Hydrotherosaurus

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title: "Hydrotherosaurus" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["late-cretaceous-plesiosaurs-of-north-america", "fossil-taxa-described-in-1943", "elasmosauridae", "taxa-named-by-samuel-paul-welles", "sauropterygian-genera"] description: "Extinct genus of reptiles" topic_path: "general/late-cretaceous-plesiosaurs-of-north-america" source: "https://en.wikipedia.org/wiki/Hydrotherosaurus" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Extinct genus of reptiles ::

| fossil_range = Maastrichtian, ~ | image = Hydrotherosaurus_NT.jpg | image_caption = Restoration | genus = Hydrotherosaurus | parent_authority = Welles, 1943 | species = alexandrae | authority = Welles, 1943

Hydrotherosaurus (meaning "fisherman lizard") is an extinct genus of elasmosaurid plesiosaur from the Upper Cretaceous (Maastrichtian stage) Moreno Formation of Fresno County, California, USA. The only known species, H. alexandrae, was named for Annie Montague Alexander in 1943 by Samuel Paul Welles.

Discovery and naming

The first remains of Hydrotherosaurus to be found were several vertebrae discovered by Frank C. Paiva on his own property in the Panoche Hills of Fresno County, California, which were taken to Berkeley in the spring of 1937 by W. M. Tucker, who was the Chairman of the Department of Geology of Fresno State College. Afterwards, Fresno State College and the University of California Museum of Paleontology organized a joint expedition which would uncover a nearly complete fossilized skeleton of the animal, with only parts of the skull, shoulder girdle, flippers and certain vertebral elements missing. This specimen, designated UCMP 33912, was collected in a ravine on the property of the Sun Ray Gypsum Mine, about 22 miles west of Mendota, California, and the deposits from which it originates are part of the Moreno Formation. After being excavated, UCMP 33912 was mounted for display, first at the Golden Gate International Exposition, then at the University of California Berkeley, where it remained until being put into storage in the 1960s.

In 1943, American paleontologist Samuel Paul Welles found that the specimen was notably different from other known plesiosaurs and declared it as the type specimen of a new genus and species which he named Hydrotherosaurus alexandrae. The generic name means "fisherman lizard", while the specific name honors Annie Montague Alexander, who has done plenty of research on the vertebrates of the western United States.

Description

::figure[src="https://upload.wikimedia.org/wikipedia/commons/8/87/Hydrotherosaurus_Scale.svg" caption="Size comparison"] ::

Hydrotherosaurus was a medium-sized plesiosaur, measuring approximately 7.8 - long. It has one of the longest necks relative to total length among elasmosaurids, with 60 vertebrae in total. It had a small head that measured about 33 cm long, a streamlined body, and four large flippers that were specially designed to help the huge animal balance, move, and accelerate itself. Like other elasmosaurs, Hydrotherosaurus is thought to have been an active, fish-eating animal.

Classification

The following cladogram shows the phylogenetic position of Hydrotherosaurus within Elasmosauridae: | style=font-size:85%; line-height:85%; | label1=Elasmosauridae | 1={{clade | 1={{clade | 1=Callawayasaurus | 2=Eromangasaurus | 3=Wapuskanectes | 4=Speeton Clay plesiosaur | label2=Euelasmosaurida | 2={{clade | 1=Lagenanectes | 2={{clade | 1={{clade | 1=Traskasaura | 2=Nakonanectes | 2={{clade | 1={{clade | 1={{clade | 1=Libonectes | 2={{clade | 1=Jucha | 2=Fluvionectes | 2={{clade | 1=Thalassomedon | 2=Styxosaurus | 2={{clade | label1=Elasmosaurinae | 1={{clade | 1=Terminonatator | 2={{clade | 1=Albertonectes | 2=Elasmosaurus | 2={{clade | 1=Zarafasaura | 2={{clade | 1=Cardiocorax | label2=Weddellonectia | 2={{clade | 1=Tuarangisaurus | 2={{clade | 1=Futabasaurus | 2={{clade | 1={{clade | 1=Chubutinectes | 2={{clade | 1=Aphrosaurus | 2=Hydrotherosaurus | 2={{clade | 1={{clade | 1=Morenosaurus | 2={{clade | 1=Vegasaurus | 2=Kawanectes | 2={{clade | 1=Marambionectes | label2=Aristonectinae | 2={{clade | 1=Wunyelfia | 2={{clade | 1=Aristonectes parvidens | 2=Aristonectes quiriquinensis | 3=Kaiwhekea | 4=MLP 99 XII 1 5 | 5={{clade | 1=Alexandronectes | 2=Morturneria

References

References

1. Tomiya, Susumu. (2012-03-09). "Cataloging the archives: Unearthing a type".
2. Sachs, Sven. (2005). "Remarks on the pectoral girdle of ''Hydrotherosaurus alexandrae'' (Plesiosauria: Elasmosauridae)". PalArch.
3. Welles, Samuel Paul. (1943). "Elasmosaurid plesiosaurs with a description of the new material from California and Colorado". University of California Memoirs.
4. (2016). "A Small Body Sized Non-Aristonectine Elasmosaurid (Sauropterygia, Plesiosauria) from the Late Cretaceous of Patagonia with Comments on the Relationships of the Patagonian and Antarctic Elasmosaurids". Ameghiniana.
5. (2020). "A new elasmosaurid plesiosaurian from the Early Cretaceous of Russia marks an early attempt at neck elongation". Zoological Journal of the Linnean Society.
6. Otero, Rodrigo A.. (2016-03-15). "Taxonomic reassessment of ''Hydralmosaurus'' as ''Styxosaurus'': new insights on the elasmosaurid neck evolution throughout the Cretaceous". PeerJ.
7. Hilton, Richard P.. (2003). "Dinosaurs and Other Mesozoic Reptiles of California". University of California Press.
8. (2025). "A name for the Provincial Fossil of British Columbia: a strange new elasmosaur taxon from the Santonian of Vancouver Island". Journal of Systematic Palaeontology.

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