Temporal range: 156.2–152 Ma Late Jurassic
The genus and type species was based upon ribs, vertebrae, a fore-paddle and fragments of the pectoral girdle discovered in Wyoming, USA in 1895. The species named as Megalneusaurus rex (meaning "great swimming lizard King") in 1898. However some of this material has since been lost, although new material has been discovered from the same site. Based upon the bones very large size, it appears to have grown to a size comparable to Liopleurodon.
Material from southern Alaska have been referred to Megalneusaurus, although this material is from an individual of much smaller size.
Megalneusaurus is said to reach lengths of 7.6–9.7 m (25–32 ft), though there are some estimates that propose a length of 11 meters.
While no stomach contents of Megalneusaurus have been discovered, it is reasonable to assume that it ate medium sized marine reptiles such as the ichthyosaur Ophthalmosaurus, and the cryptocleid plesiosaur, Pantosaurus.
- Knight WC. 1895 A new Jurassic plesiosaur from Wyoming. Science 2: 449.
- Knight WC. 1898. Some new Jurassic vertebrates from Wyoming. American Journal of Science 4: 378-381.
- Wahl WR, Ross M, Massare JA. 2007. Rediscovery of Wilbur Knight’s Megalneusaurus rex site: new material from an old pit. Paludicola 6 (2): 94-104.
- Weems RE, Blodgett RB. 1996. The pliosaurid Megalneusaurus: a newly recognized occurrence in the Upper Jurassic Neknek Formation of the Alaska Peninsula. U.S. Geological Survey Bulletin 2152: 169-175.
- "Mesozoic Stratigraphy in the Thermopolis Area". Big Horn Basin Foundation. Retrieved 2007-02-06
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