Fossil range: Late Cretaceous
Scientific classification








Cope, 1868


  • E. ischiadicus
  • E. kurskensis
  • E. marshii
  • E. nobilis
  • E. orskensis
  • E. platyurus
  • E. serdobensis
  • E. serpentinus

Elasmosaurus is an extinct genus of plesiosaur with an extremely long neck that lived in the Late Cretaceous.



Elasmosaurus skull and neck, North American Museum of Ancient Life.

It was about 14 m (46 ft) in length and weighed over 2,000 kg (2.2 tons), making it the second longest plesiosaur. It had a large body and four flippers for limbs. More than half of its length was neck, which had more than 70 vertebrae, more than any other animal. It had a relatively small head with sharp teeth.


Elasmosaurus platyurus was described in March, 1868 by Edward Drinker Cope from a fossil discovered and collected by Dr. Theophilus Turner, a military doctor, in western Kansas, USA. Although other specimens of elasmosaurs have been found in various locations in North America, Carpenter (1999) determined that Elasmosaurus platyurus was the only representative of the genus.

Cope Elasmosaurus

Anterior portion of the "head-on-the-wrong-end" version of Elasmosaurus platyurus. Cope did not include the hind paddles in this figure in part due to his erroneous belief that Elasmosaurus was propelled by its extremely long “tail”

When E. D. Cope received the specimen in early March, 1868, he had a pre-conceived idea of what it should look like, and mistakenly placed the head on the wrong end (e.g. 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 ever seen a plesiosaur of the size of Elasmosaurus.

Although popular legend notes that it was Othniel Charles Marsh who pointed out the error, there is no factual justification for this account (see below). However, this event is often cited as one of the causes of their long-lasting and acrimonious rivalry, known as the Bone Wars. In fact, although Marsh personally collected at least one plesiosaur from Kansas, and had several more from Kansas in the Yale Peabody collection, he never published a single paper on them (Everhart, 2005).

Although Cope verbally announced the discovery of Elasmosaurus platyurus in March 1868, he did not publish the "preprint" of his erroneous reconstruction of Elasmosaurus until August 1869. While much smaller, long-necked plesiosaurs from the Jurassic of England were well known at the time, this was the first time anyone had ever seen a Cretaceous elasmosaur. Cope's reconstruction showed it to have a long sinuous 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 actually Joseph Leidy who pointed out the problem in his Remarks on Elasmosaurus platyurus address at the Academy of Natural Sciences of Philadelphia meeting on March 8, 1870.


Like most plesiosaurs, Elasmosaurus was incapable of raising anything more than its head above the water as it is commonly depicted in art and media. The weight of its long neck places the center of gravity behind the front flippers. Thus the Elasmosaurus could only raise both its head and neck above the water if it was in shallow water, where it could rest its body on the bottom. Of course this would probably be a fatal situation for the animal. The Elasmosaurus could probably not raise the head too high up due to the weight of the neck, the limited musculature and the limited movement between the vertebrae.

The head and neck of the Elasmosaurus most likely acted as a rudder. If the animal moved the head/neck in a certain direction it would cause the rest of the body to move in that direction. Thus Elasmosaurus could not have swum in one direction while moving its head and neck either horizontally or vertically.

Elasmosaurus was a slow swimmer and is theorized to have stalked schools of fish. The long neck would allow Elasmosaurus to have concealed itself below the school of fish. It then would have moved its head slowly and approached its prey from below. The eye of the animal could have had stereoscopic vision which would help it find small prey. Hunting from below would also be help by using sunlight to silhouette the prey and be concealed in the dark water. Elasmosaurus probably ate small bony fish, belemnites (similar to squid), and ammonites (molluscs). It swallowed small stones in order to aid its digestion.

Elasmosaurus is believed to have lived mostly in open ocean. The paddles of Elasmosaurus and other plesiosaurs are so rigid 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 modern sea snakes.


  • Where the Elasmosaurs roam Original article published as: Everhart, M. J. 2002. Where the Elasmosaurs roam...... Prehistoric Times 53:24-27
  • Carpenter, K. 1999. Revision of North American elasmosaurs from the Cretaceous of the western interior. Paludicola 2(2):148-173.
  • Cope, E. D. 1868. Remarks on a new enaliosaurian, Elasmosaurus platyurus. Proceedings of the Academy of Natural Sciences of Philadelphia 20:92-93. (for meeting of March 24, 1868)
  • Cope, E. D. 1869. Synopsis of the Extinct Batrachia and Reptilia of North America, Part I. Transactions American Philadelphia Society New Series, 14:1-235, 51 figs., 11 pls. (pre-print dated August, 1869)
  • Everhart, M. J. 2005a. Oceans of Kansas - A Natural History of the Western Interior Sea. Indiana University Press, 320 pp.
  • Everhart, M. J. 2005b. Elasmosaurid remains from the Pierre Shale (Upper Cretaceous) of western Kansas. Possible missing elements of the type specimen of Elasmosaurus platyurus Cope 1868? PalArch 4(3): 19-32.
  • Leidy, J. 1870. [Remarks on Elasmosaurus platyurus]. Proceedings of the Academy of Natural Sciences of Philadelphia 22: 9-10.

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