Thalassiosaurus

Styxosaurus is a genus of elasmosaurid plesiosaur that lived during the Late Cretaceous period. Three species are known: S. snowii, S. browni, and S. rezaci. Fossils are known from North America and are mainly found in the Niobrara and Pierre Shale formations. Older fossils are known from the Graneros Shale.