Editing Plesiosaurus (section)

==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/>