View source for Plesiosaurus

{{Short description|Extinct genus of reptiles}}
{{speciesbox
| name = ''Plesiosaurus''
| fossil_range = [[Early Jurassic]], {{Fossil range|199.5|192.9}}|
| image = Plesiosaurus dolichodeirus NHM.jpg
| image_upright = 1.1
| image_caption = Mary Anning's plesiosaur: the ''Plesiosaurus'' [[type specimen]] on exhibit at [[Natural History Museum, London|Natural History Museum]] (specimen NHMUK OR 22656)

| display_parents = 3
| genus = Plesiosaurus
| parent_authority = [[Henry De la Beche|De la Beche]] & [[William Daniel Conybeare|Conybeare]], 1821
| authority = Conybeare, 1824
| species = dolichodeirus
}}

'''''Plesiosaurus''''' (Greek: ''{{lang|grc|πλησίος}}'' (''{{lang|grc-Latn|plesios}}''), near to + ''{{lang|grc|σαῦρος}}'' (''{{lang|grc-Latn|sauros}}''), lizard) is a genus of extinct, large marine [[sauropterygia]]n [[reptile]] that lived during the [[Early Jurassic]]. It is known by nearly complete skeletons from the [[Lias Group|Lias]] of England. It is distinguishable by its small head, long and slender neck, broad turtle-like body, a short tail, and two pairs of large, elongated paddles. It lends its name to the order [[Plesiosauria]], of which it is an early, but fairly typical member. It contains only one species, the [[type species|type]], '''''Plesiosaurus dolichodeirus'''''.  Other species once assigned to this genus, including ''P. brachypterygius'', ''P. guilielmiimperatoris'', and ''P. tournemirensis'' have been reassigned to new genera, such as ''[[Hydrorion]]'', ''[[Seeleyosaurus]]'' and ''[[Occitanosaurus|Microcleidus]]''.

==Discovery==


[[File:Mary Anning Plesiosaurus.jpg|thumb|left|upright|Letter concerning the discovery of the genus from [[Mary Anning]]]]

The first complete skeleton of ''Plesiosaurus'' was discovered by early paleontologist and fossil hunter [[Mary Anning]] in [[Sinemurian]] ([[Early Jurassic]])-age rocks of the lower [[Lias Group]] in December 1823 near [[Lyme Regis]] in [[Dorset]], [[England]].<ref>Torrens 1995</ref><ref name=Storrs146>Storrs 1997 pp. 146</ref> Additional fossils of ''Plesiosaurus'' were found in rocks of the Lias Group of [[Dorset]] for many years,<ref>Andrew 1896</ref><ref>Lydekker 1889</ref><ref>Owen 1865</ref> "until the cessation of quarrying activities in the Lias Group, early in this [20th] century."<ref name=Storrs146/> although less complete remains were used by [[Henry De la Beche]] and [[William Conybeare (geologist)|William Conybeare]] to name the species two years earlier in 1821,<ref name=Conybeare1821>De la Beche, H. T. & W. D. Conybeare. (1821). Notice of the discovery of a new fossil animal, forming a link between the ''[[Ichthyosaurus]]'' and [[crocodile]], together with general remarks on the osteology of the ''Ichthyosaurus''. ''Transactions of the Geological Society of London'' 5: 559–594</ref> and despite being discovered first, Conybeare's remains were not the [[holotype]]; Anning's were.

''Plesiosaurus'' was one of the first of the "[[antediluvian]] reptiles" to be discovered and excited great interest in 19th-century England. It was so-named ("near lizard") by [[William Daniel Conybeare|William Conybeare]] and [[Henry De la Beche]], to indicate that it was more like a normal reptile than ''[[Ichthyosaurus]]'', which had been found in the same rock strata just a few years earlier. ''Plesiosaurus'' is the archetypical genus of Plesiosauria and the first to be described, hence lending its name to the order. Conybeare and De la Beche coined the name for scattered finds from the Bristol region, [[Dorset]], and [[Lyme Regis]] in 1821.<ref name=Conybeare1821 /> The [[type species]] of ''Plesiosaurus'', ''P. dolichodeirus'', was named and described by Conybeare in 1824 on the basis of Anning's original finds.

==Description==
===Skull and dentition===

[[File:Plesiosaurus Scale.svg|thumb|''Plesiosaurus'' with a human to scale.]]
Compared to other plesiosaur genera, ''Plesiosaurus'' has a small head. The skull is much narrower than long,<ref name=Storrs166>Storrs 1997 pp. 166</ref> reaching its greatest width just behind the eyes (the [[postorbital]] bar).<ref name=Storrs165>Storrs 1997 pp. 165</ref> The [[anatomical terms of location|anterior]] portion is "bluntly triangular".<ref name=Storrs165/> In lateral view, the skull reaches its highest point at the rear of the [[skull roof|skull table]].<ref name=Storrs167>Storrs 1997 pp. 167</ref> "The external nostrils overlie the internal nares".<ref name=Storrs165/> They are not positioned at the tip of the snout, but farther back, nearer the eyes than the tip of the skull.<ref name=Storrs166/> Unlike the nostrils of ''[[Rhomaleosaurus]]'',<ref>Cruickshank 1991</ref> they do not appear to be adapted for underwater olfaction.<ref name=Storrs165/> The [[orbit (anatomy)|orbits]] (eye sockets) are roughly circular and are positioned about halfway along the length of the skull.<ref name=Storrs165/> They face up and to the sides.<ref name=Storrs166/><ref name=Storrs167/> Just posterior to the orbits are the [[supratemporal fenestra]]e, which are about the same size as the orbits and also roughly circular.<ref name=Storrs165/> Between the four openings is the [[Pineal gland#Miscellaneous anatomy|pineal foramen]], and between the temporal fenestrae is a narrow sagittal ridge.<ref name=Storrs165/> As in other plesiosaurs, the [[Pterygoid bone|pterygoid]]s of the palate are fused to the [[Occipital bone#In other animals|basioccipital]] of the [[braincase]],<ref name=Storrs165/> although the union is not as robust as in the pliosaurs ''Rhomaleosaurus'' and ''[[Pliosaurus]]''.<ref name=Storrs165/><ref>Taylor and Cruickshank 1993</ref> "The palatal bones are thin, but there is no suborbital fenestra."<ref name=Storrs165/>

The two [[ramus of the mandible|rami of the lower jaw]] make a "V" shape with an angle of about 45°.<ref name=Storrs166/> The specialized region where they meet, the [[Symphysis menti|symphysis]], is robust. The two rami are fused at the symphysis, making a pointed, shallow scoop-like shape.<ref name=Storrs169>Storrs 1997 pp. 169.</ref>

The teeth of ''Plesiosaurus'' are "simple, needle-like cones" that are "slightly curved and circular in transverse section". They are sharply pointed with fine striations running from tip to base, and point forward (procumbent). This procumbency becomes more pronounced near the leading end of the skull, where they may be only 10–15° above horizontal.<ref name=Storrs166/> There are 20 to 25 teeth per upper jaw tooth row,<ref name=Storrs165/> and 24 per low jaw tooth row.<ref name=Storrs166/> Up to four teeth of a lower jaw's tooth row are found in the symphyseal region.<ref name=Storrs169/>

===Vertebral column===
[[File:Conybeare Plesiosaur 1824.jpg|thumb|left|Illustration of the skeletal anatomy of a ''Plesiosaurus dolichodeirus'' from Conybeare's 1824 paper that described an almost complete plesiosaur skeleton found by Mary Anning in 1823]]
''Plesiosaurus'' was a moderately sized plesiosaur that grew to {{cvt|2.87|-|3.5|m|ft}} in length.<ref>{{cite journal|first1=W.J.|last1=Sollas|year=1881|title=On a new species of ''Plesiosaurus'' (''P. Conybeari'') from the Lower Lias of Charmouth; with observations on ''P. megacephalus'', Stutchbury, and ''P. brachycephalus'', Owen|journal=Quarterly Journal of the Geological Society of London|volume=37|issue=1–4 |pages=440–480|doi=10.1144/GSL.JGS.1881.037.01-04.42 |s2cid=129977015 |url=https://ia800708.us.archive.org/view_archive.php?archive=/22/items/crossref-pre-1909-scholarly-works/10.1144%252Fgsl.jgs.1873.029.01-02.46.zip&file=10.1144%252Fgsl.jgs.1881.037.01-04.42.pdf}}</ref><ref name=Storrs149>Storrs 1997 pp. 149</ref> There are approximately 40 [[cervical vertebrae]] (neck vertebrae), with different specimens preserving 38 to 42 cervical vertebrae.<ref name=Storrs170>Storrs 1997 pp. 170</ref>  Of the rest of the vertebral column, there are a handful (four or five in the [[holotype]] specimen) of "pectoral" vertebrae from the neck-torso transition,<ref name=Storrs170/> approximately 21 dorsal or back vertebrae, three or more [[sacrum|sacral vertebrae]], and at least 28 [[caudal vertebra]]e.<ref name=Storrs171>Storrs 1997 pp. 171</ref> Generally, the [[body of vertebra|centra]] of the cervical vertebrae are relatively elongated, being slightly longer than tall. The width, however, is usually greater than or equal to the length. The articular surfaces of the cervical centra are "slightly concave and kidney-shaped, with rounded, slightly rugose edges." Small holes called foramina subcentralia are found on the ventral surface of the centra. Some of the dorsals have rugose articular edges, like the cervicals; this feature is typically absent from the caudals.<ref name=Storrs170/>

Ribs are found from the neck to the tail. Cervical ribs are hatchet-shaped and have two articular heads.<ref name=Storrs170/> Dorsal ribs are thick and have only one head. Sacral ribs are "short, robust, and blunt or knob-like on both ends." Caudal ribs have different [[morphology (biology)|morphologies]] depending on their location along the tail, with anterior examples being pointed and more distal examples being "broad and blunt."<ref name=Storrs170>Storrs 1997 pp. 170</ref> ''Plesiosaurus'' also has [[gastralium|gastralia]], also known as "belly ribs." Nine or more sets of gastralia are present between the [[pectoral girdle|shoulder]] and [[pelvis]]. Each set is composed of seven elements: a bone on the midline flanked by three lateral elements.<ref name=Storrs171/>

===Limbs===
[[File:Plesiosaurus dolichodeirus.png|thumb|right|[[Life restoration]]]] 
The shoulder girdle is only partly known but appears to be typical for plesiosaurs. It includes fused [[clavicle]]s at the anterior end, [[scapula]]e (shoulder blades), and large [[coracoid]]s. The scapulae and coracoids both contribute to the [[glenoid cavity|glenoids]] (arm sockets). A pair of oval holes called pectoral fenestrae are found midway along the scapular/coracoid contacts.<ref name=Storrs171/> The forelimbs are elongate and relatively narrow compared to those of most plesiosaurs. The [[humerus]] (upper arm bone) has distinctive curvature, which appears to be a retained [[symplesiomorphy|primitive feature]] among [[sauropterygia]]ns. Mature ''Plesiosaurus'' also have a distinctive groove along the ventral surface of the humerus. The forearm includes a flat, broad, crescent-shaped [[ulna]] and a "robust and pillar-like" [[radius (bone)|radius]]. The wrist includes six bones.<ref name=Storrs173>Storrs 1997 pp. 173</ref> The hand paddle has five digits; the [[phalanx bones|phalangeal formula]] is uncertain, but the count for one large individual, from "[[thumb]]" to fifth "finger", is 4-8-9-8-6.<ref name=Storrs176>Storrs 1997 pp. 176</ref>

The pelvis includes equant [[pubis (bone)|pubic bones]], [[ischium|ischia]],<ref name=Storrs176/> and blade-shaped [[ilium (bone)|ilia]] connecting the pelvis to the vertebral column.<ref name=Storrs178>Storrs 1997 pp. 178</ref> The [[acetabulum]] is formed by surfaces on the pubic bones and ischia. Similar to the pectoral girdle, there is a pair of holes between the ischia and pubic bones.<ref name=Storrs176/> The hindlimbs are long and narrow,<ref name=Storrs178/> and in adults, they are much smaller than the forelimbs.<ref name=Storrs176/> The [[femur|thigh bones]] are straight. The lower hindlimb includes two roughly equal-sized bones, the robust [[tibia]] and the semilunate-shaped [[fibula]]. There are six bones in the ankle. The foot paddle includes five digits. Like the hand, the phalangeal formula is uncertain, but is at least 3-7-9-8-7 from innermost to outer "toe".<ref name=Storrs178/>

==Classifications==
[[File:Plesiosaurus dolichodeirus.jpg|thumb| Specimen referred to ''Plesiosaurus dolichodeirus'' (NHMUK OR 36183)]]
[[File:View of dinosaurs in the Dinosaur Trail in Crystal Palace Park ^8 - geograph.org.uk - 4491056.jpg|thumb|right|Historically important sculpture (left) in [[Crystal Palace Park]]]]
[[File:Plesiosaurus2.jpg|thumb|right|Modern restoration]] 
''Plesiosaurus'' has historically been a [[wastebasket taxon]]. This is due in part to few anatomical or taxonomic studies of the relevant fossils. Uncritical taxonomic work resulted in hundreds of species representing most of the world and most of the Mesozoic being assigned to ''Plesiosaurus.'' None of the younger [[Jurassic]] or [[Cretaceous]] species belong to ''Plesiosaurus''. Review of the Early Jurassic species indicates that the only English species properly assigned to ''Plesiosaurus'' is ''P. dolichodeirus''.<ref name=Storrs146/> Several other European Early Jurassic species have been assigned to new genera. ''P. brachypterygius'', ''P. guilielmiimperatoris'' and ''P. tournemirensis'', for example,  were assigned to the new genera ''[[Hydrorion]]'', ''[[Seeleyosaurus]]'' and ''[[Occitanosaurus]]''.

The following [[cladogram]] follows an analysis by Benson ''et al.'', 2012, and shows the placement of ''Plesiosaurus'' within Plesiosauria.<ref name=Bensonetal12>{{Cite journal | last1 = Benson | first1 = R. B. J. | last2 = Evans | first2 = M. | last3 = Druckenmiller | first3 = P. S. | editor1-last = Lalueza-Fox | editor1-first = Carles | title = High Diversity, Low Disparity and Small Body Size in Plesiosaurs (Reptilia, Sauropterygia) from the Triassic–Jurassic Boundary | doi = 10.1371/journal.pone.0031838 | journal = PLOS ONE | volume = 7 | issue = 3 | pages = e31838 | year = 2012 | pmid =  22438869| pmc =3306369 | bibcode = 2012PLoSO...731838B | doi-access = free }}</ref>

<div class="noprint">
{{clade| style=font-size:85%;line-height:85%
|label1=[[Pistosauria]]
|1={{clade
   |1="''Pistosaurus'' postcranium"
   |2=''[[Pistosaurus]]''
   |3=''[[Yunguisaurus liae]]''
   |4={{clade
      |1=''[[Augustasaurus hagdorni]]''
      |label2=[[Plesiosauria]]
      |2={{clade
         |1=''[[Bobosaurus forojuliensis]]''
         |2={{clade
            |1=''[[Anningasaura lymense]]''
            |2={{clade
               |label1=[[Rhomaleosauridae]]
               |1={{clade
                  |1=''[[Stratesaurus taylori]]''
                  |2={{clade
                     |1=''[[Macroplata tenuiceps]]''
                     |2={{clade
                        |1=''[[Avalonnectes arturi]]''
                        |2={{clade
                           |1=''[[Eurycleidus arcuatus]]'' 
                           |2={{clade
                              |1=''[[Meyerasaurus victor]]'' 
                              |2={{clade
                                 |1=''[[Maresaurus coccai]]''
                                 |2={{clade
                                    |1=''[[Atychodracon megacephalus]]''
                                    |2={{clade
                                       |1=''[[Archaeonectrus rostratus]]'' 
                                       |2={{clade
                                          |1=''[[Rhomaleosaurus cramptoni]]'' 
                                          |2={{clade
                                             |1=''[[Rhomaleosaurus thorntoni]]''
                                             |2=''[[Rhomaleosaurus zetlandicus]]'' }} }} }} }} }} }} }} }} }} }}
               |label2=[[Neoplesiosauria]]
               |2={{clade
                  |label1=[[Pliosauridae]]
                  |1={{clade
                     |1=''[[Thalassiodracon hawkinsi]]'' 
                     |2={{clade
                        |1=''[[Hauffiosaurus]] spp.'' 
                        |2={{clade
                           |1=''[[Attenborosaurus conybeari]]'' 
                           |2=advanced pliosaurids (''[[Peloneustes]]'') }} }} }}
                  |label2=[[Plesiosauroidea]]
                  |2={{clade
                     |1=''[[Eoplesiosaurus antiquior]]''
                     |2={{clade
                        |1='''''Plesiosaurus dolichodeirus'''''
                        |2={{clade
                           |1={{clade
                              |1=''[[Plesiopterys wildi]]''
                              |2=''[[Cryptoclidus eurymerus]]''}} 
                           |label2=[[Microcleididae]]
                           |2={{clade
                              |1=''[[Eretmosaurus rugosus]]'' 
                              |2=''[[Westphaliasaurus simonsensii]]''
                              |3={{clade
                                 |1=''[[Seeleyosaurus guilelmiimperatoris]]'' 
                                 |2={{clade
                                    |1=''[[Microcleidus tournemiensis]]''
                                    |2={{clade
                                       |1=''[[Hydrorion|Microcleidus brachypterygius]]'' 
                                       |2=''[[Microcleidus homalospondylus]]''}} }} }} }} }} }} }} }} }} }} }} }} }} }}
</div>

==Palaeobiology==
[[File:Plesiosaurus 3DB.jpg|thumb|Restoration]]
''Plesiosaurus'' fed mainly on clams and snails, and is thought to have eaten [[belemnite]]s, fish and other prey as well.<ref>{{Cite news|url=http://news.bbc.co.uk/1/hi/sci/tech/4341204.stm|title = Plesiosaur bottom-feeding shown|date = 17 October 2005}}</ref> Its U-shaped jaw and sharp teeth would have been like a fish trap. It propelled itself by the paddles, the tail being too short to be of much use. Its neck could have been used as a rudder when navigating during a chase. ''Plesiosaurus'' gave live birth to live young in the water like most [[sea snake]]s. The young might have lived in [[estuary|estuaries]] before moving out into the open ocean.
It has been postulated that the long neck of ''Plesiosaurus'' would have been a hindrance when trying to speed up, any bend in the neck creating turbulences.<ref>{{Cite web|url=https://www.sciencedaily.com/releases/2017/07/170705132908.htm|title = Sticking your neck out: How did plesiosaurs swim with such long necks?}}</ref> If that is the case then ''Plesiosaurus'' would have had to keep its neck straight to achieve good acceleration, something that would make hunting difficult. For this reason it may be possible that these animals would actually lie in wait for prey to come close instead of trying to pursue them.

==Palaeoenvironment==
[[File:Plesiosaurus sp.jpg|thumb|left|Tentatively referred specimen in [[Calgary]]]] 
Unequivocal specimens of ''Plesiosaurus dolichodeirus'' are limited to the Lyme Regis area of Dorset.<ref>Storrs 1997 pp. 148</ref> It appears to be the most common species of plesiosaur in the Lias Group of England.<ref>Storrs 1997 pp. 179</ref> ''Plesiosaurus'' is best represented from the "upper part of the [[Blue Lias]], the 'Shales with Beef,' and the lower Black Ven Marls" the latter of which form part of the [[Charmouth Mudstone]]; using the Lias Group [[ammonite]] fossil zones, these rocks date to the early [[Sinemurian]] stage. Some other ''Plesiosaurus'' fossils are from later Sinemurian rocks. The oldest specimen may be a skull thought to come from late [[Rhaetian]] or early [[Hettangian]] rocks.<ref name=Storrs180>Storrs 1997 pp.180</ref>

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

==References==
===Notes===
{{Reflist}}

===Sources===
* Andrews, C. W. 1896. "On the structure of the plesiosaurian skull". ''Quarterly Journal of the Geological Society'', London, 52, 246–253.
* Brown, D. S. 1981. "The English Upper Jurassic Plesiosauroidea (Reptilia) and a review of the phylogeny and classification of the Plesiosauria". ''Bulletin of the British Museum (Natural History): Geology'', 35, (4), 253–347.
* Cruickshank, A. R. I.; Small, P. G.; and Taylor, M. A. 1991. "Dorsal nostrils and hydrodynamically driven underwater olfaction in plesiosaurs". ''Nature'', 352, 62–64.
*Lydekker, R. 1889. Catalogue of the fossil Reptilia and Amphibia in the British Museum (Natural History), Part II. Containing the Orders Ichthyopterygia and Sauropterygia. British Museum (Natural History)
* [[Richard Owen]], Fossil Reptili of the Liassic Formations, pt iii. (Monogr. Palaeont. Soc., 1865)
* Persson, P. O. 1963. A revision of the classification of the Plesiosauria with a synopsis of the stratigraphical and geographical distribution of the group. Lunds Universitets Årsskrift, N. F. Avd. 2. 59, 1–59.
* Storrs, G. W. 1991. "Anatomy and relationships of ''Corosaurus alcovensis'' (Diapsida: Sauropterygia) and the Triassic Alcova Limestone of Wyoming". ''Bulletin of the Peabody Museum of Natural History'', 44, 1–151.
* Storrs, G. W. and Taylor, M. A. 1996. "Cranial anatomy of a new plesiosaur genus from the lowermost Lias (Rhaetian/Hettangian) of Street, Somerset, England". ''Journal of Vertebrate Paleontology'', 16, (3), 403–420.
* Storrs, G. W. 1997. "Morphological and taxonomic clarification of the genus ''Plesiosaurus''". 145–190. In Callaway, J. M and Nicholls, E. L. (eds.). ''Ancient Marine Reptiles''. Academic press. London.
*Taylor, M. A. and Cruickshank, A. R. I. 1993. Cranial anatomy and functional morphology of Pliosaurus brachyspondylus (Reptilia: Plesiosauria) from the Upper Jurassuc of Westbury, Wiltshire. Philosophical Transactions of the Royal Society of London, Series B, 341, 399–418. 
* Torrens, Hugh 1995. "Mary Anning (1799–1847) of Lyme; 'The Greatest Fossilist the World Ever Knew'". ''The British Journal for the History of Science'', 25 (3): 257–284

==External links==
{{Wikibooks|Wikijunior:Dinosaurs/Plesiosaur}}
{{Commons category|Plesiosaurus}}
* [http://plesiosauria.com/plesiosaurus Genus Plesiosaurus] – The Plesiosaur Directory
* [https://web.archive.org/web/20060712103044/http://www.palaeos.com/Vertebrates/Units/220Lepidosauromorpha/220.600.html Plesiosauroidea] – Palaeos
* [https://web.archive.org/web/20060902043146/http://www.fmnh.helsinki.fi/users/haaramo/Metazoa/Deuterostoma/Chordata/Reptilia/Sauropterygia/Plesiosauria.htm Plesiosauria ] – Mikko's Phylogeny Archive

{{Sauropterygia|Pliosauroidea}}
{{Taxonbar|from=Q857081}}

[[Category:Early Jurassic plesiosaurs of Europe]]
[[Category:Plesiosauroids]]
[[Category:Fossil taxa described in 1821]]
[[Category:Taxa named by William Conybeare]]
[[Category:Sauropterygian genera]]
[[Category:Taxa named by Henry De la Beche]]