plesiosaur

{{main|Timeline of plesiosaur research}}

File:Stukely plesiosaur.jpg

Skeletal elements of plesiosaurs are among the first fossils of extinct reptiles recognised as such. 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.Richard Verstegan, 1605, A restitution of decayed intelligence or Nationum Origo, R. Bruney, Antwerpen 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.Lhuyd, E., 1699, Lithophylacii Brittannici Ichnographia, sive Lapidum aliorumque Fossilium Brittanicorum singulari figurà insignium, Londen Other naturalists during the seventeenth century added plesiosaur remains to their collections, such as John Woodward; these were only much later understood to be of a plesiosaurian nature and are today partly preserved in the Sedgwick Museum.{{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}}

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.{{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 }} The specimen is today on display at the 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.

File:Conybeare Plesiosaur 1824.jpg

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 and Baptist Noel Turner, active in the Vale of Belvoir, whose collections were in 1795 described by John Nicholls in the first part of his The History and Antiquities of the County of Leicestershire.Nicholls, J., 1795, The History and Antiquities of the County of Leicestershire. Volume I, John Nicholls, Londen 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.

File:Mary Anning Plesiosaurus.jpg

In the early nineteenth century, plesiosaurs were still poorly known and their special build was not understood. No systematic distinction was made with 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,{{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 }} was described by 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.{{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 }} The name should thus be rather read as "approaching the Sauria" or "near reptile" than as "near lizard".{{cite web|url=http://oceansofkansas.com/Creisler_Plesiosaurs2.html|title=Plesiosaur_Names|website=oceansofkansas.com}} Parts of the specimen are still present in the Oxford University Museum of Natural History.

Soon afterwards, the 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. 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 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,{{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 }} 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 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). 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.{{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| page = e65989 | doi=10.1371/journal.pone.0065989 | pmid=23741520 | pmc=3669260| bibcode = 2013PLoSO...865989B | doi-access = free }}

File:Great Sea-Dragons.jpg

Plesiosaurs became better known to the general public through two lavishly illustrated publications by the collector Thomas Hawkins: Memoirs of Ichthyosauri and Plesiosauri of 1834{{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|url-status=dead |archive-url=https://web.archive.org/web/20050830021622/http://www.plesiosaur.com/references/hawkins/hawkins.pdf|archive-date=2005-08-30}} and The Book of the Great Sea-Dragons of 1840. Hawkins entertained a very idiosyncratic view of the animals,{{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}} seeing them as monstrous creations of the devil, during a pre-Adamitic phase of history.{{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}} Hawkins eventually sold his valuable and attractively restored specimens to the British Museum of Natural History.Christopher McGowan, 2001, The Dragon Seekers, Cambridge, Massachusetts, Perseus Publishing

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 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".{{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 }} 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.{{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 }} In 1835, Henri Marie Ducrotay de Blainville named the order Plesiosauria itself.{{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 }}

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

In 1867, physician Theophilus Turner near Fort Wallace in Kansas uncovered a plesiosaur skeleton, which he donated to Cope.{{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 }} 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.{{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 }} 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.{{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 }} After having published a description of this animal,{{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 }} followed by an illustration in a textbook about reptiles and amphibians,{{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}} 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.{{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 }} 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.{{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 }} 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.{{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 }} 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.Ellis (2003), p. 129

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.Williston, S.W., 1914, Water Reptiles of the Past and Present. Chicago University Press. Chicago, Illinois. 251 pp Despite treating sea reptiles in general, it would for many years remain the most extensive general text on plesiosaurs.{{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}} 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.

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.Smith, A.S., 2003, Cladistic analysis of the Plesiosauria (Reptilia: Sauropterygia). Masters thesis in palaeobiology, University of Bristol, 91 pp 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,{{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 }} 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.{{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|url-status=dead|archive-url=https://web.archive.org/web/20110715093428/http://www.plesiosaur.com/plesiosaurs/liopleurodon.php|archive-date=15 July 2011}}{{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|url-status=dead|archive-url=https://web.archive.org/web/20110903081504/http://www.plesiosaur.com/plesiosaurs/aramberri.php|archive-date=3 September 2011}}

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 million years ago as indicated by the ammonites 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.{{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|pages=107|doi=10.55468/GC217 |s2cid=251120888 |doi-access=free}}{{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|url-status=dead}}{{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}}

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.{{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 }}

In 2006, a combined team of American and Argentinian investigators (the latter from the 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.{{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 |url-status=dead |archive-url=https://web.archive.org/web/20130718145659/http://www.dna.gov.ar/DIVULGAC/CPLESIO.HTM |archive-date=2013-07-18 }} (In Spanish) The fossil is currently on display at the geological museum of South Dakota School of Mines and Technology.{{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}}

In 2008, fossil remains of an undescribed plesiosaur that was named Predator X, now known as Pliosaurus funkei, were unearthed in Svalbard.{{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}} It had a length of {{cvt|12|m}}, and its bite force of {{convert|149|kN}} is one of the most powerful known.{{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/|url-status=dead|archive-date=21 March 2009|title=PREDATOR X - Naturhistorisk Museum|date=21 March 2009}}

In December 2017, a large skeleton of a plesiosaur was found in the continent of Antarctica, the oldest creature on the continent, and the first of its species in Antarctica.{{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}}

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}}

File:Ceresiosaurus12.jpgs still had functional legs.]]

The Plesiosauria have their origins within the Sauropterygia, a group of perhaps archelosaurian 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.{{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}} The Pistosauria became warm-blooded and viviparous, giving birth to live young.{{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 }} Early, basal, members of the group, traditionally called "pistosaurids", were still largely coastal animals. Their shoulder girdles remained weak, their 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.{{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 }}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

File:Augustasaurus BW.jpg, 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, 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.{{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 }}

File:Simolestes1DB.jpg]]

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 metres (sixteen feet) long. By the Toarcian, about 180 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}}.{{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 |pages=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 }}

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 metres (ten feet) long and reached a length of up to {{convert|17|m|abbr=off|spell=in}} and a weight of ten tonnes. 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.

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 metres (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.

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.

All plesiosaurs became extinct as a result of the K-T event at the end of the Cretaceous period, approximately {{period start|Paleogene}} million years ago.{{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}}

In modern phylogeny, clades are defined groups that 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 clade". In 2008, Patrick Druckenmiller and 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.{{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}} 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 clade". Such a definition has the advantage that it is easier to include all species with a certain 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".{{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 }} 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 cryptoclidian were included, while elasmosaurids 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.{{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 }}

File:Plesiosaur skeleton, New Walk Museum.JPG fossil]]

The following cladogram follows an analysis by Benson & Druckenmiller (2014).

File:Macrocephalus.jpg

{{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 50 px

|label4=Pliosauridae

|4={{clade

|1=Thalassiodracon

|2={{clade

|1=Hauffiosaurus

|2={{clade

|1=Attenborosaurus 50 px

|2={{clade

|1=Marmornectes

|2=Thalassophonea 50 px}} }} }} }}

|label5=Plesiosauroidea

|5={{clade

|1=Plesiosaurus

|2={{clade

|1=Eretmosaurus

|2=Westphaliasaurus

|3=Microcleididae 50 px

|4={{clade

|1=Plesiopterys

|label2=Cryptoclidia

|2={{clade

|1=Cryptoclididae 50 px

|label2=Xenopsaria

|2={{clade

|1=Elasmosauridae 50 px

|label2=Leptocleidia

|2={{clade

|1=Leptocleididae 50 px

|2=Polycotylidae 50 px }} }} }} }} }} }} }} }} }} }} }}

File:Fossil-Plesiosaurier-Skelett.jpg in the Museum am Löwentor, Stuttgart, seen from below]]

In general, plesiosaurians varied in adult length from between {{convert|1.5|m|ft}} to about {{convert|15|m}}. The group thus contained some of the largest marine apex predators in the fossil record, roughly equalling the longest ichthyosaurs, mosasaurids, sharks and toothed whales in size. Some plesiosaurian remains, such as a {{convert|2.875|m|ft|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{{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}} and Liopleurodon), indicated a length of {{convert|17|m}}. 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}} metres or less is more likely.McHenry, Colin Richard (2009). "Devourer of Gods: the palaeoecology of the Cretaceous pliosaur Kronosaurus queenslandicus" (PDF): 1–460 In 2023, the length of its mandible was estimated to be 2.6 m, suggesting a length of {{convert|11.8|m}} 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}}.{{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 | pages = e65989 | year = 2013 | pmid = 23741520| pmc = 3669260| bibcode = 2013PLoSO...865989B| doi-access = free }} 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}} long.{{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}} 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.{{Cite web |last=Zhao |first=Ruizhe Jackevan |date=2024-02-19 |title=Body reconstruction and size estimation of plesiosaurs |url=https://doi.org/10.1101/2024.02.15.578844 |access-date=2025-03-11 |website=doi.org}}

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 flippers.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 Plesiosaurs were related to the earlier Nothosauridae,Storrs, Glenn W.; 1990. Phylogenetic Relationships of Pachypleurosaurian and Nothosauriform Reptiles (Diapsida: Sauropterygia); Journal of Vertebrate Paleontology; 10 (Supplement to Number 3) 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 ichthyosaurs and the later mosasaurs, in which the tail provided the main propulsion.{{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 }}

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, 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 ribs that each had a middle and an outer section. This arrangement immobilised the entire trunk.

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, 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.

File:Elasmosaur.jpg elasmosaur", 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.{{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 }} 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, 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.

The skull of plesiosaurs showed the "euryapsid" condition, lacking the lower temporal fenestrae, 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 bones were very large, with a midline crest, while the squamosal bones 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 rings, the form of which shows that they were relatively flat, an adaptation to diving. The anteriorly placed internal nostrils, the choanae, 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.{{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 }}{{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 }} 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 glands.{{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 }} The distance between the eye sockets and the nostrils was so limited because the nasal bones were strongly reduced, even absent in many species. The premaxillae directly touched the frontal bones; in the elasmosaurids, they even reached back to the parietal bones. Often, the lacrimal bones were also lacking.

File:SeeleysaurusDB.jpg 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 carina. With some species, the front teeth were notably longer, to grab prey.{{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}}

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{{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}}), that the trailing edge of the flippers extended considerably behind the limb bones,{{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 }} and that the tail bore a vertical fin, as reported by Wilhelm Dames in his description of Plesiosaurus guilelmiimperatoris (presently Seeleyosaurus).{{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 }} The possibility of a tail fluke has been confirmed by recent studies on the caudal neural spine form of Pantosaurus, Cryptoclidus and Rhomaleosaurus zetlandicus.Wilhelm, B.C., 2010, Novel anatomy of cryptoclidid plesiosaurs with comments on axial locomotion. Ph.D thesis, Marshall University, Huntington, WV. USA{{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 }}{{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 }} A 2020 study claims that the caudal fin was horizontal in configuration.{{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}} 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 melanosomes and keratinocytes.{{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 }}{{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 |at=e18960 |doi=10.7717/peerj.18960 |doi-access=free |pmc=11967415 }}

File:Plesiosaur on land.jpg]]

File:Gastroliths.jpg

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 jaws 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.{{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 }} 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.{{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}}{{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/ }} Such a lifestyle has in 2017 been suggested for Morturneria.{{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| page = e1347570| doi = 10.1080/02724634.2017.1347570 | bibcode = 2017JVPal..37E7570O | s2cid = 91144814 | hdl = 11336/75235 | hdl-access = free }} "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.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

The short-necked "pliosauromorphs" were top carnivores, or apex predators, in their respective foodwebs.{{cite web |title=The Plesiosaur Directory|url=http://www.plesiosauria.com/diet.php |access-date=20 April 2013}} They were pursuit predators{{cite journal | last= Massare | first= J.A. | year = 1992 | title = Ancient mariners | journal = Natural History | volume = 101 | pages = 48–53 }} or ambush predators of various sized prey and opportunistic feeders; their teeth could be used to pierce soft-bodied prey, especially fish.{{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 }} 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{{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 }} and the fossilized remains of a mosasaur's stomach contents that are thought to be the remains of a plesiosaur.{{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 }}

Skeletons have also been discovered with gastroliths, 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.Williston, Samuel Wendel; 1904. The stomach stones of the plesiosaurs Science 20; 565{{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 }} However, the total weight of the gastroliths found in various specimens appears to be insufficient to modify the buoyancy of these large reptiles.{{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 }} The first plesiosaur gastroliths, found with Mauisaurus gardneri (a nomen nudum{{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. }}), were reported by Harry Govier Seeley in 1877.{{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 }} 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.{{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 }} 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 centimetres 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.{{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 }} The type specimen of Scalamagnus (MNA V10046) is associated with 289 gastroliths, 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.{{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 }}

File:Plesiosaur swimming animation.ogv

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 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,{{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 }} and in 1908 by Othenio Abel.{{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 }} 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.{{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 }}

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.{{cite journal | last= Tarlo | first= L.B. | year = 1957 | title = The scapula of Pliosaurus macromerus Phillips | journal = Palaeontology | volume = 1 | pages = 193–199 }}{{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 }} 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.{{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 }} 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.{{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 }}

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.{{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 }}{{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 }} This modified flying model became a popular interpretation. Less attention was given to an alternative hypothesis by 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.{{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 }}

In 2010, 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.{{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 }} 

File:Plesiosaurus2.jpg

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.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 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. He rejected Robinson's hypothesis that elastic energy was stored in the ribcage, considering the ribs too stiff for this.{{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 }}

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 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.{{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 }} 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. 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.{{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 }} 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.{{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| page = 20170951 | doi = 10.1098/rspb.2017.0951 | pmid = 28855360 | pmc = 5577481 }}

File:Kronosaurus2 NT.jpg 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. 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.

Sustained speed may be estimated by calculating the 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.{{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 }} 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 metres per second, or about fourteen kilometres per hour, considerably exceeding the known speeds of extant dolphins and whales.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 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 metre per second (1.8 km/h) for a cold-blooded plesiosaur and one and a half metres per second (5.4 km/h) for an endothermic plesiosaur. Even the highest estimate is about a third lower than the speed of extant Cetacea.{{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 }}

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.

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, further reducing speed but improving roll.{{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 }}

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 humeri and 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.{{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 }}

Descending would have been helped by a negative 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{{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 }} or even as means to attain neutral buoyancy, swallowing or spitting them out again as needed.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 They might also have been used to increase stability.{{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 }}

The relatively large eyes of the Cryptocleididae have been seen as an adaptation to deep diving.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].

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.

Traditionally, it was assumed that extinct reptile groups were 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.{{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| page = e11613 | doi = 10.1371/journal.pone.0011613 | pmid=20657768 | pmc=2908119| bibcode = 2010PLoSO...511613K | doi-access = free }} 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.{{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 }} 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.{{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|pages=e4955|doi=10.7717/peerj.4955|issn=2167-8359|pmc=5994164|pmid=29892509 |doi-access=free }} 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 endothermy at lower RMRs, with inferred body temperatures of ca. {{Cvt|26|C}}.{{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}}

File:Polycotylus NT.jpg

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 turtles. 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 with four to eight tiny skeletons.{{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 }} In 1896, he described this discovery in more detail.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 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.{{cite journal | last= Thulborn | first= R.A. | year = 1982 | title = Liassic plesiosaur embryos reinterpreted as shrimp burrows | journal = Palaeontology | volume = 25 | pages = 351–359 }}

An actual plesiosaur specimen found in 1987 eventually proved that plesiosaurs gave birth to live young:{{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 |volume=333 |issue=6044 |pages=870–873 |bibcode=2011Sci...333..870O |doi=10.1126/science.1205689 |pmid=21836013 |s2cid=36165835}} 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 metres (five feet) long and thus large compared to its mother of five metres (sixteen feet) length, indicating a K-strategy in reproduction.{{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}} Little is known about growth rates or a possible sexual dimorphism.

From the parental care indicated by the large size of the young, it can be deduced that social behaviour in general was relatively complex. 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.{{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 }}{{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 }}

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.{{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 }}

Plesiosaur fossils have been found on every continent, including Antarctica.{{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 }}

ImageSize = width:1000px height:2000

PlotArea = left:10px bottom:50px top:10px right:10px

Period = from:-252.2 till:-25

TimeAxis = orientation:horizontal

ScaleMajor = unit:year increment:10 start:-252.2

ScaleMinor = unit:year increment:1 start:-252.2

TimeAxis = orientation:hor

AlignBars = justify

Colors =

#legends

id:CAR value:claret

id:ANK value:rgb(0.4,0.3,0.196)

id:HER value:teal

id:HAD value:green

id:OMN value:blue

id:black value:black

id:white value:white

id:mesozoic value:rgb(0.54,0.54,0.258)

id:triassic value:rgb(0.51,0.17,0.57)

id:earlytriassic value:rgb(0.6,0.22,0.61)

id:middletriassic value:rgb(0.73,0.53,0.71)

id:latetriassic value:rgb(0.78,0.65,0.8)

id:jurassic value:rgb(0.2,0.7,0.79)

id:earlyjurassic value:rgb(0,0.69,0.89)

id:middlejurassic value:rgb(0.52,0.81,0.91)

id:latejurassic value:rgb(0.74,0.89,0.97)

id:cretaceous value:rgb(0.5,0.78,0.31)

id:earlycretaceous value:rgb(0.63,0.78,0.65)

id:latecretaceous value:rgb(0.74,0.82,0.37)

BarData=

bar:eratop

bar:space

bar:periodtop

bar:space

bar:NAM1

bar:NAM2

bar:NAM3

bar:NAM4

bar:NAM5

bar:NAM6

bar:NAM7

bar:NAM8

bar:NAM9

bar:NAM10

bar:NAM11

bar:NAM12

bar:NAM13

bar:NAM14

bar:NAM15

bar:NAM16

bar:NAM17

bar:NAM18

bar:NAM19

bar:NAM20

bar:NAM21

bar:NAM22

bar:NAM23

bar:NAM24

bar:NAM25

bar:NAM26

bar:NAM27

bar:NAM28

bar:NAM29

bar:NAM30

bar:NAM31

bar:NAM32

bar:NAM33

bar:NAM34

bar:NAM35

bar:NAM36

bar:NAM37

bar:NAM38

bar:NAM39

bar:NAM40

bar:NAM41

bar:NAM42

bar:NAM43

bar:NAM44

bar:NAM45

bar:NAM46

bar:NAM47

bar:NAM48

bar:NAM49

bar:NAM50

bar:NAM51

bar:NAM52

bar:NAM53

bar:NAM54

bar:NAM55

bar:NAM56

bar:NAM57

bar:NAM58

bar:NAM59

bar:NAM60

bar:NAM61

bar:NAM62

bar:NAM63

bar:NAM64

bar:NAM65

bar:NAM66

bar:NAM67

bar:NAM68

bar:NAM69

bar:NAM70

bar:NAM71

bar:NAM72

bar:NAM73

bar:NAM74

bar:NAM75

bar:NAM76

bar:NAM77

bar:NAM78

bar:NAM79

bar:NAM80

bar:NAM81

bar:NAM82

bar:NAM83

bar:NAM84

bar:NAM85

bar:NAM86

bar:NAM87

bar:NAM88

bar:NAM89

bar:NAM90

bar:NAM91

bar:NAM92

bar:NAM93

bar:NAM94

bar:NAM95

bar:NAM96

bar:NAM97

bar:NAM98

bar:NAM99

bar:NAM100

bar:NAM101

bar:NAM102

bar:NAM103

bar:NAM104

bar:NAM105

bar:NAM106

bar:NAM107

bar:NAM108

bar:NAM109

bar:NAM110

bar:NAM111

bar:NAM112

bar:NAM113

bar:NAM114

bar:NAM115

bar:NAM116

bar:NAM117

bar:NAM118

bar:NAM119

bar:NAM120

bar:NAM121

bar:NAM122

bar:NAM123

bar:NAM124

bar:NAM125

bar:NAM126

bar:NAM127

bar:NAM128

bar:NAM129

bar:NAM130

bar:NAM131

bar:NAM132

bar:NAM133

bar:NAM134

bar:NAM135

bar:NAM136

bar:NAM137

bar:NAM138

bar:NAM139

bar:NAM140

bar:NAM141

bar:NAM142

bar:NAM143

bar:NAM144

bar:space

bar:period

bar:space

bar:era

PlotData=

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

shift:(7,-4)

bar:periodtop

from: -252.2 till: -247.2 color:earlytriassic text:Early

from: -247.2 till: -235 color:middletriassic text:Middle

from: -235 till: -199.6 color:latetriassic text:Late

from: -199.6 till: -175.6 color:earlyjurassic text:Early

from: -175.6 till: -161.2 color:middlejurassic text:Middle

from: -161.2 till: -145 color:latejurassic text:Late

from: -145 till: -99.6 color:earlycretaceous text:Early

from: -99.6 till: -66 color:latecretaceous text:Late

bar:eratop

from: -252.2 till: -199.6 color:triassic text:Triassic

from: -199.6 till: -145 color:jurassic text:Jurassic

from: -145 till: -66 color:cretaceous text:Cretaceous

PlotData=

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

from: -247.2 till: -235 color:middletriassic text:Middle

from: -235 till: -199.6 color:latetriassic text:Late

from: -199.6 till: -175.6 color:earlyjurassic text:Early

from: -175.6 till: -161.2 color:middlejurassic text:Middle

from: -161.2 till: -145 color:latejurassic text:Late

from: -145 till: -99.6 color:earlycretaceous text:Early

from: -99.6 till: -65.5 color:latecretaceous text: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

{{Main|List of plesiosaur-bearing stratigraphic units}}

The following is a list of geologic formations that have produced plesiosaur fossils.

{{Main | Loch Ness Monster}}

{{See also | Sea monster}}

File:When Dinosaurs Ruled the Earth (1970) trailer - Plesiosaurus 1.png]]

The belief that plesiosaurs are dinosaurs is a common misconception, and plesiosaurs are often erroneously depicted as dinosaurs in popular culture.{{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}}{{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}}

It has been suggested that legends of sea serpents and modern sightings of supposed 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 sharks instead.Anonymous (AP Report). Japanese scientist says that sea creature could be related to a shark species. The New York Times, 26 July 1977.{{cite web|url=http://paleo.cc/paluxy/plesios.htm|title=Sea-Monster or Shark: An Alleged Plesiosaur Carcass|website=paleo.cc}}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.

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,The Great Sea Serpent, Antoon Cornelis Oudemans, 2009, Cosimo Inc {{ISBN|978-1-60520-332-4}} p. 321. 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 million years ago.{{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}} Frequent explanations for the sightings include waves, floating inanimate objects, tricks of the light, swimming known animals and practical jokes.{{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|url-status=dead}} 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.Ellis (2003), pp. 1–3.

• List of plesiosaurs
• List of plesiosaur type specimens

{{notelist}}

{{Reflist}}

• {{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 Zool. Fennica | volume = 213 | pages = 1–63 }}
• ( ), 1997: in Reports of the National Center for Science Education, 17.3 (May/June 1997) pp 16–28.
• 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 }}

{{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: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