Elasmosaurus

Elasmosaurus () is a genus of plesiosaur that lived in North America during the Campanian stage of the Late Cretaceous period, at about 80.6 to 77million years ago. The first specimen was discovered in 1867 near Fort Wallace, Kansas, US, and was sent to the American paleontologist Edward Drinker Cope, who named it E.platyurus in 1868. The generic name means "thin-plate reptile", and the specific name means "flat-tailed". Cope originally reconstructed the skeleton of Elasmosaurus with the skull at the end of the tail, an error which was made light of by the paleontologist Othniel Charles Marsh, and became part of their "Bone Wars" rivalry. Only one incomplete Elasmosaurus skeleton is definitely known, consisting of a fragmentary skull, the spine, and now lost pectoral and pelvic girdles, and a single species is recognized today; other species are now considered invalid or have been moved to other genera. A fragmentary specimen from Germany may belong to this genus.

Measuring in length, Elasmosaurus would have had a streamlined body with paddle-like limbs, a short tail, a small head, and an extremely long neck. The neck alone was around long. Along with its relative Albertonectes, it was one of the longest-necked animals to have lived, with the second largest number of neck vertebrae known, 72, at least 3 fewer than Albertonectes. The skull would have been slender and triangular, with large, fang-like teeth at the front, and smaller teeth towards the back. It had six teeth in each premaxilla of the upper jaw, and may have had 14 teeth in the maxilla and 19 in the dentary of the lower jaw. Most of the neck vertebrae were compressed sideways, and bore a longitudinal crest or keel along the sides.