Styxosaurus is a genus of plesiosaur of the family Elasmosauridae. Styxosaurus lived during the Santonian and Campanian ages of the Cretaceous period.


Styxosaurus was a large plesiosaur, one of several species of a group collectively called elasmosaurs that appeared in the Late Cretaceous. Elasmosaurs typically have a neck that is at least half the length of the body, and composed of 60-72 vertebrae.

Styxosaurus was around 11–12 metres (36–39 ft) long, with about half of the length being composed of its 5.25 metres (17.2 ft) neck. Its sharp teeth were conical and were adapted to puncture and hold rather than to cut; like other plesiosaurs, Styxosaurus swallowed its food whole.


Styxosaurus snowii is from a group called elasmosaurs, and is closely related to Elasmosaurus platyurus, which was found in Kansas, USA, in 1867.

The first Styxosaurus to be described was initially called Cimoliasaurus snowii by S.W. Williston in 1890. The specimen included a complete skull and more than 20 cervical vertebrae ( KUVP 1301) that were found near Hell Creek in western Kansas by Judge E.P. West.

The name was later changed to Elasmosaurus snowii by Williston in 1906 and then to Styxosaurus snowii by Welles in 1943.

A second species, Styxosaurus browni, was named by Welles in 1952. Although synonymized with Hydralmosaurus serpentinus in 1999, it has been recently revalidated.