Styxosaurus

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

Etymology
Styxosaurus is named for the mythological River Styx (Στυξ), which separated the Greek underworld from the world of the living. The -saurus part comes from the Greek sauros (σαυρος), meaning "lizard" or "reptile."

The type specimen was found on Hell Creek in Logan County, Kansas and is the source of the genus name coined by Samuel Paul Welles, who described the genus, in 1943.

Description
The holotype specimen of Styxosaurus snowii was described by S.W. Williston[2][3] from a complete skull and 20 vertebrae.[2]

Another more complete specimen - SDSMT 451 (about 36 feet (11 m) long) was discovered near Iona, South Dakota in 1945. The specimen was originally described and named Alzadasaurus pembertoni by Welles and Bump (1949) and remained so until it was synonymized with S. snowii by Carpenter.[4] Its chest cavity contained about 250 gastroliths, or "stomach stones". Although it is mounted at the School of Mines as if its head were looking up and out of the water, such a position would be physically impossible.[5]

[6] While most predators do not use gastroliths for grinding of food, almost all reasonably complete elasmosaur specimens include gastroliths. Although it is possible Styxosaurus may have used the stones as ballast, a Styxosaurus specimen found in the Pierre Shale of western Kansas included ground up fish bones mixed with the gastroliths.[7] In addition, the weight of the gastroliths found in elasmosaurs specimens is always much less than 1% of the estimated weight of the living animal.[8]

While crocodiles and some other animals may use gastroliths for ballast today, it appears likely that elasmosaurs used them as a gastric mill. See Henderson (2006) contra Wings (2004).