| Megalneusaurus | |
|---|---|
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| Illustration of some of the holotype fossils | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | †Sauropterygia |
| Order: | †Plesiosauria |
| Family: | †Pliosauridae |
| Genus: | †Megalneusaurus Knight, 1898 |
| Type species | |
| Megalneusaurus rex | |
| Synonyms | |
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Megalneusaurus is an extinct genus of large pliosaurs that lived during the Oxfordian and Kimmeridgian stages of the Late Jurassic in what is now North America. It was provisionally described as a species of Cimoliosaurus by the geologist Wilbur Clinton Knight in 1895, before being given its own genus by the same author in 1898. The only species identified to date is M. rex, known from several specimens identified in the Redwater Shale Member, within the Sundance Formation, Wyoming, United States. A specimen discovered in the Naknek Formation in southern Alaska was referred to the genus in 1994. In Ancient Greek, the generic name literally translates to "large swimming lizard", due to the measurement of the fossils of the holotype specimen.
Estimated to be around 7–9 metres (23–30 ft) long, Megalneusaurus is the largest known North American pliosaur dating from the Jurassic. As its name suggests, the genus was considered the largest sauropterygian identified before the discovery of some Kronosaurus fossils in 1930. Based on comparisons made with other pliosaurs, Megalneusaurus is generally viewed as a pliosaurid. Based on stomach contents, the animal fed on cephalopods and fish, although it is not excluded that it would have attacked and fed on contemporary plesiosaurs.