In Ancient Greek, the generic name literally translates to "large swimming lizard", due to the measurement of the fossils of the holotype specimen.

In 1895, geologist Wilbur Clinton Knight discovered and exhumed fossils of a partially articulated but incomplete skeleton of a pliosaur near the small town of Ervey, Wyoming, United States.

[6] The generic name is formed from the words in Ancient Greek μέγας (mégas, "great") and νηκτός (nêktós, "swimmer") prefixed onto σαῦρος (saûros, "lizard").

[6][5]: 358 Due to the discovery of other large pliosaurs found elsewhere in the world,[9] the attention of Megalneusaurus by paleontologists has generally been "forgotten",[1] being only very briefly mentioned in 20th and early 21st century scientific literature.

Based on this same casting, paleontologist William Wahl of the Wyoming Dinosaur Center, began to take a particular interest in this taxon, which led him to open an investigation to find the location of the original type locality of Megalneusaurus, previously unmentioned in Knight's work.

It was therefore during the summer of 1996 that the type locality was finally found, corresponding to the Redwater Shale Member, located in the upper part of the Sundance formation, in the eastern Rocky Mountains.

[1][2][8] About 20 m (66 ft) from the area where the first known fossils of Megalneusaurus were discovered, a large bone fragment probably coming from the shoulder or pelvic girdle was exhumed and described in an article published in 2007.

These two bone fragments were collected from the Kejulik River, located on the Alaska Peninsula, and consist of the proximal and distal ends of the same large humerus, which would subsequently be cataloged as USNM 418489.

The stratigraphic unit to which this specimen was discovered corresponds to the Snug Harbor Siltstone Member of the Naknek Formation, dating from between the Oxfordian and Kimmeridgian stages of the Late Jurassic.

[1][2][9] Most studies and descriptions qualify Megalneusaurus as the largest pliosaur to have been identified in the North American continent of any period,[1][2] but some related genera such as Megacephalosaurus, dating from the Upper Cretaceous, are of comparable size.

[2] In his thesis published in 2009, Australian paleontologist Colin McHenry estimates the size of the animal at between 10 and 12 m (33 and 39 ft) long based on measurements of the femurs given by Knight in 1898.

[1][19] Based on a skeleton preserved in the Museum of Comparative Zoology of Harvard attributed to Kronosaurus, Bakker suggests that the animal would have had a skull measuring 3.3 m (11 ft) long, although the relevant material is not known.

[10]: 37 [11]: 341 [15][2][1][19] In 2009, McHenry questioned the validity of Megalneusaurus, judging the holotype specimen to be non diagnostic and possibly representing a nomen dubium or another taxon better known in other localities around the world.

Similar stomach contents have also been documented in other contemporary plesiosaurs, such as Tatenectes and Pantosaurus, proving that these were common food sources for marine reptiles of the Sundance Formation.

It is nevertheless uncertain to deduce the depth to which the animal would have descended because it is also possible that the avascular necrosis would have been caused by a few very deep dives, or by a large number of relatively shallow descents.

The vertebrae, however, do not show such damage, being, during the lifetime of plesiosaurs, probably protected by a superior blood supply, made possible by the arteries penetrating the bone through the two foramina subcentralia, large openings in their lower face.

[20] From the Yukon and Northwest Territories of Canada, where it was connected to the open ocean, this sea spanned inland southwards to New Mexico and eastward to the Dakotas.