Hydrotherosaurus is a long-necked elasmosaurid from California, USA. It is represented by one of the most complete elasmosaurid skeletons ever discovered, so Hydrotherosaurus is one of the best known members of this family. The almost complete type skeleton of Hydrotherosaurus was discovered in the Panoche Hills by Mr. Frank C. Paiva in 1937, and was excavated by the University of California Museum of Palaeontology, Berkeley, and Fresno State College. A cast of the holotype skeleton is on public display in the California Academy of Sciences.
The taxon was named and described in detail by Samuel P. Welles (1943, 1952) and differs most obviously from other elasmosaurids in its cervical vertebral count of 60. Welles (1943) derived the genus name from the word “fisherman”, hence “fisherman lizard”. However, the genus translates more literally as “water beast reptile”.
