Editing Elasmosaurus (section)

===Vertebrae===

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

The [[atlas bone|atlas]] and [[axis bone]] complex, consisting of the first two neck vertebrae and articulated with the back of the skull, was long, low, and horizontally rectangular in side-view. The centra, or "bodies", of these vertebrae were co-ossified in the holotype specimen, which indicates it was an adult. The neural arches of these vertebrae were very thin and rather high, which gave the neural canal (the opening through the middle of the vertebrae) a triangular outline when seen from the back. The lower part of the neural canal was narrow towards the back by the axis, where it was half the breadth of the centrum. It became broader towards the front, where it was almost the same breadth as the centrum of the atlas. The neural arches were also more robust there than in the axis, and the neural canal was higher. The neural spine was low and directed upwards and back. The centra of the atlas and axis were of equal length, and had a quadratic shape in side view. The surface (or facet) where the axis articulated with the next vertebra had an oval outline, and an excavation for the neural canal in the middle of its upper edge. A distinct keel ran along the lower middle of the atlas and axis vertebrae.<ref name=Sachs2005/>

[[File:Cervical vertebrae of Elasmosaurus.png|thumb|left|alt=Gray vertebrae seen from different angles|[[Cervical vertebrae]] from different areas of the holotype's neck shown from the left, behind, and below]]
Most of the neck vertebrae were compressed sideways, especially at the middle of the neck. A crest (also termed ridge or keel) ran longitudinally along the side of the neck vertebrae (a feature typical of elasmosaurids), visible from the third to the fifty-fifth vertebrae, at the hind part of the neck. This crest was positioned at the middle of the centrum in the front vertebrae, and at the upper half of the centrum from the 19th vertebra and onwards. The crest would have served to anchor the musculature of the neck. The centra differed in shape depending on the position of the vertebrae in the neck; that of the third vertebra was about as long as it was broad, but the centra became longer than broad from the fourth vertebra and onwards. The centra became more elongated at the middle of the neck, but became shorter again at the back of the neck, with the length and breadth being about equal at the 61st vertebra, and those of the hindmost vertebrae being broader than long. The articular surfaces of the vertebrae in the front of the neck were broad oval, and moderately deepened, with rounded, thickened edges, with an excavation (or cavity) at the upper and lower sides. Further back in the front part of the neck, around the 25th vertebra, the lower edge of the articular facets became more concave, and the facet shaped like a quadrate with rounded edges. By the 63rd vertebra, the articular facet was also quadratic in shape with rounded edges, whereas the centra of the hindmost vertebrae had a broad oval outline.<ref name="Focus"/><ref name="Count"/><ref name=Sachs2005/>

The neural arches of the neck vertebrae were well fused to the centra, leaving no visible sutures, and the neural canal was narrow in the front vertebrae, becoming more prominently developed in the hind vertebrae, where it was as broad as high, and almost circular. The pre-and post-[[zygapophyses]] of the neck vertebrae, processes that articulated adjacent vertebrae so they fit together, were of equal length; the former reached entirely over the level of the centrum whereas the latter reached only with their back half. The neural spines of the neck vertebrae appear to have been low, and almost semi-circular by the 20th vertebra. The facets where the [[Cervical rib|neck ribs]] articulated with the neck vertebrae were placed on the lower sides of the centra, but were only placed higher in the last three vertebrae, reaching around the middle of the sides. The neck ribs were semicircular to quadratic in side view, and were directed rather straight down. The bottom of each neck vertebrae had pairs of nutritive [[foramina]] (openings) at the middle, separated by a ridge, which became progressively more prominent and thickened towards the back of the neck.<ref name=Sachs2005/>

[[File:Pectoral vertebrae of Elasmosaurus.png|thumb|alt=Three connected gray vertebrae on a white background|Vertebrae from the [[pectoral region]] of the holotype specimen]]
The vertebrae that transitioned between the neck and back vertebrae in the [[pectoral region]] of plesiosaurs, close to the front margin of the [[forelimb girdle]], are often termed pectoral vertebrae. ''Elasmosaurus'' had three pectoral vertebrae, which is a common number for elasmosaurids. The rib facets of the pectoral vertebrae were triangular in shape and situated on transverse processes, and the centra bore pairs of nutritive foramina in the middle of the lower sides. The back vertebrae had rib facets level with the neural canal, and the front and back part of the transverse processes here had distinct ridges on their margins. Here the rib facets where placed higher than the transverse processes, separating the two, and were oval to rectangular in outline. The pre-zygapophyses here were shorter than those in the neck and pectoral vertebrae, and only reached above the level of the centrum with the front third of their length. The post-zygapophyses reached over the level of the centrum with the back half of their length. Back vertebrae are not useful for distinguishing between elasmosaurids, since they are not diagnostic at the genus level.<ref name="Count"/><ref name=Sachs2005/>

''Elasmosaurus'' had four [[sacral vertebrae]] (the fused vertebrae that form the sacrum connected to the pelvis), a number typical of elasmosaurids. The transverse processes here were very short, and the rib facets increased in size from the first to the fourth sacral vertebra. A ridge ran along the top of these vertebrae, and the lower sides of the centra were rounded, and bore pairs of nutritive foramina, separated by low ridges. The first tail (or caudal) vertebra could be distinguished by the preceding sacral vertebra by having smaller rib facets, and by being positioned in the lower half of the centrum. These vertebrae were almost circular in shape, and the first two bore a narrow keel in the middle of the upper side. The rib facets of the tail vertebrae were located on the lower side of the centra, and their oval shape became larger and broader from the third vertebra and onwards, but became smaller from the 14th vertebra. Here, the pre-zygapophyses also reached over the level of the centra for most of their length, while the post-zygapophyses reached over this level by half their length. The lower part of the centra were rounded from the first to the third tail vertebrae, but concave from the fourth to the 18th. The usual number of tail vertebrae in elasmosaurids is 30.<ref name=Sachs2005/> Since the last tail-vertebrae of elasmosaurids were fused into a structure similar to the [[pygostyle]] of birds, it is possible this supported a tail-fin, but the shape it would have had is unknown.<ref name="Focus"/>