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Temporal range: Late Cretaceous, Maastrichtian
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: Sauropterygia
Order: Plesiosauria
Superfamily: Plesiosauroidea
Family: Elasmosauridae
Genus: Hydrotherosaurus
Welles, 1943
Species: † H. alexandrae
Binomial name
Hydrotherosaurus alexandrae
Welles, 1943

Hydrotherosaurus (meaning "water beast lizard") is an extinct genus of elasmosaurid plesiosaur from the Upper Cretaceous (Maastrichtian stage) of Fresno County, California, measuring up to 13 m in length. The species H. alexandrae was named for its discoverer, Annie Montague Alexander by Samuel Paul Welles.


  • Welles, S. P. 1943. Elasmosaurid plesiosaurs with a description of the new material from California and Colorado. University of California Memoirs 13:125-254. figs.1-37., pls.12-29.

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