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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 – 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.

Research history
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. The discovery consists of a specimen, cataloged as UW 4602, which preserves ribs, vertebrae, two more or less partial flippers and part of the shoulder girdle, from a specimen that would have reached adulthood. The same year, Knight formalized his discovery via an announcement that was published by Science Magazine, where he briefly described certain fossils. As the fossils were not exhumed from their geological matrix at the time of publication of his announcement, Knight was uncertain about the generic positioning of the specimen. He therefore erected a new species of Cimoliosaurus, C. rex, to which he provisionally classified it pending a more in-depth description. It was three years later, in 1898, that Knight described the specimen in more detail and erected the distinct genus Megalneusaurus to include it, the species thus becoming M. rex. The generic name is formed from the words in Ancient Greek μέγας (mégas, "great") and νηκτός (nêktós, "swimmer") prefixed onto σαῦρος (saûros, "lizard"). The specific name rex means "king" in Latin. Although no description of the meaning of this etymology was given in his 1898 description, Knight would likely have named the taxon thus due to the rather considerable measurements of this specimen for the time, which he judges according to his words to be "the largest known animal of the order Sauropterygia".

Due to the discovery of other large pliosaurs found elsewhere in the world, the attention of Megalneusaurus by paleontologists has generally been "forgotten", being only very briefly mentioned in 20th and early 21st century scientific literature. In 1995, the Tate Geological Museum of Casper planned to create an exhibit showcasing marine reptiles discovered in the Sundance Formation. During the collections, researchers came across a cast of the original forelimb originally discovered by Knight, the actual fossil being preserved at the Geological Museum of the University of Wyoming. 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. However, Knight having written a considerable number of letters for many paleontologists of his time, these are then used by researchers as clues to make the reunion. 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. This precise part of the formation dates from the Oxfordian stage of the Late Jurassic. After the rediscovery of this place, new excavations were launched in the field and additional fossils were found. About 20 m 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. In 2008, it was a fully articulated and almost complete forelimb coming directly from the holotype specimen which was exhumed, and it is described in detail in 2010. Fossils coming possibly from two other specimens have also been reported. The first is an isolated neural arch, cataloged as UW 24238, while the second is a propodial (upper limb bone), cataloged as WDC SS019. Other fossil finds referred to Megalneusaurus were made in this area over an additional period from 2009 until 2011, but these have not yet been officially described.

Megalneusaurus was then historically only known from Wyoming, but another specimen referred to the genus was discovered in southern Alaska. It was in June 1922 that W. R. Smith of the United States Geological Survey received two bone fragments from a certain Jack Mason. 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. It was only in 1994 that Robert E. Weems and Robert B. Blodgett described this fossil in detail to which they referred it to the genus on the basis of comparisons made with the holotype specimen. However, as their description and comparison is only based on an isolated humerus, they refer it under the name Megalneusaurus sp., the authors being uncertain as to whether the specimen would belong to another species or to the type species.

Size
Megalneusaurus is the largest Jurassic pliosaurid identified in North America. Most studies and descriptions qualify Megalneusaurus as the largest pliosaur to have been identified in the North American continent of any period, but some related genera such as Megacephalosaurus, dating from the Upper Cretaceous, are of comparable size. The taxon was even considered to be the largest known pliosaur in the world until some fossils of the Australian pliosaurid Kronosaurus were described in 1930. Several estimates of the size of Megalneusaurus have been given throughout research. In 2006, Wahl gave the animal a length of approximately 13 m, before the same author and his colleagues reduced its size to just 10 m the following year. In his thesis published in 2009, Australian paleontologist Colin McHenry estimates the size of the animal at between 10 and 12 m long based on measurements of the femurs given by Knight in 1898. However, most recent estimates reduce the size of Megalneusaurus to between 7.6 and 9.2 m long. The referred specimen from Alaska is smaller in measurement than the holotype specimen, although no estimate of its size has been given.

Morphology
The majority of the anatomy of Megalneusaurus is only known from the holotype specimen and concerns only the postcranial parts of the animal, no cranial materials being known. In addition, additional parts including cervical, dorsal and caudal vertebrae, a large part of the shoulder girdle and ribs, have also been lost, the only remaining descriptions of them being made by Knight in 1898.

Classification
When Knight made the discovery of the first known fossils of Megalneusaurus official in 1895, they were first classified in the genus Cimoliosaurus, a plesiosauroid currently classified in the family Elasmosauridae. However, this classification was deliberately provisional, because the fossils were at that time unprepared to allow the specimen to have taxonomic clarification. It was in 1898 that Knight discovered that it was in fact a pliosaur, not hesitating to compare it to the pliosaurids Pliosaurus and Peloneustes. However, Knight does not give any more precise classification of this genus. Studies published from the second half of the 20th century agree that Megalneusaurus would most likely be a pliosaurid. In 2009, McHenry questioned the validity of Megalneusaurus, judging the holotype specimen to be non diagnostic and possibly representing a taxon better known in other localities around the world. However, the name Megalneusaurus is still preserved in studies published subsequently, without mentioning McHenry's opinion.

Feeding
The stomach contents of the holotype specimen of M. rex contain numerous hooklets of coleoid cephalopods as well as a few rare fragments of fish bones. 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. Being a large pliosaurid, Megalneusaurus would likely have fed on other contemporary plesiosaurs, particularly juveniles or subadults. A juvenile plesiosaur fossil identified in this formation also shows bite marks on the bones of one of its flippers, although their origins have not been formally determined.

Paleopathology
Both humeri of the M. rex holotype specimen are affected by avascular necrosis. A large number of other more or less related plesiosaur fossils also show this disease in both the humerus and the femur. This is the result of ascent too quickly after a deep diving. 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.

Wyoming
Megalneusaurus comes from the Oxfordian-aged (Upper Jurassic) rocks of the Redwater Shale Member of the Sundance Formation. This member is about 30 - 60 m thick. While mainly composed of grayish green shale, it also has layers of yellow limestone and sandstone, the former layers containing plentiful fossils of marine life. The Sundance Formation represents a shallow epicontinental sea known as the Sundance Sea. 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. When Megalneusaurus was alive, most of the Sundance Sea was less than 40 m deep. Based on δ18O isotope ratios in belemnite fossils, the temperature in the Sundance Sea would have been 13 - 17 C below and 16 - 20 C above the thermocline.

The paleobiota of the Sundance Formation includes foraminiferans and algae, in addition to a variety of animals. Many invertebrates are known from the Sundance Formation, represented by crinoids, echinoids, serpulid worms, ostracods, malacostracans, and mollusks. The mollusks include cephalopods such as ammonites and belemnites, bivalves such as oysters and scallops, and gastropods. Fish from the formation are represented by hybodont and neoselachian chondrichthyans as well as teleosts (including Pholidophorus). Marine reptiles are uncommon, but are represented by four species. Of all the plesiosaurs, Megalneusaurus is the only pliosaurid identified in the Sundance Formation. The other plesiosaurs known from this formation are the cryptoclidids Tatenectes and Pantosaurus. Besides plesiosaurs, marine reptiles are also represented by the ichthyosaur Ophthalmosaurus (or, possibly, Baptanodon) natans, the most abundant marine reptile of the Sundance Formation.