. The media published exaggerated reports claiming it was {{convert|25|m}} long, and weighed up to {{convert|150000|kg}}, which would have made it among the largest predators of all time.<ref name="L">{{Cite web|last=Forrest|first=Richard|title=Liopleurodon|url=http://www.plesiosaur.com/plesiosaurs/liopleurodon.php|publisher=The Plesiosaur Site|access-date=18 September 2017|archive-url=https://web.archive.org/web/20110715093428/http://www.plesiosaur.com/plesiosaurs/liopleurodon.php|archive-date=15 July 2011}}</ref><ref>{{cite web|last=Forrest|first=Richard|url=http://www.plesiosaur.com/plesiosaurs/aramberri.php|title=The 'Monster of Aramberri'|publisher=The Plesiosaur Site|access-date=18 September 2017|archive-url=https://web.archive.org/web/20110903081504/http://www.plesiosaur.com/plesiosaurs/aramberri.php|archive-date=3 September 2011}}</ref>

In 2004, what appeared to be a completely intact juvenile plesiosaur was discovered, by a local fisherman, at [[Bridgwater Bay]] National Nature Reserve in Somerset, UK. The fossil, dating from 180&nbsp;million years ago as indicated by the [[ammonite]]s associated with it, measured {{convert|1.5|m}} in length, and may be related to ''[[Rhomaleosaurus]]''. It is probably the best preserved specimen of a plesiosaur yet discovered.<ref name=CollardPlesiosaur>{{cite journal|last=Larkin|first=Nigel|author2=O'Connor, Sonia|author3=Parsons, Dennis |title=The virtual and physical preparation of the Collard plesiosaur from Bridgwater Bay, Somerset, UK|journal=Geological Curator|year=2010|volume=9|issue=3|page=107|doi=10.55468/GC217 |s2cid=251120888 |doi-access=free}}</ref><ref name=CollardPlesiosaurSite>{{cite web|last=Forrest|first=Richard|title=The Collard Plesiosaur|url=http://www.plesiosaur.com/plesiosaurs/collard.php|access-date=31 October 2012|archive-url=https://web.archive.org/web/20130117230152/http://www.plesiosaur.com/plesiosaurs/collard.php|archive-date=2013-01-17}}</ref><ref name=LarkinCollardPreparation>{{cite web|last=Larkin|first=Nigel |title=Preparing and conserving an important six-foot long Plesiosaur skeleton for Somerset Museum|url=http://www.natural-history-conservation.com/plesiosaur.htm|access-date=31 October 2012}}</ref>

In 2005, the remains of three plesiosaurs (''[[Dolichorhynchops herschelensis]]'') discovered in the 1990s near [[Herschel, Saskatchewan]] were found to be a new species, by Dr. Tamaki Sato, a Japanese vertebrate paleontologist.<ref>{{Cite journal|last=Sato|first=Tamaki|date=205|title=A new Polycotylid Plesiosaur (Reptilia: Sauropterygia) from the Upper Cretaceous Bearpaw Formation in Saskatchewan, Canada.|journal=Journal of Paleontology|volume=79: 969–980|issue=5 |page=969 |doi=10.1666/0022-3360(2005)079[0969:ANPPRS]2.0.CO;2 |bibcode=2005JPal...79..969S }}</ref>

In 2006, a combined team of American and Argentinian investigators (the latter from the [[Instituto Antártico Argentino|Argentinian Antarctic Institute]] and the [[La Plata Museum]]) found the skeleton of a juvenile plesiosaur measuring {{convert|1.5|m}} in length on [[Vega Island]] in Antarctica.<ref>{{cite web|url=http://www.dna.gov.ar/DIVULGAC/CPLESIO.HTM |title=Hallazgo de un ejemplar completo de plesiosaurio joven |access-date=2013-04-22 |archive-url=https://web.archive.org/web/20130718145659/http://www.dna.gov.ar/DIVULGAC/CPLESIO.HTM |archive-date=2013-07-18 }} (In Spanish)</ref> The fossil is currently on display at the geological museum of [[South Dakota School of Mines and Technology]].<ref>{{cite journal|url= http://www.nature.com/news/2006/061211/full/news061211-4.html|first=H.|last=Ledford|year=2006|title=Rare reptile fossil found in Antarctica|journal=Nature News|pages=news061211–4|doi=10.1038/news061211-4|s2cid=85361720|doi-access=free|url-access=subscription}}</ref>

In 2008, fossil remains of an undescribed plesiosaur that was named [[Predator X]], now known as ''[[Pliosaurus funkei]]'', were unearthed in [[Svalbard]].<ref>{{Cite web|title=Scientists discover massive Jurassic marine reptile|url=https://phys.org/news/2008-03-scientists-massive-jurassic-marine-reptile.html|access-date=2022-01-28|website=phys.org|language=en}}</ref> It had a length of {{cvt|12|m}}, and its bite force of {{convert|149|kN}} is one of the most powerful known.<ref>{{cite web|url=http://www.nhm.uio.no/pliosaurus/english/|archive-url=https://web.archive.org/web/20090321104455/http://www.nhm.uio.no/pliosaurus/english/|archive-date=21 March 2009|title=PREDATOR X - Naturhistorisk Museum|date=21 March 2009}}</ref>

In December 2017, a large skeleton of a plesiosaur was found on the continent of Antarctica, the oldest creature on the continent, and the first of its species from Antarctica.<ref name=Antc>{{cite magazine|url=http://www.newsweek.com/plesiosaur-sea-monster-antarctica-756598|title=Plesiosaur: Ancient Sea Monster Discovered in Antarctica|magazine=[[Newsweek]]|date=2017-12-22|access-date=2017-12-23|first=Katherine|last=Hignett}}</ref>

Not only has the number of field discoveries increased, but also, since the 1950s, plesiosaurs have been the subject of more extensive theoretical work. The methodology of [[cladistics]] has, for the first time, allowed the exact calculation of their evolutionary relationships. Several hypotheses have been published about the way they hunted and swam, incorporating general modern insights about [[biomechanics]] and [[ecology]]. The many recent discoveries have tested these hypotheses and given rise to new ones.{{original research inline|date=July 2021}}

== Evolution ==
[[File:Ceresiosaurus12.jpg|thumb|[[Nothosaur]]s still had functional legs.]]
The Plesiosauria have their origins within the [[Sauropterygia]], a group of perhaps [[archelosauria]]n reptiles that returned to the sea. An advanced sauropterygian subgroup, the carnivorous [[Eusauropterygia]] with small heads and long necks, split into two branches during the [[Upper Triassic]]. One of these, the [[Nothosauroidea]], kept functional elbow and knee joints; but the other, the [[Pistosauria]], became more fully adapted to a sea-dwelling lifestyle. Their vertebral column became stiffer and the main propulsion while swimming no longer came from the tail but from the limbs, which changed into flippers.<ref>{{cite book|last=Rieppel|first=O.|year=2000|title= Sauropterygia I.|series=Handbuch der Paläoherpetologie|volume=12A|publisher=Verlag Dr. Friedrich Pfeil|language=de|pages=1–134}}</ref> The Pistosauria became warm-blooded and [[viviparous]], giving birth to live young.<ref>{{cite journal | last1 = Cheng | first1 = Y-N. | last2 = Wu | first2 = X-C. | last3 = Ji | first3 = Q. | year = 2004 | title = Chinese marine reptiles gave live birth to young | journal = Nature | volume = 432 | issue = 7015| pages = 383–386 | doi = 10.1038/nature03050 | pmid = 15549103 | bibcode = 2004Natur.432..383C | s2cid = 4391810 | url = http://doc.rero.ch/record/15288/files/PAL_E2587.pdf }}</ref> Early, [[basal (phylogenetics)|basal]], members of the group, traditionally called "[[Pistosauridae|pistosaurids]]", were still largely coastal animals. Their shoulder girdles remained weak, their [[pelvis|pelves]] could not support the power of a strong swimming stroke, and their flippers were blunt. Later, a more advanced pistosaurian group split off: the Plesiosauria. These had reinforced shoulder girdles, flatter pelves, and more pointed flippers. Other adaptations allowing them to colonise the open seas included stiff limb joints; an increase in the number of phalanges of the hand and foot; a tighter lateral connection of the finger and toe phalanx series, and a shortened tail.<ref>{{cite journal | last= Storrs | first= G.W. | year = 1993 | title = Function and phylogeny in sauropterygian (Diapsida) evolution | journal = American Journal of Science | volume = 293A | pages = 63–90 | bibcode = 1993AmJS..293...63S | doi = 10.2475/ajs.293.A.63 }}</ref><ref name="Rieppel1997">Rieppel, O., 1997, "Introduction to Sauropterygia", In: Callaway, J. M. & Nicholls, E. L. (eds.), ''Ancient marine reptiles'' pp 107–119. Academic Press, San Diego, California</ref>
[[File:Augustasaurus BW.jpg|left|thumb|Basal Pistosauria, like ''[[Augustasaurus]]'', already bore a strong resemblance to Plesiosauria.]]
From the earliest [[Jurassic]], the [[Hettangian]] stage, a rich radiation of plesiosaurs is known, implying that the group must already have diversified in the [[Late Triassic]]; of this diversification, however, only a few (very) basal forms have been discovered, the most derived ''[[Rhaeticosaurus]]''. The subsequent evolution of the plesiosaurs is very contentious. The various cladistic analyses have not resulted in a consensus about the relationships between the main plesiosaurian subgroups. Traditionally, plesiosaurs have been divided into the long-necked [[Plesiosauroidea]] and the short-necked [[Pliosauroidea]]. However, modern research suggests that some generally long-necked groups might have had short-necked members. To avoid confusion between the [[phylogeny]], the evolutionary relationships, and the [[Morphology (biology)|morphology]], the way the animal is built, long-necked forms are therefore called "plesiosauromorph" and short-necked forms are called "pliosauromorph", without the "plesiosauromorph" species necessarily being more closely related to each other than to the "pliosauromorph" forms.<ref>{{cite journal | last= O'Keefe | first= F.R. | year = 2002 | title = The evolution of plesiosaur and pliosaur morphotypes in the Plesiosauria (Reptilia: Sauropterygia) | journal = Paleobiology | volume = 28 | issue = 1 | pages = 101–112 | doi=10.1666/0094-8373(2002)028<0101:teopap>2.0.co;2| bibcode = 2002Pbio...28..101O | s2cid = 85753943 | url = http://doc.rero.ch/record/14731/files/PAL_E1857.pdf }}</ref>
[[File:Simolestes1DB.jpg|thumb|Illustration of the pliosaur ''[[Simolestes vorax]]'']]

The [[latest common ancestor]] of the Plesiosauria was probably a rather small short-necked form. During the earliest Jurassic, the subgroup with the most species was the [[Rhomaleosauridae]], a possibly very basal split-off of species which were also short-necked. Plesiosaurs in this period were at most five meters (sixteen feet) long. By the [[Toarcian]], about 180&nbsp;million years ago, other groups, among them the [[Plesiosauridae]], became more numerous and some species developed longer necks, resulting in total body lengths of up to {{convert|10|m|abbr=off|spell=in}}.<ref name=Bensonetal12>{{cite journal|year=2012 |title=High Diversity, Low Disparity and Small Body Size in Plesiosaurs (Reptilia, Sauropterygia) from the Triassic–Jurassic Boundary |journal=PLOS ONE |volume=7 |issue=3 |article-number=e31838 |doi=10.1371/journal.pone.0031838 |pmid=22438869 |pmc=3306369 |author1=Roger B. J. Benson |author2=Mark Evans |author3=Patrick S. Druckenmiller |editor1-last=Lalueza-Fox |editor1-first=Carles|bibcode=2012PLoSO...731838B|doi-access=free }}</ref>

In the middle of the Jurassic, very large [[Pliosauridae]] evolved. These were characterized by a large head and a short neck, such as ''[[Liopleurodon]]'' and ''[[Simolestes]]''. These forms had skulls up to three meters (ten feet) long and reached a length of up to {{convert|17|m|abbr=off|sp=us|spell=in}} and a weight of ten tons. The pliosaurids had large, conical teeth and were the dominant marine carnivores of their time. During the same time, approximately 160 million years ago, the [[Cryptoclididae]] were present, shorter species with a long neck and a small head.<ref name="Ketchum2010"/>

The [[Leptocleididae]] radiated during the [[Early Cretaceous]]. These were rather small forms that, despite their short necks, might have been more closely related to the Plesiosauridae than to the Pliosauridae. Later in the Early Cretaceous, the [[Elasmosauridae]] appeared; these were among the longest plesiosaurs, reaching up to fifteen meters (fifty feet) in length due to very long necks containing as many as 76 vertebrae, more than any other known vertebrate. Pliosauridae were still present as is shown by large predators, such as ''[[Kronosaurus]]''.<ref name="Ketchum2010"/>

At the beginning of the [[Late Cretaceous]], the [[Ichthyosauria]] became extinct; perhaps a plesiosaur group evolved to fill their niches: the [[Polycotylidae]], which had short necks and peculiarly elongated heads with narrow snouts. During the Late Cretaceous, the elasmosaurids still had many species.<ref name="Ketchum2010"/>

All plesiosaurs became [[extinction|extinct]] as a result of the [[K-T event]] at the end of the Cretaceous period, approximately {{period start|Paleogene}} million years ago.<ref>{{cite book|last= Bakker|first= R.T.|year= 1993|chapter= Plesiosaur Extinction Cycles — Events that Mark the Beginning, Middle and End of the Cretaceous|editor1= Caldwell, W.G.E. |editor2=Kauffman, E.G.|title= Evolution of the Western Interior Basin|publisher= Geological Association of Canada|pages= 641–664}}</ref>

===Relationships===
In modern [[phylogeny]], [[clade]]s are defined groups that [[Monophyly|contain all species belonging to a certain branch of the evolutionary tree]]. One way to define a clade is to let it consist of the [[last common ancestor]] of two such species and all its descendants. Such a clade is called a "[[node-based taxon|node clade]]". In 2008, [[Patrick Druckenmiller]] and [[Anthony Russell (paleontologist)|Anthony Russell]] in this way defined Plesiosauria as the group consisting of the last common ancestor of ''[[Plesiosaurus]] dolichocheirus'' and ''[[Peloneustes]] philarchus'' and all its descendants.<ref>{{cite journal|last1=Druckenmiller|first1=P. S.|last2=Russell| first2=A. P.|year=2008|title=A phylogeny of Plesiosauria (Sauropterygia) and its bearing on the systematic status of Leptocleidus Andrews, 1922|journal=Zootaxa|volume=1863|pages=1–120|doi=10.11646/zootaxa.1863.1.1}}</ref> ''Plesiosaurus'' and ''Peloneustes'' represented the main subgroups of the Plesiosauroidea and the Pliosauroidea and were chosen for historical reasons; any other species from these groups would have sufficed. 

Another way to define a clade is to let it consist of all species more closely related to a certain species that one in any case wishes to include in the clade than to another species that one to the contrary desires to exclude. Such a clade is called a "[[stem-based taxon|stem clade]]". Such a definition has the advantage that it is easier to include all species with a certain [[Morphology (biology)|morphology]]. Plesiosauria was in 2010 by [[Hillary Ketchum]] and [[Roger Benson]] defined as such a [[stem-based taxon]]: "all taxa more closely related to ''[[Plesiosaurus dolichodeirus]]'' and ''[[Pliosaurus brachydeirus]]'' than to ''[[Augustasaurus hagdorni]]''". Ketchum and Benson (2010) also coined a new clade Neoplesiosauria, a [[node-based taxon]] that was defined by as "''Plesiosaurus dolichodeirus'', ''Pliosaurus brachydeirus'', their most recent common ancestor and all of its descendants".<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 of the Cambridge Philosophical Society |volume=85 |issue=2 |pages=361–392 |doi=10.1111/j.1469-185X.2009.00107.x |pmid=20002391|s2cid=12193439 }}</ref> The clade Neoplesiosauria very likely is materially identical to Plesiosauria ''sensu'' Druckenmiller & Russell, thus would designate exactly the same species, and the term was meant to be a replacement of this concept.

Benson ''et al.'' (2012) found the traditional Pliosauroidea to be [[paraphyletic]] in relation to Plesiosauroidea. Rhomaleosauridae was found to be outside Neoplesiosauria, but still within Plesiosauria. The early [[Carnian]] pistosaur ''[[Bobosaurus]]'' was found to be one step more advanced than ''[[Augustasaurus]]'' in relation to the Plesiosauria and therefore it represented by definition the basalmost known plesiosaur. This analysis focused on basal plesiosaurs and therefore only one derived pliosaurid and one [[cryptoclidia]]n were included, while [[elasmosaurid]]s were not included at all. A more detailed analysis published by both Benson and Druckenmiller in 2014 was not able to resolve the relationships among the lineages at the base of Plesiosauria.<ref name=plesiosaurturnover>{{Cite journal | last1 = Benson | first1 = R. B. J. | last2 = Druckenmiller | first2 = P. S. | doi = 10.1111/brv.12038 | title = Faunal turnover of marine tetrapods during the Jurassic-Cretaceous transition | journal = Biological Reviews | pages = 1–23| year = 2013 | pmid = 23581455| volume=89| issue = 1 | s2cid = 19710180 }}</ref>
[[File:Plesiosaur skeleton, New Walk Museum.JPG|thumb|''[[Atychodracon]]'' fossil]]

The following [[cladogram]] follows an analysis by Benson & Druckenmiller (2014).<ref name=plesiosaurturnover/>
[[File:Macrocephalus.jpg|thumb|Cast of "Plesiosaurus" ''macrocephalus'', yet to receive a valid genus name]]
{{clade| style=font-size:85%;line-height:85%
|1={{clade
   |1="''Pistosaurus''" postcranium
   |2=''[[Yunguisaurus]]''
   |3={{clade
      |1=''[[Augustasaurus]]''
      |2={{clade
         |1=''[[Bobosaurus]]''
         |label2='''Plesiosauria'''
         |2={{clade
            |1=''[[Stratesaurus]]''
            |2=''[[Eoplesiosaurus]]''
            |3=[[Rhomaleosauridae]] <span style="{{MirrorH}}">[[File:Macroplata BW.jpg|50 px]]</span>
            |label4=[[Pliosauridae]]
            |4={{clade
               |1=''[[Thalassiodracon]]''
               |2={{clade
                  |1=''[[Hauffiosaurus]]''
                  |2={{clade
                     |1=''[[Attenborosaurus]]'' <span style="{{MirrorH}}">[[File:Attenborosaurus BW.jpg|50 px]]</span>
                     |2={{clade
                       |1=''[[Marmornectes]]''
                       |2=[[Thalassophonea]] <span style="{{MirrorH}}">[[File:Kronosaurus.jpg|50 px]]</span>}} }} }} }}
            |label5=[[Plesiosauroidea]]
            |5={{clade
               |1=''[[Plesiosaurus]]''
               |2={{clade
                  |1=''[[Eretmosaurus]]''
                  |2=''[[Westphaliasaurus]]''
                  |3=[[Microcleididae]] <span style="{{MirrorH}}">[[File:Microcleidus1DB.jpg|50 px]]</span>
                  |4={{clade
                     |1=''[[Plesiopterys]]''
                     |label2=[[Cryptoclidia]]
                     |2={{clade
                        |1=[[Cryptoclididae]] [[File:Muraenosaurus BW.jpg|50 px]]
                        |label2=[[Xenopsaria]]
                        |2={{clade
                           |1=[[Elasmosauridae]] [[File:Styxosaurus BW.jpg|50 px]]
                           |label2=[[Leptocleidia]]
                           |2={{clade
                              |1=[[Leptocleididae]] [[File:Leptocleidus BW.jpg|50 px]]
                              |2=[[Polycotylidae]]  <span style="{{MirrorH}}">[[File:Dolichorhynchops BW.jpg|50 px]]</span> }} }} }} }} }} }} }} }} }} }} }}

== Description ==

===Size===
[[File:Fossil-Plesiosaurier-Skelett.jpg|thumb|Plesiosaur skeleton of ''[[Meyerasaurus]]'' in the [[Museum am Löwentor]], [[Stuttgart]], seen from below]]
In general, plesiosaurians varied in adult length from between {{convert|1.5|m|ft|sp=us}} to about {{convert|15|m|sp=us}}. The group thus contained some of the largest marine [[apex predator]]s in the [[fossil record]], roughly equalling the longest [[Ichthyosauria|ichthyosaurs]], [[mosasaurids]], [[sharks]] and [[toothed whales]] in size. Some plesiosaurian remains, such as a {{convert|2.875|m|ft|sp=us|adj=mid|-long}} set of highly reconstructed and fragmentary lower jaws preserved in the [[Oxford University Museum]] and referable to ''[[Pliosaurus rossicus]]'' (previously referred to ''[[Stretosaurus]]''<ref>{{cite journal| last= Tarlo | first= L.B.H. | year = 1959 | title = ''Stretosaurus'' gen nov., a giant pliosaur from the Kimmeridge Clay | journal = Palaeontology | volume = 2 | issue = 2| pages = 39–55}}</ref> and ''[[Liopleurodon]]''), indicated a length of {{convert|17|m|sp=us}}. However, it was recently argued that its size cannot be currently determined due to their being poorly reconstructed and a length of {{convert|12.7|m|sp=us}} or less is more likely.<ref name=McHenry2009>McHenry, Colin Richard (2009). "Devourer of Gods: the palaeoecology of the Cretaceous pliosaur ''Kronosaurus queenslandicus''" (PDF): 1–460</ref> In 2023, the length of its mandible was estimated to be 2.6m, suggesting a length of {{convert|11.8|m|sp=us}} and a weight of over {{Convert|20|t|LT ST}}. MCZ 1285, a specimen currently referable to ''[[Kronosaurus queenslandicus]]'', from the [[Early Cretaceous]] of [[Australia]], was estimated to have a skull length of {{cvt|2.21|-|2.85|m}}.<ref name=McHenry2009/><ref name=Pliosaurus2013>{{Cite journal| last1 = Benson | first1 = R. B. J. | last2 = Evans | first2 = M. | last3 = Smith | first3 = A. S. | last4 = Sassoon | first4 = J. | last5 = Moore-Faye | first5 = S. | last6 = Ketchum | first6 = H. F. | last7 = Forrest | first7 = R. | editor1-last = Butler | editor1-first = Richard J | title = A Giant Pliosaurid Skull from the Late Jurassic of England | doi = 10.1371/journal.pone.0065989 | journal = PLOS ONE | volume = 8 | issue = 5 | article-number = e65989 | year = 2013 | pmid =  23741520| pmc = 3669260| bibcode = 2013PLoSO...865989B| doi-access = free }}</ref> A series of neck vertebrae from the Kimmeridge Clay Formation indicate a pliosaur, probably ''Pliosaurus'', that may have been up to {{convert|14.4|m|sp=us}} long.<ref name=M23>{{cite journal|last1=Martill|first1=David M.|last2=Jacobs|first2=Megan L.|last3=Smith|first3=Roy E.|title=A truly gigantic pliosaur (Reptilia, Sauropterygia) from the Kimmeridge Clay Formation (Upper Jurassic, Kimmeridgian) of England|journal=Proceedings of the Geologists' Association|year=2023|volume=134|issue=3|pages=361–373|doi=10.1016/j.pgeola.2023.04.005|bibcode=2023PrGA..134..361M |s2cid=258630597 |doi-access=free}}</ref> A later estimate suggests a length of {{cvt|10.7|-|11.8|m}} for these cervicals, as well as {{cvt|11.7|-|13|m}} for another specimen known from an isolated cervical.<ref>{{Cite journal |last=Zhao |first=Ruizhe Jackevan |date=2025 |title=Body reconstruction and size estimation of plesiosaurs |website=doi.org |doi=10.1101/2024.02.15.578844 |doi-access=free }}</ref>

===Skeleton===
The typical plesiosaur had a broad, flat, body and a short [[tail]]. Plesiosaurs retained their ancestral two pairs of limbs, which had evolved into large [[Flipper (anatomy)|flipper]]s.<ref>Caldwell, Michael W; 1997b. Modified perichondral ossification and the evolution of paddle-like limbs in Ichthyosaurs and Plesiosaurs; ''Journal of Vertebrate Paleontology'' 17 (3); 534–547</ref> Plesiosaurs were related to the earlier [[Nothosauridae]],<ref>Storrs, Glenn W.; 1990. Phylogenetic Relationships of Pachypleurosaurian and Nothosauriform Reptiles (Diapsida: Sauropterygia); Journal of Vertebrate Paleontology; 10 (Supplement to Number 3)</ref> that had a more crocodile-like body. The flipper arrangement is unusual for aquatic animals in that probably all four limbs were used to propel the animal through the water by up-and-down movements. The tail was most likely only used for helping in directional control. This contrasts to the [[Ichthyosauria|ichthyosaurs]] and the later [[mosasaur]]s, in which the tail provided the main propulsion.<ref name="Smith2008">{{cite journal | last= Smith | first= Adam Stuart | year = 2008 | title = Fossils explained 54: Plesiosaurs | journal = Geology Today | volume = 24 | issue = 2| pages = 71–75 | doi=10.1111/j.1365-2451.2008.00659.x| s2cid = 247668864 }}</ref>

To power the flippers, the [[shoulder girdle]] and the [[pelvis]] had been greatly modified, developing into broad bone plates at the underside of the body, which served as an attachment surface for large muscle groups, able to pull the limbs downwards. In the shoulder, the [[coracoid]] had become the largest element covering the major part of the breast. The [[scapula]] was much smaller, forming the outer front edge of the trunk. To the middle, it continued into a [[clavicle]] and finally a small [[interclavicular bone]]. As with most [[tetrapods]], the shoulder joint was formed by the scapula and coracoid. In the pelvis, the bone plate was formed by the [[ischium]] at the rear and the larger [[pubic bone]] in front of it. The [[Ilium (bone)|ilium]], which in land vertebrates bears the weight of the hindlimb, had become a small element at the rear, no longer attached to either the pubic bone or the thighbone. The hip joint was formed by the ischium and the pubic bone. The pectoral and pelvic plates were connected by a [[plastron]], a bone cage formed by the paired [[belly rib]]s that each had a middle and an outer section. This arrangement immobilised the entire trunk.<ref name="Smith2008"/>

To become flippers, the limbs had changed considerably. The limbs were very large, each about as long as the trunk. The forelimbs and hindlimbs strongly resembled each other. The [[humerus]] in the upper arm, and the [[femur]] in the upper leg, had become large flat bones, expanded at their outer ends. The elbow joints and the knee joints were no longer functional: the lower arm and the lower leg could not flex in relation to the upper limb elements, but formed a flat continuation of them. All outer bones had become flat supporting elements of the flippers, tightly connected to each other and hardly able to rotate, flex, extend or spread. This was true of the [[ulna]], [[Radius (bone)|radius]], [[metacarpals]] and fingers, as well of the [[tibia]], [[fibula]], [[metatarsals]] and toes. Furthermore, in order to elongate the flippers, the number of phalanges had increased, up to eighteen in a row, a phenomenon called [[hyperphalangy]]. The flippers were not perfectly flat, but had a lightly convexly curved top profile, like an [[airfoil]], to be able to "fly" through the water.<ref name="Smith2008"/>
[[File:Traskasaura (mounted skeleton, Canadian Museum of Nature).jpg|thumb|Cast of ''[[Traskasaura]]'', [[Canadian Museum of Nature]]]]
While plesiosaurs varied little in the build of the trunk, and can be called "conservative" in this respect, there were major differences between the subgroups as regards the form of the neck and the skull. Plesiosaurs can be divided into two major morphological types that differ in head and [[neck]] size. "Plesiosauromorphs", such as [[Cryptoclididae]], [[Elasmosauridae]], and [[Plesiosauridae]], had long necks and small heads. "Pliosauromorphs", such as the [[Pliosauridae]] and the [[Rhomaleosauridae]], had shorter necks with a large, elongated head. The neck length variations were not caused by an elongation of the individual cervical vertebrae, but an increase in their number. ''[[Elasmosaurus]]'' has seventy-two neck vertebrae; the known record is held by the elasmosaurid ''[[Albertonectes]]'', with seventy-six cervicals.<ref>{{cite journal | last1 = Kubo | first1 = Tai | last2 = Mitchell | first2 = Mark T. | last3 = Henderson | first3 = Donald M. | year = 2012 | title = ''Albertonectes vanderveldei'', a new elasmosaur (Reptilia, Sauropterygia) from the Upper Cretaceous of Alberta | journal = Journal of Vertebrate Paleontology | volume = 32 | issue = 3| pages = 557–572 | doi=10.1080/02724634.2012.658124| bibcode = 2012JVPal..32..557K | s2cid = 129500470 }}</ref> The large number of joints suggested to early researchers that the neck must have been very flexible; indeed, a swan-like curvature of the neck was assumed to be possible - in [[Icelandic language|Icelandic]], plesiosaurs are even called ''Svaneðlur'', "swan lizards". However, modern research has confirmed an earlier conjecture of Williston that the long plate-like spines on top of the vertebrae, the ''processus spinosi'', strongly limited vertical neck movement. Although horizontal curving was less restricted, in general, the neck must have been rather stiff and certainly was incapable of being bent into serpentine coils. This is even more true of the short-necked "pliosauromophs", which had as few as eleven cervical vertebrae. With early forms, the [[amphicoelous]] or amphiplat neck vertebrae bore double-headed neck ribs; later forms had single-headed ribs. In the remainder of the [[vertebral column]], the number of [[dorsal vertebrae]] varied between about nineteen and thirty-two; of the sacral vertebrae, between two and six, and of the tail vertebrae, between about twenty-one and thirty-two. These vertebrae still possessed the original processes inherited from the land-dwelling ancestors of the Sauropterygia and had not been reduced to fish-like simple discs, as happened with the vertebrae of ichthyosaurs. The tail vertebrae possessed chevron bones. The dorsal vertebrae of plesiosaurs are easily recognisable by two large ''foramina subcentralia'', paired vascular openings at the underside.<ref name="Smith2008"/>

The skull of plesiosaurs showed the "[[Euryapsida|euryapsid]]" condition, lacking the lower [[temporal fenestra]]e, the openings at the lower rear sides. The upper temporal fenestrae formed large openings at the sides of the rear skull roof, the attachment for muscles closing the lower jaws. Generally, the [[parietal bone]]s were very large, with a midline crest, while the [[squamosal bone]]s typically formed an arch, excluding the parietals from the [[occiput]]. The eye sockets were large, in general pointing obliquely upwards; the pliosaurids had more sideways directed eyes. The eyes were supported by [[scleral ring]]s, the form of which shows that they were relatively flat, an adaptation to diving. The anteriorly placed internal nostrils, the ''[[choana]]e'', have palatal grooves to channel water, the flow of which would be maintained by hydrodynamic pressure over the posteriorly placed, in front of the eye sockets, external [[nares]] during swimming. According to one hypothesis, during its passage through the nasal ducts, the water would have been 'smelled' by olfactory epithelia.<ref>{{cite journal | last1 = Cruickshank | first1 = A.R.I. | last2 = Small | first2 = P.G. | last3 = Taylor | first3 = M.A. | year = 1991 | title = Dorsal nostrils and hydrodynamically driven underwater olfaction in plesiosaurs | journal = Nature | volume = 352 | issue = 6330| pages = 62–64 | doi=10.1038/352062a0| bibcode = 1991Natur.352...62C | s2cid = 4353612 }}</ref><ref>{{cite journal | last1 = Brown | first1 = D. S. | last2 = Cruickshank | first2 = A. R. I. | year = 1994 | title = The skull of the Callovian plesiosaur ''Cryptoclidus eurymerus'' and the sauropterygian cheek | journal = Palaeontology | volume = 37 | issue = 4| pages = 941–953 }}</ref> However, more to the rear, a second pair of openings is present in the palate; a later hypothesis holds that these are the real ''choanae'' and the front pair in reality represented paired [[salt gland]]s.<ref>{{cite journal | last1 = Buchy | first1 = M C. | last2 = Frey | first2 = E. | last3 = Salisbury | first3 = S.W. | year = 2006 | title = The internal cranial anatomy of the Plesiosauria (Reptilia, Sauropterygia): evidence for a functional secondary palate | journal = Lethaia | volume = 39 | issue = 4| pages = 289–303 | doi=10.1080/00241160600847488| bibcode = 2006Letha..39..289B | url = http://doc.rero.ch/record/16186/files/PAL_E3394.pdf }}</ref> The distance between the eye sockets and the nostrils was so limited because the [[nasal bone]]s were strongly reduced, even absent in many species. The [[premaxilla]]e directly touched the [[frontal bone]]s; in the elasmosaurids, they even reached back to the [[parietal bone]]s. Often, the [[lacrimal bone]]s were also lacking.<ref name="Rieppel1997"/>
[[File:SeeleysaurusDB.jpg|thumb|''[[Seeleyosaurus]]'' with a tail fin]]
The tooth form and number was very variable. Some forms had hundreds of needle-like teeth. Most species had larger conical teeth with a round or oval cross-section. Such teeth numbered four to six in the premaxilla and about fourteen to twenty-five in the [[maxilla]]; the number in the lower jaws roughly equalled that of the skull. The teeth were placed in tooth-sockets, had vertically wrinkled enamel and lacked a true cutting edge or ''[[wikt:carina|carina]]''. With some species, the front teeth were notably longer, to grab prey.<ref>{{Cite web|title=Analysing The Skeleton: The Plesiosaur Diet|url=https://www.open.edu/openlearn/nature-environment/natural-history/nature-environment/natural-history/analysing-the-skeleton-the-plesiosaur-diet|access-date=2022-01-28|website=Analysing The Skeleton: The Plesiosaur Diet|language=en}}</ref>

===Soft tissues===
Soft tissue remains of plesiosaurs are rare, but sometimes, especially in [[shale]] deposits, they have been partly preserved, e.g. showing the outlines of the body. An early discovery in this respect was the holotype of ''Plesiosaurus conybeari'' (presently ''[[Attenborosaurus]]''). From such finds it is known that the skin was smooth, without apparent scales but with small wrinkles (although Frey ''et al''., (2017) reported that ''[[Mauriciosaurus]]'' had millimetric scale-like structures across the body that they interpret as scales<ref name="Frey2017">{{cite journal |last1=Frey |first1=Eberhard |last2=Mulder |first2=Eric W. A. |last3=Stinnesbeck |first3=Wolfgang |last4=Rivera-Sylva |first4=Héctor E. |last5=Padilla-Gutiérrez |first5=José Manuel |last6=González-González |first6=Arturo Homero |title=A new polycotylid plesiosaur with extensive soft tissue preservation from the early Late Cretaceous of northeast Mexico. |journal=Boletín de la Sociedad Geológica Mexicana |date=April 2017 |volume=69 |url=https://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1405-33222017000100087 |access-date=21 January 2025}}</ref>), that the trailing edge of the flippers extended considerably behind the limb bones,<ref>{{cite journal | last= Huene | first= F. von | year = 1923 | title = Ein neuer Plesiosaurier aus dem oberen Lias Württembergs | journal = Jahreshefte des Vereins für vaterländische Naturkunde in Württemberg | volume = 79 | pages = 1–21 }}</ref> and that the tail bore a vertical fin, as reported by [[Wilhelm Dames]] in his description of ''Plesiosaurus guilelmiimperatoris'' (presently ''[[Seeleyosaurus]]'').<ref>{{cite journal | last= Dames | first= W | year = 1895 | title = Die Plesiosaurier der Süddeutschen Liasformation | journal = Abhandlungen der Königlich Preussischen Akademie der Wissenschaften zu Berlin | volume = 1895 | pages = 1–81 }}</ref> The possibility of a tail fluke has been confirmed by recent studies on the caudal neural spine form of ''[[Pantosaurus]]'', ''[[Cryptoclidus]]'' and ''[[Rhomaleosaurus zetlandicus]]''.<ref>Wilhelm, B.C., 2010, ''Novel anatomy of cryptoclidid plesiosaurs with comments on axial locomotion''. Ph.D thesis, Marshall University, Huntington, WV. USA</ref><ref>{{cite journal | last1 = Wilhelm | first1 = B.C. | last2 = O'Keefe | first2 = F. | year = 2010 | title = A new partial skeleton of ''Pantosaurus striatus'', a cryptocleidoid Plesiosaur from the Upper Jurassic Sundance Formation of Wyoming | url = https://mds.marshall.edu/bio_sciences_faculty/58| journal = Journal of Vertebrate Paleontology | volume = 30 | issue = 6| pages = 1736–1742 | doi=10.1080/02724634.2010.521217| s2cid = 36408899 | url-access = subscription }}</ref><ref>{{cite journal | last= Smith | first= Adam S. | year = 2013 | title = Morphology of the caudal vertebrae in ''Rhomaleosaurus zetlandicus'' and a review of the evidence for a tail fin in Plesiosauria | journal = Paludicola | volume = 9 | issue = 3| pages = 144–158 }}</ref> A 2020 study claims that the caudal fin was horizontal in configuration.<ref name="ReferenceA">{{Cite journal |doi = 10.1134/S1062359019070100|title = Peculiarities of the Structure and Locomotor Function of the Tail in Sauropterygia|year = 2019|last= Sennikov|first= A. G.|journal = Biology Bulletin|volume = 46|issue = 7|pages = 751–762| bibcode=2019BioBu..46..751S |s2cid = 211217453}}</ref> In 2025, a subadult specimen of ''[[Plesiopterys]]'' (MH 7) from the Lower Jurassic [[Posidonia Shale]] ([[Germany]]) was reported to have preserved traces of scaly skin on the right front flipper and smooth skin on the tail, with the tail integument also showing preservation of dark-colored [[melanosome]]s and [[keratinocyte]]s.<ref>{{cite journal|last1=Marx |first1=M. |last2=Sjövall |first2=P. |last3=Kear |first3=B. P. |last4=Jarenmark |first4=M. |last5=Eriksson |first5=M. E. |last6=Sachs |first6=S. |last7=Nilkens |first7=K. |last8=Op De Beeck |first8=M. |last9=Lindgren |first9=J. |year=2025 |title=Skin, scales, and cells in a Jurassic plesiosaur |journal=Current Biology |volume=35 |issue=5 |pages=1113–1120.e3 | doi=10.1016/j.cub.2025.01.001 |doi-access=free }}</ref><ref>{{Cite journal |last1=Marx |first1=M. |last2=Sachs |first2=S. |last3=Kear |first3=B. P. |last4=Eriksson |first4=M. E. |last5=Nilkens |first5=K. |last6=Lindgren |first6=J. |year=2025 |title=A new specimen of ''Plesiopterys wildi'' reveals the diversification of cryptoclidian precursors and possible endemism within European Early Jurassic plesiosaur assemblages |journal=PeerJ |volume=13 |article-number=e18960 |doi=10.7717/peerj.18960 |pmid=40183068 |doi-access=free |pmc=11967415 }}</ref>

== Paleobiology ==
[[File:Plesiosaur on land.jpg|thumb|Painting of a plesiosaur on land, by [[Heinrich Harder]]]]
[[File:Gastroliths.jpg|thumb|upright|Plesiosaur gastroliths]]

===Food===
The probable food source of plesiosaurs varied depending on whether they belonged to the long-necked "plesiosauromorph" forms or the short-necked "pliosauromorph" species.

The extremely long necks of "plesiosauromorphs" have caused speculation as to their function from the very moment their special build became apparent. Conybeare had offered three possible explanations. The neck could have served to intercept fast-moving fish in a pursuit. Alternatively, plesiosaurs could have rested on the sea bottom, while the head was sent out to search for prey, which seemed to be confirmed by the fact the eyes were directed relatively upwards. Finally, Conybeare suggested the possibility that plesiosaurs swam on the surface, letting their necks plunge downwards to seek food at lower levels. All these interpretations assumed that the neck was very flexible. The modern insight that the neck was, in fact, rather rigid, with limited vertical movement, has necessitated new explanations. One hypothesis is that the length of the neck made it possible to surprise schools of fish, the head arriving before the sight or pressure wave of the trunk could alert them. "Plesiosauromorphs" hunted visually, as shown by their large eyes, and perhaps employed a directional sense of olfaction. Hard and soft-bodied cephalopods probably formed part of their diet. Their [[jaw]]s were probably strong enough to bite through the hard shells of this prey type. Fossil specimens have been found with cephalopod shells still in their stomach.<ref>{{cite journal | last1 = McHenry | first1 = C.R. | last2 = Cook | first2 = A.G. | last3 = Wroe | first3 = S. | year = 2005 | title = Bottom-feeding plesiosaurs | journal = Science | volume = 310 | issue = 5745| page = 75 | doi=10.1126/science.1117241 | pmid=16210529| s2cid = 28832109 }}</ref> The [[bony fish]] ([[Osteichthyes]]), which further diversified during the Jurassic, were likely prey as well. A very different hypothesis claims that "plesiosauromorphs" were bottom feeders. The stiff necks would have been used to plough the sea bottom, eating the [[benthos]]. This would have been proven by long furrows present in ancients seabeds.<ref>{{cite news|title=Plesiosaur bottom-feeding shown |url=http://news.bbc.co.uk/1/hi/sci/tech/4341204.stm |work=BBC News |date=17 October 2005 |access-date=21 May 2012}}</ref><ref>{{cite journal | last= Geister | first= J | year = 1998 | title = Lebensspuren von Meersauriern und ihren Beutetieren im mittleren Jura (Callovien) von Liesberg, Schweiz | journal = Facies | volume = 39 | issue = 1| pages = 105–124 | doi=10.1007/bf02537013| s2cid = 127249009 | url = https://boris.unibe.ch/89981/ }}</ref> Such a lifestyle has in 2017 been suggested for ''[[Morturneria]]''.<ref>{{cite journal | last1 = O'Keefe | first1 = F. | last2 = Otero | first2 = R. | last3 = Soto-Acuña | first3 = S. | last4 = O'Gorman | first4 = J. | last5 = Godfrey | first5 = S. | last6 = Chatterjee | first6 = S. | year = 2017 | title = Cranial anatomy of ''Morturneria seymourensis'' from Antarctica, and the evolution of filter feeding in plesiosaurs of the Austral Late Cretaceous | journal = Journal of Vertebrate Paleontology | volume =  37| issue = 4| article-number =  e1347570| doi = 10.1080/02724634.2017.1347570 | bibcode = 2017JVPal..37E7570O | s2cid = 91144814 | hdl = 11336/75235 | hdl-access = free }}</ref> "Plesiosauromorphs" were not well adapted to catching large fast-moving prey, as their long necks, though seemingly streamlined, caused enormous [[skin friction]]. [[Sankar Chatterjee]] suggested in 1989 that some [[Cryptocleididae]] were suspension feeders, filtering [[plankton]]. ''[[Aristonectes]]'' e.g. had hundreds of teeth, allowing it to sieve small [[Crustacea]] from the water.<ref>Chatterjee, S. and Small, B.J., 1989, "New plesiosaurs from the Upper Cretaceous of Antarctica", In: Crame, J. (ed) ''Origins and Evolution of Antarctic Biota'', pp. 197-215, Geological Society Publishing House, London</ref>

The short-necked "pliosauromorphs" were top carnivores, or [[apex predator]]s, in their respective [[foodweb]]s.<ref>{{cite web |title=The Plesiosaur Directory|url=http://www.plesiosauria.com/diet.php |access-date=20 April 2013}}</ref> They were pursuit predators<ref>{{cite journal | last= Massare | first= J.A. | year = 1992 | title = Ancient mariners | journal = Natural History | volume = 101 | pages = 48–53 }}</ref> or ambush predators of various sized prey and opportunistic feeders; their teeth could be used to pierce soft-bodied prey, especially fish.<ref>{{cite journal |author=J A Massare |year=1987 |title=Tooth morphology and prey preference of Mesozoic marine reptiles |journal=J. Vertebr. Paleontol. |volume=7 |issue= 2|pages=121–137 |doi=10.1080/02724634.1987.10011647|bibcode=1987JVPal...7..121M }}</ref> Their heads and teeth were very large, suited to grab and rip apart large animals. Their morphology allowed for a high swimming speed. They too hunted visually.

Plesiosaurs were themselves prey for other carnivores, as shown by bite marks left by a shark that have been discovered on a fossilized plesiosaur fin<ref>{{cite journal | last= Everhart | first= M. J. | 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 = Vertebrate Paleontology | volume = 2 | issue = 2| pages = 14–24 }}</ref> and the fossilized remains of a mosasaur's stomach contents that are thought to be the remains of a plesiosaur.<ref>{{cite journal | last= Everhart | first= M. J. | year = 2004 | title = Plesiosaurs as the food of mosasaurs; new data on the stomach contents of a ''Tylosaurus proriger'' (Squamata; Mosasauridae) from the Niobrara Formation of western Kansas | journal = The Mosasaur | volume = 7 | pages = 41–46 }}</ref>

Skeletons have also been discovered with [[gastrolith]]s, stones, in their stomachs, though whether to help break down food, especially cephalopods, in a muscular gizzard, or to vary [[buoyancy]], or both, has not been established.<ref>Williston, Samuel Wendel; 1904. The stomach stones of the plesiosaurs ''Science'' 20; 565</ref><ref>{{cite journal | last= Everhart | first= M. J. | year = 2000 | title = Gastroliths associated with plesiosaur remains in the Sharon Springs Member of the Pierre Shale (Late Cretaceous), western Kansas | journal = Kansas Acad. Sci. Trans | volume = 103 | issue = 1–2| pages = 58–69 | doi = 10.2307/3627940 | jstor = 3627940 }}</ref> However, the total weight of the gastroliths found in various specimens appears to be insufficient to modify the buoyancy of these large reptiles.<ref>{{cite journal | last1 = Cerda | first1 = A | last2 = Salgado | first2 = L | year = 2008 | title = Gastrolitos en un plesiosaurio (Sauropterygia) de la Formación Allen (Campaniano-Maastrichtiano), provincia de Río Negro, Patagonia, Argentina | journal = Ameghiniana | volume = 45 | pages = 529–536 }}</ref> The first plesiosaur gastroliths, found with ''[[Mauisaurus]] gardneri'' (a ''nomen nudum''<ref>{{cite journal|url=https://www.researchgate.net/publication/232672883 | title = The nature of ''Mauisaurus haasti'' Hector, 1874 (Reptilia: Plesiosauria) | journal=Journal of Vertebrate Paleontology | pages =  588–601| year = 2005 | volume=25| issue = 3 | doi = 10.1671/0272-4634(2005)025[0588:TNOMHH]2.0.CO;2 | s2cid = 130607702 | last1 = Hiller | first1 = Norton | last2 = Mannering | first2 = Al A. | last3 = Jones | first3 = Craig M. | last4 = Cruickshank | first4 = Arthur R. I. }}</ref>), were reported by [[Harry Govier Seeley]] in 1877.<ref>{{cite wikisource | last=Seeley | first=Harry Govier | title=On Mauisaurus Gardneri (Seeley), an Elasmosaurian from the Base of the Gault at Folkestone | wslink=Quarterly Journal of the Geological Society of London/Volume 33/On Mauisaurus Gardneri (Seeley), an Elasmosaurian from the Base of the Gault at Folkestone | work=Quarterly Journal of the Geological Society of London | volume=33 | year=1877 | doi=10.1144/gsl.jgs.1877.033.01-04.32 | pages=541–546 | firsticon=yes | noicon=yes }}</ref> The number of these stones per individual is often very large. In 1949, a fossil of ''[[Alzadasaurus]]'' (specimen SDSM 451, later renamed to ''[[Styxosaurus]]'') showed 253 of them.<ref>{{cite journal | last1 = Welles | first1 = S.P. | last2 = Bump | first2 = J.D. | year = 1949 | title = ''Alzadasaurus pembertoni'', a new elasmosaur from the Upper Cretaceous of South Dakota | journal = Journal of Paleontology | volume = 23 | issue = 5| pages = 521–535 }}</ref> The size of individual stones is often considerable. In 1991 an elasmosaurid specimen, KUVP 129744, was investigated, containing a gastrolith with a diameter of seventeen centimeters and a weight of 1,300 grams; and a somewhat shorter stone of 1,490 grams. In total, forty-seven gastroliths were present, with a combined weight of 13 kilograms. The size of the stones has been seen as an indication that they were not swallowed by accident, but deliberately, the animal perhaps covering large distances in search of a suitable rock type.<ref>{{cite journal | last= Everhart | first= M.J. | year = 2000 | title = Gastroliths associated with plesiosaur remains in the Sharon Springs Member of the Pierre Shale (late Cretaceous), Western Kansas | journal = Kansas Academy of Sciences Transactions | volume = 103 | issue = 1–2| pages = 58–69 }}</ref> The type specimen of ''[[Scalamagnus]]'' (MNA V10046) is associated with 289 [[gastrolith]]s, which is unusual in comparison to most polycotylid skeletons that generally lack gastroliths. Ranging from less than 0.1 grams to 18.5 grams, the total mass of the gastroliths was about 518 grams. About three-quarters of the stones weighed less than 2 grams, with the mean mass and median mass of the stones respectively estimated at 1.9 grams and 0.8 grams. The gastroliths had high mean value and variability in sphericity, suggesting that this individual was obtaining its stones from rivers located along the western side of the Western Interior Seaway.<ref>{{cite journal|first1=R.L.|last1=Schmeisser|first2=D.D.|last2=Gillette|year=2009|title=Unusual occurrence of gastroliths in a polycotylid plesiosaur from the Upper Cretaceous Tropic Shale, southern Utah|journal=PALAIOS|volume=24|issue=7 |pages=453–459|doi=10.2110/palo.2008.p08-085r|bibcode=2009Palai..24..453S |s2cid=128969768 }}</ref>

===Locomotion===

====Flipper movement====
[[File:Plesiosaur swimming animation.ogv|thumb|thumbtime=0:13|3D animation showing the most likely swimming motions]] 
The distinctive four-flippered body-shape has caused considerable speculation about what kind of stroke plesiosaurs used. The only modern group with four flippers are the sea turtles, which only use the front pair for propulsion. Conybeare and Buckland had already compared the flippers with bird wings. However, such a comparison was not very informative, as the mechanics of bird flight in this period were poorly understood. By the middle of the nineteenth century, it was typically assumed that plesiosaurs employed a rowing movement. The flippers would have been moved forward in a horizontal position, to minimise friction, and then axially rotated to a vertical position in order to be pulled to the rear, causing the largest possible [[Reaction (physics)|reactive force]]. In fact, such a method would be very inefficient: the recovery stroke in this case generates no thrust and the rear stroke generates an enormous turbulence. In the early twentieth century, the newly discovered principles of bird flight suggested to several researchers that plesiosaurs, like turtles and penguins, made a flying movement while swimming. This was e.g. proposed by [[Eberhard Fraas]] in 1905,<ref>{{cite journal | last= Fraas | first= E | year = 1905 | title = Reptilien und Säugetiere in ihren Anpassungserscheinungen an das marine Leben | journal = Jahresheften des Vereins für vaterländische Naturkunde in Württemberg | volume = 29 | pages = 347–386 }}</ref> and in 1908 by [[Othenio Abel]].<ref>{{cite journal | last= Abel | first= O | year = 1908 | title = Die Anpassungsformen der Wirbeltiere an das Meeresleben | journal = Schriften des Vereines zur Verbreitung Naturwissenschaftlicher Kenntnisse in Wien | volume = 48 | issue = 14| pages = 395–422 }}</ref> When flying, the flipper movement is more vertical, its point describing an oval or "8". Ideally, the flipper is first moved obliquely to the front and downwards and then, after a slight retraction and rotation, crosses this path from below to be pulled to the front and upwards. During both strokes, down and up, according to [[Bernoulli's principle]], forward and upward thrust is generated by the convexly curved upper profile of the flipper, the front edge slightly inclined relative to the water flow, while turbulence is minimal. However, despite the evident advantages of such a swimming method, in 1924 the first systematic study on the musculature of plesiosaurs by [[David Meredith Seares Watson]] concluded they nevertheless performed a rowing movement.<ref>{{cite journal | last= Watson | first= D.M.S. | year = 1924 | title = The elasmosaurid shoulder-girdle and fore-limb | journal = Proceedings of the Zoological Society of London | volume = 1924 | issue = 2| pages = 885–917 | doi = 10.1111/j.1096-3642.1924.tb03320.x }}</ref>

During the middle of the twentieth century, Watson's "rowing model" remained the dominant hypothesis regarding the plesiosaur swimming stroke. In 1957, [[Lambert Beverly Halstead]], at the time using the family name Tarlo, proposed a variant: the hindlimbs would have rowed in the horizontal plane but the forelimbs would have paddled, moved to below and to the rear.<ref>{{cite journal | last= Tarlo | first= L.B. | year = 1957 | title = The scapula of ''Pliosaurus macromerus'' Phillips | journal = Palaeontology | volume = 1 | pages = 193–199 }}</ref><ref>{{cite journal | last= Halstead | first= L.B. | year = 1989 | title = Plesiosaur locomotion | journal = Journal of the Geological Society | volume = 146 | issue = 1| pages = 37–40 | doi=10.1144/gsjgs.146.1.0037| bibcode = 1989JGSoc.146...37H| s2cid = 219541473 }}</ref> In 1975, the traditional model was challenged by [[Jane Ann Robinson]], who revived the "flying" hypothesis. She argued that the main muscle groups were optimally placed for a vertical flipper movement, not for pulling the limbs horizontally, and that the form of the shoulder and hip joints would have precluded the vertical rotation needed for rowing.<ref>{{cite journal | last= Robinson | first= J.A. | year = 1975 | title = The locomotion of plesiosaurs | journal = Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen | volume = 149 | issue = 3| pages = 286–332 }}</ref> In a subsequent article, Robinson proposed that the [[kinetic energy]] generated by the forces exerted on the trunk by the strokes, would have been stored and released as [[elastic energy]] in the ribcage, allowing for an especially efficient and dynamic propulsion system.<ref>{{cite journal | last= Robinson | first= J.A. | year = 1977 | title = Intercorporal force transmission in plesiosaurs | journal = Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen | volume = 153 | issue = 1| pages = 88–128 }}</ref>

In Robinson's model, both the downstroke and the upstroke would have been powerful. In 1982, she was criticised by [[Samuel Tarsitano]], [[Eberhard Frey]] and [[Jürgen Riess]], who claimed that, while the muscles at the underside of the shoulder and pelvic plates were clearly powerful enough to pull the limbs downwards, comparable muscle groups on the top of these plates to elevate the limbs were simply lacking, and, had they been present, could not have been forcefully employed, their bulging carrying the danger of hurting the internal organs. They proposed a more limited flying model in which a powerful downstroke was combined with a largely unpowered recovery, the flipper returning to its original position by the momentum of the forward moving and temporarily sinking body.<ref>{{cite journal | last1 = Tarsitano | first1 = S. | last2 = Riess | first2 = J. | year = 1982 | title = Plesiosaur locomotion — underwater flight versus rowing | journal = Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen | volume = 164 | issue = 1–2| pages = 193–194 | doi = 10.1127/njgpa/164/1982/188 }}</ref><ref>{{cite journal | last1 = Frey | first1 = E. | last2 = Reiss | first2 = J. | year = 1982 | title = Considerations concerning plesiosaur locomotion | journal = Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen | volume = 164 | issue = 1–2| pages = 188–192 | doi = 10.1127/njgpa/164/1982/193 }}</ref> This modified flying model became a popular interpretation. Less attention was given to an alternative hypothesis by [[Stephen Godfrey (biologist)|Stephen Godfrey]] in 1984, which proposed that both the forelimbs and hindlimbs performed a deep paddling motion to the rear combined with a powered recovery stroke to the front, resembling the movement made by the forelimbs of sea-lions.<ref>{{cite journal | last= Godfrey | first= Stephen J. | year = 1984 | title = Plesiosaur subaqueous locomotion: a reappraisal | journal = Neues Jahrbuch für Geologie und Paläontologie | volume = 1984 | issue = 11| pages = 661–672 | doi = 10.1127/njgpm/1984/1984/661 }}</ref>

In 2010, [[Frank Sanders (biologist)|Frank Sanders]] and [[Kenneth Carpenter]] published a study concluding that Robinson's model had been correct. Frey & Riess would have been mistaken in their assertion that the shoulder and pelvic plates had no muscles attached to their upper sides. While these muscle groups were probably not very powerful, this could easily have been compensated by the large muscles on the back, especially the ''[[latissimus dorsi]]'', which would have been well developed in view of the high spines on the backbone. Furthermore, the flat build of the shoulder and hip joints strongly indicated that the main movement was vertical, not horizontal.<ref name="Sanders2010">{{cite journal | last1 = Sanders | first1 = F. | last2 = Carpenter | first2 = K. | last3 = Reed | first3 = B. | last4 = Reed | first4 = J. | year = 2010 | title = Plesiosaur swimming reconstructed from skeletal analysis and experimental results | doi = 10.1660/062.113.0201 | journal = Transactions of the Kansas Academy of Science | volume = 113 | issue = 1/2| pages = 1–34 | s2cid = 86491931 }}</ref> 

====Gait====
[[File:Plesiosaurus2.jpg|thumb|Frey & Riess favoured an "alternating" gait.]]
Like all [[tetrapods]] with limbs, plesiosaurs must have had a certain [[gait]], a coordinated movement pattern of the, in this case, flippers. Of all the possibilities, in practice attention has been largely directed to the question of whether the front pair and hind pair moved simultaneously, so that all four flippers were engaged at the same moment, or in an alternate pattern, each pair being employed in turn. Frey & Riess in 1991 proposed an alternate model, which would have had the advantage of a more continuous propulsion.<ref>Riess, J. and E. Frey, 1991. "The evolution of underwater flight and the locomotion of plesiosaurs", In: J.M.V. Rayner and R.J. Wootton (eds.) ''Biomechanics in Evolution'', Cambridge, England: Cambridge University Press, pp. 131-144</ref> In 2000, [[Theagarten Lingham-Soliar]] evaded the question by concluding that, like sea turtles, plesiosaurs only used the front pair for a powered stroke. The hind pair would have been merely used for steering. Lingham-Soliar deduced this from the form of the hip joint, which would have allowed for only a limited vertical movement. Furthermore, a separation of the propulsion and steering function would have facilitated the general coordination of the body and prevented a too extreme [[Pitch (aviation)|pitch]]. He rejected Robinson's hypothesis that elastic energy was stored in the ribcage, considering the ribs too stiff for this.<ref>{{cite journal | last= Lingham-Soliar | first= T. | year = 2000 | title = Plesiosaur locomotion: Is the four-wing problem real or merely an atheoretical exercise? | journal = Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen | volume = 217 | pages = 45–87 | doi = 10.1127/njgpa/217/2000/45 }}</ref>

The interpretation by Frey & Riess became the dominant one, but was challenged in 2004 by Sanders, who showed experimentally that, whereas an alternate movement might have caused excessive pitching, a simultaneous movement would have caused only a slight pitch, which could have been easily controlled by the hind flippers. Of the other axial movements, [[rolling]] could have been controlled by alternately engaging the flippers of the right or left side, and [[Yaw (rotation)|yaw]] by the long neck or a vertical tail fin. Sanders did not believe that the hind pair was not used for propulsion, concluding that the limitations imposed by the hip joint were very relative.<ref>{{cite journal | last1 = Sanders | first1 = F. | last2 = Carpenter | first2 = K. | last3 = Reed | first3 = B. | last4 = Reed | first4 = J. | year = 2004 | title = Plesiosaur swimming reconstructed from skeletal analysis and experimental results | journal = Journal of Vertebrate Paleontology | volume = 24 | pages = 108A–109A | doi = 10.1080/02724634.2004.10010643 | s2cid = 220415208 }}</ref> In 2010, Sanders & Carpenter concluded that, with an alternating gait, the turbulence caused by the front pair would have hindered an effective action of the hind pair. Besides, a long gliding phase after a simultaneous engagement would have been very energy efficient.<ref name="Sanders2010"/> It is also possible that the gait was optional and was adapted to the circumstances. During a fast steady pursuit, an alternate movement would have been useful; in an ambush, a simultaneous stroke would have made a peak speed possible. When searching for prey over a longer distance, a combination of a simultaneous movement with gliding would have cost the least energy.<ref>{{cite journal | last1 = Long | first1 = J. H. | last2 = Schumaker | first2 = J. | last3 = Livingston | first3 = N. | last4 = Kemp | first4 = M. | year = 2006 | title = Four flippers or two? Tetrapodal swimming with an aquatic robot | journal = Bioinspiration & Biomimetics | volume = 1 | issue = 1| pages = 20–29 | doi=10.1088/1748-3182/1/1/003| pmid = 17671301 | bibcode = 2006BiBi....1...20L | s2cid = 1869747 }}</ref> In 2017, a study by Luke Muscutt, using a robot model, concluded that the rear flippers were actively employed, allowing for a 60% increase of the propulsive force and a 40% increase of efficiency. There would not have been a single optimal phase for all conditions, the gait likely having been changed as the situation demanded.<ref>{{cite journal | last1 = Muscutt | first1 = Luke E. | last2 = Dyke | first2 = Gareth | last3 = Weymouth | first3 = Gabriel D. | last4 = Naish | first4 = Darren | last5 = Palmer | first5 = Colin | last6 = Ganapathisubramani | first6 = Bharathram | year = 2017 | title = The four-flipper swimming method of plesiosaurs enabled efficient and effective locomotion | journal = Proceedings of the Royal Society B | volume = 284 | issue = 1861| article-number = 20170951 | doi = 10.1098/rspb.2017.0951 | pmid = 28855360 | pmc = 5577481 }}</ref>

====Speed====
[[File:Kronosaurus2 NT.jpg|thumb|A short-necked [[Pliosauridae|pliosaurid]] like ''[[Kronosaurus]]'' would have been capable of overtaking a more manoeuvrable long-necked plesiosaur.]]
In general, it is hard to determine the maximum speed of extinct sea creatures. For plesiosaurs, this is made more difficult by the lack of consensus about their flipper stroke and gait. There are no exact calculations of their [[Reynolds Number]]. Fossil impressions show that the skin was relatively smooth, not scaled, and this may have reduced [[form drag]].<ref name="Sanders2010"/> Small wrinkles are present in the skin that may have prevented separation of the [[laminar flow]] in the [[boundary layer]] and thereby reduced [[skin friction]].

[[Cruise (flight)|Sustained speed]] may be estimated by calculating the [[Drag (physics)|drag]] of a simplified model of the body, that can be approached by a [[prolate spheroid]], and the sustainable level of energy output by the [[muscles]]. A first study of this problem was published by [[Judy Massare]] in 1988.<ref>{{cite journal | last= Massare | first= J.A. | year = 1988 | title = Swimming capabilities of Mesozoic marine reptiles: implications for methods of predation | journal = Paleobiology | volume = 14 | issue = 2| pages = 187–205 | doi=10.1017/s009483730001191x| bibcode = 1988Pbio...14..187M | s2cid = 85810360 }}</ref> Even when assuming a low [[hydrodynamic efficiency]] of 0.65, Massare's model seemed to indicate that plesiosaurs, if warm-blooded, would have cruised at a speed of four meters per second, or about fourteen kilometers per hour, considerably exceeding the known speeds of extant dolphins and whales.<ref name="Massare1994">Massare, J. A., 1994, "Swimming capabilities of Mesozoic marine reptiles: a review", In: L. Maddock et al. (eds.) ''Mechanics and Physiology of Animal Swimming'', Cambridge, England: Cambridge University Press pp. 133–149</ref> However, in 2002 [[Ryosuke Motani]] showed that the formulae that Massare had used, had been flawed. A recalculation, using corrected formulae, resulted in a speed of half a meter per second (1.8&nbsp;km/h) for a cold-blooded plesiosaur and one and a half meters per second (5.4&nbsp;km/h) for an [[endothermic]] plesiosaur. Even the highest estimate is about a third lower than the speed of extant [[Cetacea]].<ref>{{cite journal | last= Motani | first= R | year = 2002 | title = Swimming speed estimation of extinct marine reptiles: energetic approach revisited | journal = Paleobiology | volume = 28 | issue = 2| pages = 251–262 | doi=10.1666/0094-8373(2002)028<0251:sseoem>2.0.co;2| bibcode = 2002Pbio...28..251M | s2cid = 56387158 }}</ref>

Massare also tried to compare the speeds of plesiosaurs with those of the two other main sea reptile groups, the [[Ichthyosauria]] and the [[Mosasauridae]]. She concluded that plesiosaurs were about twenty percent slower than advanced ichthyosaurs, which employed a very effective tunniform movement, oscillating just the tail, but five percent faster than mosasaurids, which were assumed to swim with an inefficient anguilliform, eel-like, movement of the body.<ref name="Massare1994"/>

The many plesiosaur species may have differed considerably in their swimming speeds, reflecting the various body shapes present in the group. While the short-necked "pliosauromorphs" (e.g. ''[[Liopleurodon]]'') may have been fast swimmers, the long-necked "plesiosauromorphs" were built more for manoeuvrability than for speed, slowed by a strong skin friction, yet capable of a fast rolling movement. Some long-necked forms, such as the [[Elasmosauridae]], also have relatively short stubby flippers with a low [[Aspect ratio (wing)|aspect ratio]], further reducing speed but improving roll.<ref>{{cite journal | last= O'Keefe | first= F.R. | year = 2001 | title = Ecomorphology of plesiosaur flipper geometry | url =http://www.science.marshall.edu/okeefef/PDFS/OKeefe01AR.pdf | journal = Journal of Evolutionary Biology | volume = 14 | issue = 6| pages = 987–991 | doi=10.1046/j.1420-9101.2001.00347.x| citeseerx = 10.1.1.579.4702 | s2cid = 53642687 }}</ref>

====Diving====
Few data are available that show exactly how deep plesiosaurs dived. That they dived to some considerable depth is proven by traces of [[decompression sickness]]. The heads of the ''[[Humerus|humeri]]'' and ''[[femur|femora]]'' with many fossils show [[necrosis]] of the bone tissue, caused by a too rapid ascent after deep diving. However, this does not allow to deduce some exact depth as the damage could have been caused by a few very deep dives, or alternatively by a great number of relatively shallow descents. The vertebrae show no such damage: they were probably protected by a superior blood supply, made possible by the arteries entering the bone through the two ''foramina subcentralia'', large openings in their undersides.<ref>{{cite journal | last1 = Rothschild | first1 = B.M. | last2 = Storrs | first2 = G.W. | year = 2003 | title = Decompression syndrome in plesiosaurs (Sauropterygia: Reptilia) | journal = Journal of Vertebrate Paleontology | volume = 23 | issue = 2| pages = 324–328 | doi=10.1671/0272-4634(2003)023[0324:dsipsr]2.0.co;2| s2cid = 86226384 | url = http://doc.rero.ch/record/15348/files/PAL_E2692.pdf }}</ref>

Descending would have been helped by a negative [[Archimedes' principle|Archimedes Force]], i.e. being denser than water. Of course, this would have had the disadvantage of hampering coming up again. Young plesiosaurs show [[pachyostosis]], an extreme density of the bone tissue, which might have increased relative weight. Adult individuals have more spongy bone. [[Gastroliths]] have been suggested as a method to increase weight<ref>{{cite journal | last= Taylor | first= M.A. | year = 1981 | title = Plesiosaurs — rigging and ballasting | journal = Nature | volume = 290 | issue = 5808| pages = 628–629 | doi=10.1038/290628a0| bibcode = 1981Natur.290..628T | s2cid = 10700992 | doi-access = free }}</ref> or even as means to attain neutral [[buoyancy]], swallowing or spitting them out again as needed.<ref>Taylor, M.A., 1993, "Stomach stones for feeding or buoyancy? The occurrence and function of gastroliths in marine tetrapods", ''Philosophical Transactions of the Royal Society of London B'' 341: 163–175</ref> They might also have been used to increase stability.<ref>{{cite journal | last= Henderson | first= D.M. | year = 2006 | title = Floating point: a computational study of buoyancy, equilibrium, and gastroliths in plesiosaurs | journal = Lethaia | volume = 39 | issue = 3| pages = 227–244 | doi=10.1080/00241160600799846| bibcode = 2006Letha..39..227H | url = http://doc.rero.ch/record/16190/files/PAL_E3399.pdf }}</ref>

The relatively large eyes of the [[Cryptocleididae]] have been seen as an adaptation to deep diving.<ref name="berezin2019"/<ref>A. Yu. Berezin 2019 "[https://www.academia.edu/44636598/%D0%9C%D0%BE%D1%80%D1%84%D0%BE%D1%84%D1%83%D0%BD%D0%BA%D1%86%D0%B8%D0%BE%D0%BD%D0%B0%D0%BB%D1%8C%D0%BD%D1%8B%D0%B5_%D0%BE%D1%81%D0%BE%D0%B1%D0%B5%D0%BD%D0%BD%D0%BE%D1%81%D1%82%D0%B8_%D0%BF%D0%BB%D0%B5%D0%B7%D0%B8%D0%BE%D0%B7%D0%B0%D0%B2%D1%80%D0%B0_Abyssosaurus_nataliae_PLESIOSAUROIDEA_PLESIOSAURIA_%D0%B2_%D1%81%D0%B2%D1%8F%D0%B7%D0%B8_%D1%81_%D0%B0%D0%B4%D0%B0%D0%BF%D1%82%D0%B0%D1%86%D0%B8%D0%B5%D0%B9_%D0%BA_%D0%B3%D0%BB%D1%83%D0%B1%D0%BE%D0%BA%D0%BE%D0%B2%D0%BE%D0%B4%D0%BD%D0%BE%D0%BC%D1%83_%D0%BE%D0%B1%D1%80%D0%B0%D0%B7%D1%83_%D0%B6%D0%B8%D0%B7%D0%BD%D0%B8 Morphofunctional features of the plesiosaur Abyssosaurus nataliae (Plesiosauroidea: Plesiosauria) in connection with adaptations to a deep-water lifestyle.]" Ministry of National Resources and Ecology of the Russian Federation [in Russian].</ref>

====Tail role====
A 2020 study has posited that sauropterygians relied on vertical tail strokes much like [[cetaceans]]. In plesiosaurs the trunk was rigid so this action was more limited and in conjunction with the flippers.<ref name="ReferenceA"/>

===Metabolism===
Traditionally, it was assumed that extinct reptile groups were [[Ectotherm|cold-blooded]] like modern reptiles. New research during the past decades has led to the conclusion that some groups, such as [[theropod]] [[dinosaurs]] and [[pterosaurs]], were very likely [[warm-blooded]]. Whether perhaps plesiosaurs were warm-blooded as well is difficult to determine. One of the indications of a high [[metabolism]] is the presence of fast-growing [[fibrolamellar bone]]. The pachyostosis with juvenile individuals makes it hard to establish whether plesiosaurs possessed such bone, though. However, it has been possible to check its occurrence with more basal members of the more inclusive group that plesiosaurs belonged to, the [[Sauropterygia]]. A study in 2010 concluded that fibrolamellar bone was originally present with sauropterygians.<ref>{{cite journal | last= Klein | first= N | year = 2010 | title = Long Bone Histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin Provides Unexpected Implications for Phylogeny | journal = PLOS ONE | volume = 5 | issue = 7| article-number = e11613 | doi = 10.1371/journal.pone.0011613 | pmid=20657768 | pmc=2908119| bibcode = 2010PLoSO...511613K | doi-access = free }}</ref> A subsequent publication in 2013 found that the [[Nothosauridae]] lacked this bone matrix type but that basal [[Pistosauria]] possessed it, a sign of a more elevated metabolism.<ref>{{cite journal | last1 = Krahl | first1 = Anna | last2 = Klein | first2 = Nicole | last3 = Sander | first3 = P Martin | year = 2013 | title = Evolutionary implications of the divergent long bone histologies of ''Nothosaurus'' and ''Pistosaurus'' (Sauropterygia, Triassic) | journal = BMC Evolutionary Biology | volume = 13 | issue = 1 | page = 123 | doi=10.1186/1471-2148-13-123| pmid = 23773234 | pmc = 3694513 | doi-access = free | bibcode = 2013BMCEE..13..123K }}</ref> It is thus more [[parsimonious]] to assume that the more derived pistosaurians, the plesiosaurs, also had a faster metabolism. A paper published in 2018 claimed that plesiosaurs had resting metabolic rates (RMR) in the range of birds based on quantitative osteohistological modelling.<ref>{{Cite journal|last1=Fleischle|first1=Corinna V.|last2=Wintrich|first2=Tanja|last3=Sander|first3=P. Martin|date=2018-06-06|title=Quantitative histological models suggest endothermy in plesiosaurs|journal=PeerJ|language=en|volume=6|article-number=e4955|doi=10.7717/peerj.4955|issn=2167-8359|pmc=5994164|pmid=29892509 |doi-access=free }}</ref> However, these results are problematic in view of general principles of vertebrate physiology (see [[Kleiber's law]]); evidence from isotope studies of plesiosaur tooth enamel indeed suggests [[Endotherm|endothermy]] at lower RMRs, with inferred body temperatures of ca. {{Cvt|26|C}}.<ref>{{Cite journal|last1=Bernard|first1=Aurélien|last2=Lécuyer|first2=Christophe|last3=Vincent|first3=Peggy|last4=Amiot|first4=Romain|last5=Bardet|first5=Nathalie|last6=Buffetaut|first6=Eric|last7=Cuny|first7=Gilles|last8=Fourel|first8=François|last9=Martineau|first9=François|last10=Mazin|first10=Jean-Michel|last11=Prieur|first11=Abel|date=2010-06-11|title=Regulation of Body Temperature by Some Mesozoic Marine Reptiles|journal=Science|language=en|volume=328|issue=5984|pages=1379–1382|doi=10.1126/science.1187443 |issn=1095-9203|pmid=20538946|bibcode=2010Sci...328.1379B|s2cid=206525584}}</ref> Analysis isotopes of oxygen ratio suggests Plesiosauria were [[Poikilotherm|poikilothermic]] endotherms, having a body temperature range of {{Cvt|27-34|C}}.<ref>{{Cite journal |last1=Séon |first1=Nicolas |last2=Vincent |first2=Peggy |last3=Delsett |first3=Lene L. |display-authors=et al. |date=April 16, 2025 |title=Reassessment of body temperature and thermoregulation strategies in Mesozoic marine reptiles |url=https://www.cambridge.org/core/journals/paleobiology/article/reassessment-of-body-temperature-and-thermoregulation-strategies-in-mesozoic-marine-reptiles/5DB88592AC69236250A516904E20FEC4 |journal=Paleobiology |pages=1–21}}</ref>

===Reproduction===
[[File:Polycotylus NT.jpg|thumb|A ''Polycotylus'' female giving birth to her single young]]
As reptiles in general are [[oviparous]], until the end of the twentieth century it had been seen as possible that smaller plesiosaurs may have crawled up on a beach to lay eggs, like modern [[turtle]]s. Their strong limbs and a flat underside seemed to have made this feasible. This method was, for example, defended by Halstead. However, as those limbs no longer had functional elbow or knee joints and the underside by its very flatness would have generated a lot of friction, already in the nineteenth century it was hypothesised that plesiosaurs had been [[viviparous]]. Besides, it was hard to conceive how the largest species, as big as whales, could have survived a beaching. Fossil finds of ichthyosaur embryos showed that at least one group of marine reptiles had borne live young. The first to claim that similar embryos had been found in plesiosaurs was [[Harry Govier Seeley]], who reported in 1887 having acquired a [[Nodule (geology)|nodule]] with four to eight tiny skeletons.<ref>{{cite journal | last= Seeley | first= H.G. | year = 1888 | title = On the Mode of Development of the Young in ''Plesiosaurus'' | journal = Report of the British Association for the Advancement of Science; Held at Manchester September | volume = 1887 | pages = 697–698 }}</ref> In 1896, he described this discovery in more detail.<ref>Seeley, H. G.; 1896; "On a pyritous concretion from the Lias of Whitby which appears to show the external form of the body of embryos of a species of ''Plesiosaurus''", ''Annual Report of Yorkshire philosophical Society'' pp.20-29</ref> If authentic, the embryos of plesiosaurs would have been very small, like those of ichthyosaurs. However, in 1982 [[Richard Anthony Thulborn]] showed that Seeley had been deceived by a "doctored" fossil of a nest of crayfish.<ref>{{cite journal | last= Thulborn | first= R.A. | year = 1982 | title = Liassic plesiosaur embryos reinterpreted as shrimp burrows | journal = Palaeontology | volume = 25 | pages = 351–359 }}</ref>

An actual plesiosaur specimen found in 1987 eventually proved that plesiosaurs gave birth to live young:<ref name="O'Keefe2011">{{cite journal |last1=O'Keefe |first1=F.R. |last2=Chiappe |first2=L.M. |author-link2=Luis M. Chiappe |year=2011 |title=Viviparity and K-Selected Life History in a Mesozoic Marine Plesiosaur (Reptilia, Sauropterygia) |journal=[[Science (journal)|Science]] |volume=333 |issue=6044 |pages=870–873 |bibcode=2011Sci...333..870O |doi=10.1126/science.1205689 |pmid=21836013 |s2cid=36165835}}</ref> This fossil of a pregnant ''[[Polycotylus]] latippinus'' shows that these animals gave birth to a single large juvenile and probably invested parental care in their offspring, similar to modern whales. The young was 1.5 meters (five feet) long and thus large compared to its mother of five meters (sixteen feet) length, indicating a [[K-strategy]] in reproduction.<ref>{{Cite web|first=Jennifer|last=Welsh|title=Pregnant Fossil Suggests Ancient 'Sea Monsters' Birthed Live Young|url=http://www.livescience.com/15517-pregnant-plesiosaur-big-live-young.html|publisher=LiveScience|date=11 August 2011|access-date=21 May 2012}}</ref> Little is known about growth rates or a possible [[sexual dimorphism]].

===Social behavior and intelligence===
From the parental care indicated by the large size of the young, it can be deduced that social behavior in general was relatively complex.<ref name="O'Keefe2011"/> It is not known whether plesiosaurs hunted in packs. Their relative brain size seems to be typical for reptiles. Of the senses, sight and smell were important, hearing less so; elasmosaurids have lost the [[stapes]] completely. It has been suggested that with some groups the skull housed electro-sensitive organs.<ref>{{cite journal | last1 = O'Gorman | first1 = J.P. | last2 = Gasparini | first2 = Z. | year = 2013 | title = Revision of ''Sulcusuchus erraini'' (Sauropterygia, Polycotylidae) from the Upper Cretaceous of Patagonia, Argentina | journal = Alcheringa | volume = 37 | issue = 2| pages = 161–174 | doi = 10.1080/03115518.2013.736788 | bibcode = 2013Alch...37..163O | s2cid = 131429825 | hdl = 11336/2489 | hdl-access = free }}</ref><ref>{{cite journal | last1 = Foffa | first1 = D. | last2 = Sassoon | first2 = J. | last3 = Cuff | first3 = A.R. | last4 = Mavrogordato | first4 = M.N. | last5 = Benton | first5 = M.J. | year = 2014 | title = Complex rostral neurovascular system in a giant pliosaur | journal = Naturwissenschaften | volume = 101 | issue = 5| pages = 453–456 | doi=10.1007/s00114-014-1173-3| pmid = 24756202 | bibcode = 2014NW....101..453F | s2cid = 7406418 }}</ref>

===Paleopathology===
Some plesiosaur fossils show [[pathologies]], the result of illness or old age. In 2012, a [[mandible]] of ''[[Pliosaurus]]'' was described with a jaw joint clearly afflicted by [[arthritis]], a typical sign of [[senescence]].<ref>{{cite journal | last1 = Sassoon | first1 = J. | last2 = Noe | first2 = L.F. | last3 = Benton | first3 = M.J. | year = 2012 | title = Cranial anatomy, taxonomic implications and palaeopathology of an Upper Jurassic pliosaur (Reptilia: Sauropterygia) from Westbury, Wiltshire, UK | journal = Palaeontology | volume =  55| issue = 4| pages =  743–773| doi = 10.1111/j.1475-4983.2012.01151.x | bibcode = 2012Palgy..55..743S | doi-access = free }}</ref>

==Distribution==
Plesiosaur fossils have been found on every continent, including [[Antarctica]].<ref>{{cite journal | last1 = Chatterjee | first1 = Sankar | last2 = Small | first2 = Brian J. | last3 = Nickell | first3 = M. W. | year = 1984 | title = Late Cretaceous marine reptiles from Antarctica; | journal = Antarctic Journal of the United States | volume = 19 | issue = 5| pages = 7–8 }}</ref>

=== Timeline of Plesiosauria Species===

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 from: -252.2   till:  -247.2    color:earlytriassic  text:[[Early Triassic|Early]]
 from: -247.2   till:  -235    color:middletriassic  text:[[Middle Triassic|Middle]] 
 from: -235   till:  -199.6    color:latetriassic  text:[[Late Triassic|Late]]            
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 from: -252.2   till:  -199.6    color:triassic  text:[[Triassic]]         
 from: -199.6    till: -145    color:jurassic    text:[[Jurassic]]
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 align:left fontsize:M mark:(line,white) width:5 anchor:till align:left


 color:ANK bar:NAM1  from:-205    till:-204 text:[[Rhaeticosaurus mertensi]]
 color:ANK bar:NAM2  from:-201    till:-199.6 text:[[Anningasaura lymense]]
 color:ANK bar:NAM3  from:-201    till:-196.5 text:[[Thalassiodracon hawkinsi]]
 color:ANK bar:NAM4  from:-201    till:-200 text:[[Atychodracon megacephalus]]
 color:ANK bar:NAM5  from:-199.6    till:-198 text:[[Avalonnectes arturi]]
 color:ANK bar:NAM6  from:-199.6    till:-198 text:[[Eoplesiosaurus antiquior]]
 color:ANK bar:NAM7  from:-199.6    till:-198 text:[[Stratesaurus taylori]]
 color:ANK bar:NAM8  from:-199.6    till:-198 text:[[Eurycleidus arcuatus]]
 color:ANK bar:NAM9  from:-199    till:-195 text:[[Eretmosaurus rugosus]]
 color:ANK bar:NAM10  from:-199    till:-195 text:[[Macroplata tenuiceps]]
 color:ANK bar:NAM11  from:-199    till:-193 text:[[Archaeonectrus rostratus]]
 color:ANK bar:NAM12  from:-197    till:-193 text:[[Plesiopharos moelensis]]
 color:ANK bar:NAM13  from:-197    till:-192 text:[[Plesiosaurus dolichodeirus]]
 color:ANK bar:NAM14  from:-196.5    till:-189.6 text:[[Attenborosaurus conybeari]]
 color:ANK bar:NAM15  from:-195    till:-185 text:[[Eurysaurus raincourti]]
 color:ANK bar:NAM16  from:-193    till:-184 text:[[Westphaliasaurus simonsensii]]
 color:ANK bar:NAM17  from:-190    till:-182 text:[[Arminisaurus schuberti]]
 color:ANK bar:NAM18  from:-185    till:-183 text:[[Cryonectes neustriacus]]
 color:ANK bar:NAM19  from:-183    till:-182 text:[[Plesiopterys wildi]]
 color:ANK bar:NAM20  from:-183    till:-180 text:[[Meyerasaurus victor]]
 color:ANK bar:NAM21  from:-183    till:-180 text:[[Rhomaleosaurus cramptoni]]
 color:ANK bar:NAM22  from:-183    till:-176 text:[[Sthenarosaurus dawkinsi]]
 color:ANK bar:NAM23  from:-183    till:-176 text:[[Microcleidus brachypterygius]]
 color:ANK bar:NAM24  from:-183    till:-176 text:[[Microcleidus homalospondylus]]
 color:ANK bar:NAM25  from:-183    till:-176 text:[[Microcleidus tournemiensis]]
 color:ANK bar:NAM26  from:-183    till:-175 text:[[Bishanopliosaurus youngi]]
 color:ANK bar:NAM27  from:-183    till:-175 text:[[Hydrorion brachypterygius]]
 color:ANK bar:NAM28  from:-183    till:-175 text:[[Lusonectes sauvagei]]
 color:ANK bar:NAM29  from:-183    till:-175 text:[[Rhomaleosaurus thorntoni]]
 color:ANK bar:NAM30  from:-183    till:-175 text:[[Rhomaleosaurus zetlandicus]]
 color:ANK bar:NAM31  from:-183    till:-175 text:[[Seeleyosaurus guilelmiimperatoris]]
 color:ANK bar:NAM32  from:-182.7    till:-180.7 text:[[Hauffiosaurus longirostris]]
 color:ANK bar:NAM33  from:-182.7    till:-180.7 text:[[Hauffiosaurus tomistomimus]]
 color:ANK bar:NAM34  from:-182.7    till:-180.7 text:[[Hauffiosaurus zanoni]]
 color:ANK bar:NAM35  from:-180    till:-177 text:[[Rhomaleosaurus propinquus]]
 color:ANK bar:NAM36  from:-175    till:-174 text:[[Franconiasaurus brevispinus]]
 color:ANK bar:NAM37  from:-174    till:-164 text:[[Yuzhoupliosaurus chengjiangensis]]
 color:ANK bar:NAM38  from:-171    till:-169 text:[[Maresaurus coccai]]
 color:ANK bar:NAM39  from:-168    till:-167 text:[[Lorrainosaurus keileni]]
 color:ANK bar:NAM40  from:-166    till:-163 text:[[Peloneustes philarchus]]
 color:ANK bar:NAM41  from:-166    till:-160 text:[[Cryptoclidus eurymerus]]
 color:ANK bar:NAM42  from:-166    till:-160 text:[[Eardasaurus powelli]]
 color:ANK bar:NAM43  from:-166    till:-160 text:[[Pachycostasaurus dawni]]
 color:ANK bar:NAM44  from:-166    till:-155 text:[[Liopleurodon ferox]]
 color:ANK bar:NAM45  from:-165    till:-161 text:[[Picrocleidus beloclis]]
 color:ANK bar:NAM46  from:-165    till:-161 text:[[Borealonectes russelli]]
 color:ANK bar:NAM47  from:-165    till:-145 text:[[Colymbosaurus megadeirus]]
 color:ANK bar:NAM48  from:-164    till:-163 text:[[Marmornectes candrewi]]
 color:ANK bar:NAM49  from:-164    till:-160 text:[[Muraenosaurus leedsii]]
 color:ANK bar:NAM50  from:-164    till:-160 text:[[Tricleidus seeleyi]]
 color:ANK bar:NAM51  from:-164    till:-157 text:[[Tatenectes laramiensis]]
 color:ANK bar:NAM52  from:-161    till:-156 text:[[Gallardosaurus iturraldei]]
 color:ANK bar:NAM53  from:-161    till:-156 text:[[Vinialesaurus caroli]]
 color:ANK bar:NAM54  from:-161    till:-160 text:[[Anguanax zignoi]]
 color:ANK bar:NAM55  from:-160    till:-155 text:[[Pantosaurus striatus]]
 color:ANK bar:NAM56  from:-156    till:-152 text:[[Megalneusaurus rex]]
 color:ANK bar:NAM57  from:-155    till:-154 text:[[Pliosaurus brachydeirus]]
 color:ANK bar:NAM58  from:-155    till:-154 text:[[Pliosaurus kevani]]
 color:ANK bar:NAM59  from:-152    till:-150 text:[[Kimmerosaurus langhami]]
 color:ANK bar:NAM60  from:-151    till:-150 text:[[Pliosaurus carpenteri]]
 color:ANK bar:NAM61  from:-151    till:-150 text:[[Pliosaurus westburyensis]]
 color:ANK bar:NAM62  from:-150    till:-145 text:[[Djupedalia engeri]]
 color:ANK bar:NAM63  from:-150    till:-145 text:[[Spitrasaurus larseni]]
 color:ANK bar:NAM64  from:-150    till:-145 text:[[Spitrasaurus wensaasi]]
 color:ANK bar:NAM65  from:-148    till:-147 text:[[Pliosaurus funkei]]
 color:ANK bar:NAM66  from:-148    till:-147 text:[[Pliosaurus rossicus]]
 color:ANK bar:NAM67  from:-146    till:-145 text:[[Ophthalmothule cryostea]]
 color:ANK bar:NAM68  from:-141    till:-140 text:[[Brancasaurus brancai]]
 color:ANK bar:NAM69  from:-140    till:-135 text:[[Hastanectes valdensis]]
 color:ANK bar:NAM70  from:-135    till:-132 text:[[Leptocleidus capensis]]
 color:ANK bar:NAM71  from:-133    till:-130 text:[[Abyssosaurus nataliae]]
 color:ANK bar:NAM72  from:-130    till:-129 text:[[Jucha squalea]]
 color:ANK bar:NAM73  from:-130    till:-129 text:[[Makhaira rossica]]
 color:ANK bar:NAM74  from:-130    till:-129 text:[[Lagenanectes richterae]]
 color:ANK bar:NAM75  from:-129    till:-126 text:[[Acostasaurus pavachoquensis]]
 color:ANK bar:NAM76  from:-129    till:-125 text:[[Leptocleidus clemai]]
 color:ANK bar:NAM77  from:-129    till:-121 text:[[Leptocleidus superstes]]
 color:ANK bar:NAM78  from:-129    till:-128 text:[[Luskhan itilensis]]
 color:ANK bar:NAM79  from:-127    till:-125 text:[[Vectocleidus pastorum]]
 color:ANK bar:NAM80  from:-126    till:-123 text:[[Stenorhynchosaurus munozi]]
 color:ANK bar:NAM81  from:-125    till:-112 text:[[Callawayasaurus colombiensis]]
 color:ANK bar:NAM82  from:-125    till:-100 text:[[Woolungasaurus glendowerensis]]
 color:ANK bar:NAM83  from:-123    till:-121 text:[[Sachisaurus vitae]]
 color:ANK bar:NAM84  from:-120    till:-113 text:[[Opallionectes andamookaensis]]
 color:ANK bar:NAM85  from:-120    till:-100 text:[[Kronosaurus queenslandicus]]
 color:ANK bar:NAM86  from:-119    till:-112 text:[[Monquirasaurus boyacensis]]
 color:ANK bar:NAM87  from:-115    till:-114 text:[[Umoonasaurus demoscyllus]]
 color:ANK bar:NAM88  from:-115    till:-110 text:[[Sinopliosaurus weiyuanensis]]
 color:ANK bar:NAM89  from:-112    till:-111 text:[[Wapuskanectes betsynichollsae]]
 color:ANK bar:NAM90  from:-111    till:-110 text:[[Nichollssaura borealis]]
 color:ANK bar:NAM91  from:-104    till:-103 text:[[Eromangasaurus australis]]
 color:ANK bar:NAM92  from:-103    till:-98.5 text:[[Edgarosaurus muddi]]
 color:ANK bar:NAM93  from:-101    till:-89.3 text:[[Polyptychodon interruptus]]
 color:ANK bar:NAM94  from:-100.5    till:-94 text:[[Pahasapasaurus haasi]]
 color:ANK bar:NAM95  from:-100.5    till:-89.3 text:[[Brachauchenius lucasi]]
 color:ANK bar:NAM96  from:-99.6    till:-93.5 text:[[Trinacromerum bentonianum]]
 color:ANK bar:NAM97  from:-99    till:-98 text:[[Plesiopleurodon wellesi]]
 color:ANK bar:NAM98  from:-95    till:-94 text:[[Thalassomedon haningtoni]]
 color:ANK bar:NAM99  from:-95    till:-92 text:[[Eopolycotylus rankini]]
 color:ANK bar:NAM100  from:-95    till:-92 text:[[Scalamagnus tropicensis]]
 color:ANK bar:NAM101  from:-94.3    till:-89.3 text:[[Ogmodirus martini]]
 color:ANK bar:NAM102  from:-94    till:-89 text:[[Palmulasaurus quadratus]]
 color:ANK bar:NAM103  from:-94    till:-89 text:[[Plesioelasmosaurus walker]]
 color:ANK bar:NAM104  from:-94    till:-92 text:[[Mauriciosaurus fernandezi]]
 color:ANK bar:NAM105  from:-94    till:-92 text:[[Megacephalosaurus eulerti]]
 color:ANK bar:NAM106  from:-94    till:-92 text:[[Thililua longicollis]]
 color:ANK bar:NAM107  from:-94    till:-90 text:[[Manemergus anguirostris]]
 color:ANK bar:NAM108  from:-94    till:-84 text:[[Trinacromerum kirki]]
 color:ANK bar:NAM109  from:-93    till:-92 text:[[Libonectes morgani]]
 color:ANK bar:NAM110  from:-87    till:-80 text:[[Dolichorhynchops osborni]]
 color:ANK bar:NAM111  from:-86.3    till:-83.6 text:[[Futabasaurus suzukii]]
 color:ANK bar:NAM112  from:-86.3    till:-83.6 text:[[Georgiasaurus penzensis]]
 color:ANK bar:NAM113  from:-83.6    till:-82 text:[[Polycotylus latipinnis]]
 color:ANK bar:NAM114  from:-83.6    till:-82 text:[[Polycotylus sopozkoi]]
 color:ANK bar:NAM115  from:-83.5    till:-80.5 text:[[Styxosaurus browni]]
 color:ANK bar:NAM116  from:-83.5    till:-80.5 text:[[Styxosaurus snowii]]
 color:ANK bar:NAM117  from:-83    till:-72 text:[[Dolichorhynchops bonneri]]
 color:ANK bar:NAM118  from:-81    till:-80 text:[[Elasmosaurus platyurus]]
 color:ANK bar:NAM119  from:-80    till:-75 text:[[Scanisaurus nazarowi]]
 color:ANK bar:NAM120  from:-78    till:-77 text:[[Mauisaurus haasti]]
 color:ANK bar:NAM121  from:-76    till:-75 text:[[Fluvionectes sloanae]]
 color:ANK bar:NAM122  from:-75    till:-72 text:[[Dolichorhynchops herschelensis]]
 color:ANK bar:NAM123  from:-75    till:-72 text:[[Terminonatator ponteixensis]]
 color:ANK bar:NAM124  from:-74    till:-73 text:[[Albertonectes vanderveldei]]
 color:ANK bar:NAM125  from:-74    till:-73 text:[[Nakonanectes bradti]]
 color:ANK bar:NAM126  from:-74    till:-67 text:[[Leurospondylus ultimus]]
 color:ANK bar:NAM127  from:-73    till:-72 text:[[Sulcusuchus erraini]]
 color:ANK bar:NAM128  from:-73    till:-72 text:[[Vegasaurus molyi]]
 color:ANK bar:NAM129  from:-72    till:-70 text:[[Kawanectes lafquenianum]]
 color:ANK bar:NAM130  from:-72    till:-68 text:[[Tuarangisaurus keyesi]]
 color:ANK bar:NAM131  from:-72    till:-66 text:[[Alexandronectes zealandiensis]]
 color:ANK bar:NAM132  from:-71    till:-70 text:[[Cardiocorax mukulu]]
 color:ANK bar:NAM133  from:-70    till:-69.5 text:[[Serpentisuchops pfisterae]]
 color:ANK bar:NAM134  from:-70    till:-69 text:[[Kaiwhekea katiki]]
 color:ANK bar:NAM135  from:-70    till:-66 text:[[Aphrosaurus furlong]]
 color:ANK bar:NAM136  from:-70    till:-66 text:[[Aristonectes parvidens]]
 color:ANK bar:NAM137  from:-70    till:-66 text:[[Aristonectes quiriquinensis]]
 color:ANK bar:NAM138  from:-70    till:-66 text:[[Chubutinectes carmeloi]]
 color:ANK bar:NAM139  from:-70    till:-66 text:[[Cimoliasaurus magnus]]
 color:ANK bar:NAM140  from:-70    till:-66 text:[[Fresnosaurus drescheri]]
 color:ANK bar:NAM141  from:-70    till:-66 text:[[Hydrotherosaurus alexandrae]]
 color:ANK bar:NAM142  from:-70    till:-66 text:[[Morenosaurus stocki]]
 color:ANK bar:NAM143  from:-70    till:-66 text:[[Morturneria seymourensis]]
 color:ANK bar:NAM144  from:-70    till:-66 text:[[Zarafasaura oceanis]]



PlotData=
 align:center textcolor:black fontsize:M mark:(line,black) width:25 

 bar:period
 from: -252.2   till:  -247.2    color:earlytriassic  text:[[Early Triassic|Early]]
 from: -247.2   till:  -235    color:middletriassic  text:[[Middle Triassic|Middle]] 
 from: -235   till:  -199.6    color:latetriassic  text:[[Late Triassic|Late]]            
 from: -199.6    till: -175.6    color:earlyjurassic    text:[[Early Jurassic|Early]]
 from: -175.6    till: -161.2    color:middlejurassic    text:[[Middle Jurassic|Middle]]
 from: -161.2    till: -145    color:latejurassic    text:[[Late Jurassic|Late]]
 from: -145    till: -99.6    color:earlycretaceous    text:[[Early Cretaceous|Early]]
 from: -99.6    till: -65.5    color:latecretaceous    text:[[Late Cretaceous|Late]]

 bar:era
 from: -252.2   till:  -199.6    color:triassic  text:[[Triassic]]         
 from: -199.6    till: -145    color:jurassic    text:[[Jurassic]]
 from: -145    till: -65.5   color:cretaceous    text:[[Cretaceous]]

</timeline>

===Stratigraphic distribution===
{{Main|List of plesiosaur-bearing stratigraphic units}}

The following is a list of [[Geological formation|geologic formations]] that have produced plesiosaur fossils.

{| class="wikitable sortable" align="center" width="100%"
|-
! Name
! Age
! Location
! Notes
|-
| [[Agardhfjellet Formation]]
| [[Tithonian]]
| {{Flag|Norway}}
| ''[[Colymbosaurus svalbardensis]]'', ''[[Djupedalia]]'', ''[[Pliosaurus funkei]]'', ''[[Spitrasaurus]]''
|-
| [[Akrabou Formation]]
| [[Turonian]]
| {{Flag|Morocco}}
| ''[[Manemergus]]'', ''[[Thililua]], [[Libonectes]]''
|-
| [[Hasa, Jordan|Al-Hasa]], Phosphorite formation
| [[Campanian]]-[[Maastrichtian]]
| {{Flag|Jordan}}
| ''[[Plesiosaurus |Plesiosaurus mauritanicus]]''
|-
| [[Allen Formation]]
| [[Campanian]]-[[Maastrichtian]]
| {{Flag|Argentina}}
|
|-
| [[Al-Sawwanah al-Sharqiyah]], Phosphate mine
| [[Coniacian]]-[[Santonian]]
| {{Flag|Syria}}
| ''[[Elasmosauridae]]''<ref>{{cite journal |last=Adel Alhalabi |first=Wafa |display-authors=et al. |title=Recovering lost time in Syria: New Late Cretaceous (Coniacian-Santonian) elasmosaurid remains from the Palmyrides mountain chain |journal=[[Cretaceous Research]] |year=2024 |volume=159 |article-number=105871 |doi=10.1016/j.cretres.2024.105871 }}</ref><ref>{{cite web|url=https://arabic.cnn.com/travel/video/2017/08/30/v14236-tr-300817-syria-blizzosaur|script-title=ar:ماذا تعرفون عن الـ"بليزوصور"؟ شاهدوا ما تم اكتشافه في سوريا مؤخراً|website=CNN|language=ar|date=30 August 2017}}</ref>
|-
| [[Ampthill Clay Formation]]
| [[Oxfordian (stage)|Oxfordian]]
| {{Flag|UK}}
| ''[[Liopleurodon pachydeirus]]''
|-
| [[Bearpaw Formation]]
| [[Campanian]]
| {{plainlist|
*{{Flag|Canada}}
*{{Flag|US}}
}}
| ''[[Albertonectes]]'', ''[[Dolichorhynchops herschelensis]]'', ''[[Terminonatator]]''
|-
| [[Blue Lias Formation]]
| [[Rhaetian]]-[[Hettangian]]
| {{Flag|UK}}
| ''[[Anningasaura]]'', ''[[Avalonnectes]]'', ''[[Eoplesiosaurus]]'', ''[[Eurycleidus]]'', ''[[Plesiosaurus cliduchus|"Plesiosaurus" cliduchus]]'', ''[[Plesiosaurus dolichodeirus]]'', ''[[Plesiosaurus macrocephalus|"Plesiosaurus" macrocephalus]]'', ''[[Rhomaleosaurus megacephalus|"Rhomaleosaurus" megacephalus]]'', ''[[Stratesaurus]]'', ''[[Thalassiodracon]]''
|-
| [[Britton Formation]]
| [[Coniacian]]
| {{Flag|US}}
| ''[[Libonectes]]''
|-
| [[Bückeberg Formation]]
| [[Berriasian]]
| {{Flag|Germany}}
| ''[[Brancasaurus]]'', ''[[Gronausaurus]]''
|-
| [[Bulldog Shale Formation]]
| [[Aptian]]-[[Albian]]
| {{Flag|Australia}}
| ''[[Opallionectes]]'', ''[[Umoonasaurus]]''
|-
| [[Calcaire à Bélemnites]]
| [[Pliensbachian]]
| {{Flag|France}}
| ''[[Cryonectes]]''
|-
| [[Carlile Formation]]
| [[Turonian]]
| {{Flag|US}}
| ''[[Megacephalosaurus]]''
|-
| [[Charmouth Mudstone Formation]]
| [[Sinemurian]]
| {{Flag|UK}}
| ''[[Archaeonectrus]]'', ''[[Attenborosaurus]]''
|-
| [[Chichali Formation]]
|
| {{Flag|Pakistan}}
|
|-
| [[Clearwater Formation]]
| [[Albian]]
| {{Flag|Canada}}
| ''[[Nichollssaura]]'', ''[[Wapuskanectes]]''
|-
| [[Conway Formation]]
| [[Campanian]]-[[Maastrichtian]]
| {{Flag|New Zealand}}
| ''[[Mauisaurus]]'', ''[[Alexandronectes]]''
|-
| [[Coral Rag Formation]]
| [[Oxfordian (stage)|Oxfordian]]
| {{Flag|UK}}
| ''[[Pliosaurus grossouvrei|"Pliosaurus" grossouvrei]]''
|-
| [[Exter Formation]]
| [[Rhaetian]]
| {{Flag|Germany}}
| ''[[Rhaeticosaurus mertensi]]'', perhaps a basal Pliosaur<ref>{{Cite journal|author1=Tanja Wintrich |author2=Shoji Hayashi |author3=Alexandra Houssaye |author4=Yasuhisa Nakajima |author5=P. Martin Sander |year=2017 |title=A Triassic plesiosaurian skeleton and bone histology inform on evolution of a unique body plan |journal=Science Advances |volume=3 |issue=12 |article-number=e1701144 |doi=10.1126/sciadv.1701144 |pmid=29242826 |pmc=5729018 |bibcode=2017SciA....3E1144W }}</ref>
|-
| [[Favret Formation]]
| [[Anisian]]
| {{Flag|US}}
|''[[Augustasaurus]]''
|-
| [[Fencepost limestone]]
| [[Turonian]]
| {{Flag|US}}
| ''[[Trinacromerum]]''
|-
| [[Franciscan Formation]]
|
| {{Flag|US}}
|
|-
| [[Graneros Shale]]
| [[Cenomanian]]
| {{Flag|US}}
| ''[[Thalassomedon]]''
|-
| [[Greenhorn Limestone]]
| [[Turonian]]
| {{Flag|US}}
| ''[[Brachauchenius]]'', ''[[Pahasapasaurus]]''
|-
| [[Guanling Formation]]
| [[Anisian]]
| {{Flag|China}}
|
|-
| [[Hiccles Cove Formation]]
| [[Callovian]]
| {{Flag|Canada}}
| ''[[Borealonectes]]''
|-
| [[Horseshoe Canyon Formation]]
| [[Maastrichtian]]
| {{Flag|Canada}}
| ''[[Leurospondylus]]''
|-
| [[Jagua Formation]]
| [[Oxfordian (stage)|Oxfordian]]
| {{Flag|Cuba}}
| ''[[Gallardosaurus]]'', ''[[Vinialesaurus]]''
|-
| [[Jagüel Formation]]
| [[Maastrichtian]]
| {{Flag|Argentina}}
| ''[[Tuarangisaurus cabazai]]''
|-
| [[Katiki Formation]]
| [[Maastrichtian]]
| {{Flag|New Zealand}}
| ''[[Kaiwhekea]]''
|-
| [[Kimmeridge Clay]]
| [[Kimmeridgian]]
| {{Flag|UK}}
| ''[[Bathyspondylus]]'', ''[[Colymbosaurus]]'', ''[[Kimmerosaurus]]'', ''[[Plesiosaurus manselli|"Plesiosaurus" manselli]]'', ''[[Pliosaurus brachydeirus]]'', ''[[Pliosaurus brachyspondylus]]'', ''[[Pliosaurus carpenteri]]'', ''[[Pliosaurus kevani]]'', ''[[Pliosaurus macromerus]]'', ''[[Pliosaurus portentificus]]'', ''[[Pliosaurus westburyensis]]''
|-
| [[Kingsthorp]]
| [[Toarcian]]
| {{Flag|UK}}
| ''[[Rhomaleosaurus thorntoni]]''
|-
| [[Kiowa Shale]]
| [[Albian]]
| {{Flag|US}}
| ''[[Apatomerus]]''
|-
| [[La Colonia Formation]]
| [[Campanian]]
| {{Flag|Argentina}}
| ''[[Sulcusuchus]]''
|-
| [[Lake Waco Formation]]
|
| {{Flag|US}}
|
|-
| [[Los Molles Formation]]
| [[Bajocian]]
| {{Flag|Argentina}}
| ''[[Maresaurus]]''
|-
| [[Maree Formation]]
| [[Aptian]]
| {{Flag|Australia}}
|
|-
| [[Leicestershire]]
| late [[Sinemurian]]
| {{Flag|UK}}
| ''[[Eretmosaurus]]''
|-
| [[Lücking clay pit]]
| early [[Pliensbachian]]
| {{Flag|Germany}}
| ''[[Westphaliasaurus]]''
|-
| [[Marnes feuilletés]]
| [[Toarcian]]
| {{Flag|France}}
| ''[[Occitanosaurus]]''
|-
| [[Mooreville Chalk Formation]]
| [[Santonian]]–[[Campanian]]
| {{Flag|US}}
|
|-
| [[Moreno Formation]]
| [[Albian]]
| {{Flag|US}}
| ''[[Fresnosaurus]]'', ''[[Hydrotherosaurus]]'', ''[[Morenosaurus]]''
|-
| [[Muschelkalk]]
| [[Anisian]]
| {{Flag|Germany}}
| ''[[Pistosaurus]]''
|-
| [[Naknek Formation]]
| [[Kimmeridgian]]
| {{Flag|US}}
| ''[[Megalneusaurus]]''
|-
| [[Niobrara Formation]]
| [[Santonian]]
| {{Flag|US}}
| ''[[Brimosaurus]]'',<ref name="everhart-ypm1640-173">"Material: YPM 1640," in "The Occurrence of Elasmosaurids..." Everhart (2006), page 173.</ref> ''[[Dolichorhynchops osborni]]'',<ref name="everhart-table-plesiosaurs">"Table 13.1: Plesiosaurs," in Everhart (2005) ''Oceans of Kansas'', page 245.</ref> ''[[Elasmosaurus]]'',<ref name="everhart-table-plesiosaurs"/> ''[[Polycotylus]]'',<ref name="everhart-table-plesiosaurs"/> ''[[Styxosaurus snowii]]''<ref name="everhart-table-plesiosaurs"/><ref name="everhart-ypm1640">"Material: YPM 1640," in "The Occurrence of Elasmosaurids..." Everhart (2006), page 172.</ref>
|-
| [[Oxford Clay]]
| [[Callovian]]
| {{plainlist|
{{Flag|UK}}
{{Flag|France}}
}}
| ''[[Cryptoclidus]]'', ''[[Liopleurodon]]'', ''[[Marmornectes]]'', ''[[Muraenosaurus]]'', ''[[Pachycostasaurus]]'', ''[[Peloneustes]]'', ''[[Pliosaurus andrewsi|"Pliosaurus" andrewsi]]'', ''[[Picrocleidus]]'', ''[[Simolestes]]'', ''[[Tricleidus]]''
|-
| [[Oulad Abdoun Basin]]
| late [[Maastrichtian]]
| {{Flag|Morocco}}
| ''[[Zarafasaura]]''
|-
| [[Paja Formation]]
| [[Aptian]]
| {{Flag|Colombia}}
| ''[[Callawayasaurus]]'', ''[[Kronosaurus boyacensis]]''
|-
| [[Paso del Sapo Formation]]
| [[Maastrichtian]]
| {{Flag|Argentina}}
| ''[[Aristonectes]]''
|-
| [[Pierre Shale]]
| [[Campanian]]
| {{Flag|US}}
| ''[[Martinectes]]'', ''[[Hydralmosaurus]]''
|-
| [[Posidonia Shale]]
| [[Toarcian]]
| {{Flag|Germany}}
| ''[[Hauffiosaurus zanoni]]'', ''[[Hydrorion]]'', ''[[Meyerasaurus]]'', ''[[Plesiopterys]]'', ''[[Seeleyosaurus]]''
|-
| [[Rio del Lago Formation]]
| early [[Carnian]]
| {{Flag|Italy}}
| ''[[Bobosaurus]]''
|-
| [[São Gião Formation]]
| [[Toarcian]]
| {{Flag|Portugal}}
| ''[[Lusonectes]]''
|-
| [[Smoky Hill Chalk]]
| [[Campanian]]
| {{Flag|US}}
| ''[[Dolichorhynchops osborni]]''
|-
| [[Sundance Formation]]
| [[Oxfordian (stage)|Oxfordian]]
| {{Flag|US}}
| ''[[Megalneusaurus]]'', ''[[Pantosaurus]]'', ''[[Tatenectes]]''
|-
| [[Sundays River Formation]]
| [[Valanginian]]
| {{Flag|South Africa}}
| ''[[Leptocleidus capensis]]''
|-
| [[Tahora Formation]]
| [[Campanian]]
| {{Flag|New Zealand}}
| ''[[Tuarangisaurus keyesi]]''
|-
| [[Tamayama Formation]]
| [[Santonian]]
| {{Flag|Japan}}
| ''[[Futabasaurus]]''
|-
| [[Thermopolis Shale]]
| [[Albian]]
| {{Flag|US}}
|''[[Edgarosaurus]]''
|-
| [[Toolebuc Formation]]
| [[Albian]]
| {{Flag|Australia}}
| ''[[Eromangasaurus]], [[Kronosaurus queenslandicus]]''
|-
| [[Tropic Shale Formation]]
| [[Turonian]]
| {{Flag|US}}
| ''[[Brachauchenius]] sp.'' (unnamed, previously referred to ''[[Brachauchenius lucasi|B. lucasi]]''), ''[[Scalamagnus]]'', ''[[Eopolycotylus]]'', ''[[Palmulasaurus]]'', ''[[Trinacromerum]] sp.''
|-
| [[Vectis Formation]]
| [[Aptian]]
| {{Flag|UK}}
| ''[[Vectocleidus]]''
|-
| [[Wadhurst Clay Formation]]
| [[Valanginian]]
| {{Flag|UK}}
| ''[[Hastanectes]]''
|-
| [[Wallumbilla Formation]]
| [[Aptian]]-[[Albian]]
| {{Flag|Australia}}
| ''[[Styxosaurus glendowerensis]]''
|-
| [[Weald Clay]]
| [[Barremian]]
| {{Flag|UK}}
| ''[[Leptocleidus superstes]]''
|-
| [[Whitby Mudstone Formation]]
| [[Toarcian]]
| {{Flag|UK}}
| ''[[Hauffiosaurus longirostris]]'', ''[[Hauffiosaurus tomistomimus]]'', ''[[Macroplata]]'', ''[[Microcleidus homalospondylus]]'', ''[[Microcleidus macropterus]]'', ''[[Rhomaleosaurus cramptoni]]'', ''[[Rhomaleosaurus propinquus]]'', ''[[Rhomaleosaurus zetlandicus]]'', ''[[Sthenarosaurus]]''
|-
| [[Wilczek Formation]]
| [[Norian]]
| {{Flag|Russia}}
| ''[[Alexeyisaurus]]''
|-
| [[Xintiangou Formation]]
| [[Middle Jurassic]]
| {{Flag|China}}
| ''[[Yuzhoupliosaurus]]''
|-
| [[Zhenzhuchong Formation]]
|
| {{Flag|China}}
|
|-
| [[Ziliujing Formation]]
| [[Toarcian]]
| {{Flag|China}}
| ''[[Bishanopliosaurus]]'', ''[[Sinopliosaurus]]''
|}

== In contemporary culture ==
{{Main | Loch Ness Monster}}
{{See also | Sea monster}}
[[File:When Dinosaurs Ruled the Earth (1970) trailer - Plesiosaurus 1.png|thumb|A ''Plesiosaurus'' depicted in ''[[When Dinosaurs Ruled the Earth]]'']]
The belief that plesiosaurs are dinosaurs is a [[List of common misconceptions|common misconception]], and plesiosaurs are often [[Cultural depictions of dinosaurs|erroneously depicted as dinosaurs]] in popular culture.<ref name=":0">{{cite web|date=Feb 17, 2021|title=Famous Prehistoric Animals That Weren't Actually Dinosaurs|url=https://medium.com/prehistoric-tales/famous-prehistoric-animals-that-werent-actually-dinosaurs-730bb941ed50}}</ref><ref>{{Cite web|date=2015-06-19|title=The real sea monsters|url=https://www.sciencenewsforstudents.org/article/real-sea-monsters|access-date=2022-02-12|website=Science News for Students|language=en-US}}</ref>

It has been suggested that [[legend]]s of [[sea serpent]]s and modern sightings of supposed [[lake monster|monsters in lakes]] or the sea could be explained by the survival of plesiosaurs into modern times. This [[cryptozoological]] proposal has been rejected by the [[scientific community]] at large, which considers it to be based on fantasy and [[pseudoscience]]. Purported plesiosaur carcasses have been shown to be partially decomposed corpses of [[basking shark]]s instead.<ref>Anonymous (AP Report). Japanese scientist says that sea creature could be related to a shark species. ''[[The New York Times]]'', 26 July 1977.</ref><ref>{{cite web|url=http://paleo.cc/paluxy/plesios.htm|title=Sea-Monster or Shark: An Alleged Plesiosaur Carcass|website=paleo.cc}}</ref><ref>Kimura S, Fujii K, and others. The morphology and chemical composition of the horny fiber from an unidentified creature captured off the coast of New Zealand. In CPC 1978, pp. 67–74.</ref>

While the [[Loch Ness monster]] is often reported as looking like a plesiosaur, it is also often described as looking completely different. A number of reasons have been presented for it to be unlikely to be a plesiosaur. The fact that the [[osteology]] of the plesiosaur's neck makes it absolutely safe to say that the plesiosaur could not lift its head like a swan out of water as the Loch Ness monster does, the assumption that air-breathing animals would be easy to see whenever they appear at the surface to breathe,<ref>The Great Sea Serpent, Antoon Cornelis Oudemans, 2009, Cosimo Inc {{ISBN|978-1-60520-332-4}} p. 321.</ref> the fact that the loch is too small and contains insufficient food to be able to support a breeding colony of large animals, and finally the fact that the lake was formed only 10,000 years ago at the end of the last [[ice age]], and the latest fossil appearance of plesiosaurs dates to over 66&nbsp;million years ago.<ref>{{cite web|url=https://www.newscientist.com/channel/life/mg19225764.900-why-the-loch-ness-monster-is-no-plesiosaur.html|title=Life|website=New Scientist}}</ref> Frequent explanations for the sightings include [[wave]]s, floating inanimate objects, tricks of the light, swimming known animals and practical jokes.<ref>{{cite web|url=http://www.crawley-creatures.com/recent/lucy.htm|title=Crawley Creatures|website=www.crawley-creatures.com|access-date=2013-04-22|archive-url=https://web.archive.org/web/20090521091326/http://www.crawley-creatures.com/recent/lucy.htm|archive-date=2009-05-21}}</ref> Nevertheless, in the popular imagination, plesiosaurs have come to be identified with the Monster of Loch Ness. This has made plesiosaurs better known to the general public.<ref>Ellis (2003), pp. 1–3.</ref>

==See also==
* [[List of plesiosaurs]]
* [[List of plesiosaur type specimens]]

==Notes==
{{notelist}}

==References==
{{Reflist}}

==Further reading==
*{{cite book|chapter-url={{google books|plainurl=yes|jDXwfVl140IC|page=105}}|first1=J. M.|last1=Callaway|first2=E. L.|last2=Nicholls|year=1997|title=Ancient Marine Reptiles|chapter=Sauropterygia|publisher=Academic Press|isbn=978-0-12-155210-7}}
* {{cite journal | last= Carpenter | first= K | year = 1996 | title = A review of short-necked plesiosaurs from the Cretaceous of the western interior, North America | journal = Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen | volume = 201 | issue = 2| pages = 259–287 | doi = 10.1127/njgpa/201/1996/259 | url = http://doc.rero.ch/record/15069/files/PAL_E2225.pdf }}
* Carpenter, K. 1997. Comparative cranial anatomy of two North American Cretaceous plesiosaurs. Pp 91–216, in Calloway J. M. and E. L. Nicholls, (eds.), Ancient Marine Reptiles, Academic Press, San Diego.
* {{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 }}
*{{cite journal | last1 = Cicimurri | first1 = D. | last2 = Everhart | first2 = M. | year = 2001 | title = An Elasmosaur with Stomach Contents and Gastroliths from the Pierre Shale (Late Cretaceous) of Kansas| journal = Trans. Kans. Acad. Sci. | volume = 104 | issue = 3 & 4| pages = 129–143 | doi = 10.1660/0022-8443(2001)104[0129:AEWSCA]2.0.CO;2 | s2cid = 86037286 }}
* {{cite journal | last= Cope | first= 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 }}
* Ellis, R. 2003: ''Sea Dragons'' ([[Kansas University Press]])
* {{cite journal | last= Everhart | first= M. J. | year = 2002 | title = Where the elasmosaurs roamed | journal = Prehistoric Times | volume = 53 | pages = 24–27 }}
* Everhart, M.J. 2005. "Where the Elasmosaurs roamed", Chapter 7 in ''Oceans of Kansas: A Natural History of the Western Interior Sea'', [[Indiana University Press]], Bloomington, 322 p.
*Everhart, M.J. 2005. ''Oceans of Kansas: A Natural History of the Western Interior Sea''. [[Indiana University Press]], Bloomington, 322 pp.
*{{cite journal | last= Everhart | first= M.J. | year = 2005 | title = Gastroliths associated with plesiosaur remains in the Sharon Springs Member (Late Cretaceous) of the Pierre Shale, Western Kansas | url = http://www.oceansofkansas.com/Gastro2.html | journal = Kansas Acad. Sci. Trans | volume = 103 | issue = 1–2| pages = 58–69 }}
* {{cite journal | last= Everhart | first= Michael J | year = 2006 | title = The Occurrence of Elasmosaurids (Reptilia: Plesiosauria) in the Niobrara Chalk of Western Kansas | journal = Paludicila | volume = 5 | issue = 4| pages = 170–183 }}
*Hampe, O., 1992: ''Courier Forsch.-Inst. Senckenberg'' '''145''': 1-32
*{{cite journal | last= Lingham-Soliar | first= T. | year = 1995 | title = in | journal = Phil. Trans. R. Soc. Lond. | volume = 347 | pages = 155–180 }}
*{{cite journal | last= O'Keefe | first= F. R. | year = 2001 | title = A cladistic analysis and taxonomic revision of the Plesiosauria (Reptilia: Sauropterygia); | journal = [[Acta Zoologica Fennica|Acta Zool. Fennica]] | volume = 213 | pages = 1–63 }}
* ( ), 1997: in ''Reports of the National Center for Science Education'', '''17.3''' (May/June 1997) pp 16&ndash;28.
* Kaddumi, H. F., 2009. Fossils of the Harrana Fauna and the adjacent areas. Publications of the Eternal River Museum of Natural History, Jordan. 324 pp.
* Storrs, G. W., 1999. An examination of Plesiosauria (Diapsida: Sauropterygia) from the Niobrara Chalk (Upper Cretaceous) of central North America, ''University of Kansas Paleontologcial Contributions'', (N.S.), No. 11, 15 pp.
* Welles, S. P. 1943. Elasmosaurid plesiosaurs with a description of the new material from California and Colorado. ''University of California Memoirs'' 13:125-254. figs.1-37., pls.12-29.
* Welles, S. P. 1952. A review of the North American Cretaceous elasmosaurs. ''University of California Publications in Geological Science'' 29:46-144, figs. 1-25.
* Welles, S. P. 1962. A new species of elasmosaur from the Aptian of Columbia and a review of the Cretaceous plesiosaurs. University of California Publications in Geological Science 46, 96 pp.
* White, T., 1935: in ''Occasional Papers Boston Soc. Nat. Hist.'' '''8''': 219–228
* Williston, S. W. 1890. A new plesiosaur from the Niobrara Cretaceous of Kansas. ''Transactions of the Kansas Academy of Science'' 12:174-178, 2 fig.
* {{cite journal | last= Williston | first= S. W. | year = 1902 | title = Restoration of ''Dolichorhynchops osborni'', a new Cretaceous plesiosaur | journal = Kansas University Science Bulletin | volume = 1 | issue = 9| pages = 241–244, 1 plate }}
* Williston, S. W. 1903. North American plesiosaurs. Field Columbian Museum, Publication 73, Geology Series 2(1): 1–79, 29 pl.
* {{cite journal | last= Williston | first= S. W. | year = 1906 | title = North American plesiosaurs: ''Elasmosaurus'', ''Cimoliasaurus'', and ''Polycotylus'' | url = https://zenodo.org/record/1450146| journal = American Journal of Science | volume = 21 | issue = 123| pages = 221–234, 4 pl | bibcode = 1906AmJS...21..221W | doi = 10.2475/ajs.s4-21.123.221 }}
* {{cite journal | last= Williston | first= S. W. | year = 1908 | title = North American plesiosaurs: ''Trinacromerum'' | journal = Journal of Geology | volume = 16 | issue = 8| pages = 715–735 | doi=10.1086/621573 | bibcode=1908JG.....16..715W| doi-access = free }}
* {{cite journal | last1 = Bardet | first1 = Nathalie | last2 = Cappetta | first2 = Henri | last3 = Pereda Suberbiola | first3 = Xabier | title=The marine vertebrate faunas from the Late Cretaceous phosphates of Syria | publisher = Cambridge University| year = 2000 | pages = 269–290 | volume = 137 | issue = 3 | doi=10.1017/S0016756800003988 |journal=Geological Magazine | bibcode = 2000GeoM..137..269B | s2cid = 129600143 }}

== External links ==
{{Commons category}}
{{Wikiquote|Plesiosauria}}
* ''[http://www.plesiosaur.com/ The Plesiosaur Site]''. Richard Forrest.
* ''[http://plesiosauria.com/ The Plesiosaur Directory]''. Adam Stuart Smith.
** [http://plesiosauria.com/plesiosauria Plesiosauria] technical definition at the Plesiosaur Directory
* ''[https://web.archive.org/web/20091027090505/http://geocities.com/CapeCanaveral/Hangar/9020/plesiosaur/ Plesiosaur FAQ's]''. Raymond Thaddeus C. Ancog.
* ''[http://www.oceansofkansas.com/plesiosaur.html Oceans of Kansas Paleontology]''. Mike Everhart.
* "[https://web.archive.org/web/20070313002354/http://www.somerset.gov.uk/somerset/culturecommunity/museums/whatsnew/plesiosaurfossil/ Plesiosaur fossil found in Bridgwater Bay]". ''Somersert Museums County Service''. (best known fossil)
* "[https://archive.today/20110604040628/http://www.timesonline.co.uk/printFriendly/0,,1-2-527486,00.html Fossil hunters turn up 50-ton monster of prehistoric deep]". Allan Hall and Mark Henderson. ''Times Online'', December 30, 2002. (Monster of Aramberri)
* ''[http://www.the-scientist.com/news/20041118/01 Triassic reptiles had live young]''.
* [https://web.archive.org/web/20070313002354/http://www.somerset.gov.uk/somerset/culturecommunity/museums/whatsnew/plesiosaurfossil/ Bridgwater Bay juvenile plesiosaur]
* [https://blogs.scientificamerican.com/laelaps/just-how-good-is-the-plesiosaur-fossil-record/ Just How Good Is the Plesiosaur Fossil Record? ''Laelaps Blog''.]

{{Plesiosauria|Pliosauroidea}}

{{Taxonbar|from=Q269195}}
{{Authority control}}

[[Category:Plesiosaurs| ]]
[[Category:Taxa named by Henri Marie Ducrotay de Blainville]]
[[Category:Rhaetian first appearances]]
[[Category:Late Triassic taxonomic orders]]
[[Category:Early Jurassic taxonomic orders]]
[[Category:Middle Jurassic taxonomic orders]]
[[Category:Late Jurassic taxonomic orders]]
[[Category:Early Cretaceous taxonomic orders]]
[[Category:Late Cretaceous taxonomic orders]]
[[Category:Maastrichtian extinctions]]
[[Category:Articles containing video clips]]