Elasmosaurus was about 46 ft long and weighed 8 to 10 tons, which makes it one of the largest plesiosaurs.It differs from other plesiosaurs by having six teeth per premaxilla (the bones at the tip of the snout) and 71 neck vertebrae. The skull was flat, with quite a few long, sharp teeth. The lower jaws were joined at the tip to a point between the fourth and fifth teeth. The neck vertebrae immediately following the skull were long and low, and had crests on the sides. Like most elasmosaurids, Elasmosaurus had around 3 chest vertebrae.The tail had at least 18 vertebrae.
The shoulder girdle had a long bar  not found in young ones.The shoulder blade had edges of the same length for the joint with the coracoid and the articular surface for the upper arm. The front of the pelvic girdle was made up of 3 straight edges aimed at the front and sides of the animal. The ischia, a pair of bones that formed the back of the pelvis, were joined on their medial surfaces. The limbs of Elasmosaurus, like those of all plesiosaurs, were modified into four paddle-shaped fins. 
Some parts of Elasmosaurus were quite advanced for elasmosaurids and plesiosaurs on the whole. Elasmosaurus can be distinguished by its six premaxillary teeth and 71 cervical vertebrae.  Early plesiosaurs and most elasmosaurids had five teeth per premaxilla. Some elasmosaurids had more: Terminonatator had 9 and Aristonectes had 10-13.  Most plesiosaurs had less than 60 cervical vertebrae.  Other plesiosaurs with more than 60 cervicals are Styxosaurus, Hydralmosaurus, and Thalassomedon. Elasmosaurus is the one known plesiosaur with more than 70 cervicals, but, it's neck was close to the same length as Thalassomedon as the latter has proportionally longer vertebrae.  Elasmosaurus had more vertebrae than any known animal. The long, low axis centrum is not like those seen in most plesiosaurs, which have centra that are shorter in length than height, or the same size. Styxosaurus and Hydralmosaurus have the condition present in Elasmosaurus too. Another strange trait of Elasmosaurus is the same length of the edges of the shoulder blade. Most plesiosaurs had longer edges for articulation with the coracoid than for articulation with the upper arm.
Classification and species
While a lot of species of Elasmosaurus have been named since it was found, a 1999 review by Ken Carpenter showed that just one of these, the type species Elasmosaurus platyurus, was valid. Most other species assigned to the genus are either doubtful or have been classified in other genera. For example, E. serpentinus was reclassified as Hydralmosaurus, E. morgani as Libonectes, and E. snowii as Styxosaurus.  By the Late Cretaceous, plesiosaurs had evolved (or been reduced) to two distinct groups.  Elasmosaurus is the type genus for one of these groups, elasmosaurids which had very long necks with small heads, in contrast to the polycotylids which had short necks and large heads. Late Cretaceous elasmosaurids from the Western Interior of North America had traits that differ from them and are morphologically primitive.  However, Elasmosaurus and some others have some derived features. Elasmosaurus may have been closely related to Hydralmosaurus and Styxosaurus due to these advanced features. 
Elasmosaurus platyurus was described in March, 1868 by Edward Drinker Cope  from a fossil found by Dr. Theophilus Turner, a military doctor in Kansas. Though other specimens of elasmosaurs have been found in North America, Carpenter (1999) found that Elasmosaurus platyurus was the only representative of the genus.
When E. D. Cope got the specimen in early March, 1868, he had a biased idea of what it should look like, and mistakenly placed the head on the wrong end (i.e. the tail). In his defense, at the time he was an expert on lizards, which have a short neck and a long tail, and no one had seen a plesiosaur the size of Elasmosaurus. While folk legend notes that it was Othniel Charles Marsh who pointed out the error, there is no factual evidence for this account, but, this event is may have caused their long and hate-filled feud, known as the Bone Wars. In fact, although Marsh personally gathered at least one plesiosaur from Kansas, and had several more from Kansas in the Yale Peabody collection, he did not write any paper on them. 
Although Cope announced the discovery of Elasmosaurus platyurus in March 1868, he did not print the "preprint" of his wrong reconstruction of Elasmosaurus until August 1869. While much smaller, long-necked plesiosaurs from the Jurassic of England were well known at the time, but Elasmosaurus was the first known Cretaceous elasmosaur. Cope's reconstruction showed it to have a long curving tail like a lizard or a mosasaur. Note that while O.C. Marsh claimed to have pointed out Cope's error "20 years after the fact" in an 1890 newspaper article, it was in fact Joseph Leidy who pointed out the problem in his Remarks on Elasmosaurus platyurus speech at the Academy of Natural Sciences of Philadelphia meeting on March 8, 1870.
Elasmosaurus fossils have been found in the Late Cretaceous formation of the Pierre Shale of Kansas. The Pierre Shale represents a period of marine deposition from the Western Interior Seaway, a shallow continental sea that covered much of central North America in the Cretaceous.
Like most plesiosaurs, Elasmosaurus could not raise anything more than its head above the water as it is commonly shown in art and media. The weight of its long neck placed the center of gravity behind the front flippers. Thus Elasmosaurus could only have raised its head and neck above the water if in shallow water, where it could rest its body on the bottom. The weight of the neck, the small muscles, and the limited movement between the vertebrae would have kept Elasmosaurus from raising its head and neck high as well.  Nevertheless one study found that the necks of elasmosaurs were capable of 75–177° of ventral movement, 87–155° of dorsal movement, and 94–176° of lateral movement, depending on the amount of tissue between the vertebrae.  "Swan-like" S-shape neck postures would take more than 360° degrees of upright bending were not possible. The head and shoulders of the Elasmosaurus most likely acted as a rudder. If the animal moved the front of the body in one direction, it would cause the rest of the body to move in that direction. Thus, Elasmosaurus could not have swum one way and move its head and neck another way, horizontally or vertically.
Elasmosaurus was a slow swimmer and may have stalked schools of fish. The long neck would let Elasmosaurus hide under the school of fish. It then would have moved its head slowly and neared its prey from below. The eyes of the animal could have had stereoscopic vision, which would help it find small prey. Hunting from below would have helped by silhouetting the prey in the sunlight while Elasmosaurus hid in the dark waters. Elasmosaurus probably ate small bony fish, belemnites (similar to squid), and ammonites (molluscs). It ate small stones to help it digest. Elasmosaurus is believed to have lived mostly in open sea. The fins of Elasmosaurus and other plesiosaurs are so stiff and specialized for swimming that they could not have come on land to lay eggs. Thus it most likely gave live birth to its young like sea snakes. While direct evidence of reproduction in Elasmosaurus is not yet known, we know the co-existing plesiosaur Polycotylus gave birth to live young.
- Elasmosaurus platyurus on Oceans of Kansas
- Elasmosaurus The Plesiosaur Directory - Elasmosaurus
- Where the Elasmosaurs roam: Separating fact from fiction
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