Megalneusaurus

Megalneusaurus is a genus of large pliosaurid plesiosaur from the Late Jurassic of 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 known species is M. rex, known from several specimens mainly found 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. The loss of most fossils has led some paleontologists to consider the genus as dubious, although its validity is maintained by many authors. The binominal name literally means "king of large swimming lizards", due to the size of the first specimen.

Estimated to be around 7–9 meters (23–30 ft) long, Megalneusaurus is one of the largest known North American pliosaurs. As its name suggests, the genus was considered the largest sauropterygian identified before the discovery of some Kronosaurus fossils in 1930. Like some other plesiosaurs, Megalneusaurus have four flippers, a short tail, and most likely an elongated head and short neck, suggesting that it is a thalassophonean-like pliosaurid. The rear flippers were larger than those at the front.

The animal lived in the shallow waters of the Sundance Sea, an epicontinental sea covering much of North America during part of the Jurassic. Like other plesiosaurs, Megalneusaurus was well-adapted to aquatic life, using its flippers for a method of swimming known as subaqueous flight. It shared its habitat with invertebrates, fish, ichthyosaurs, and other plesiosaurs, including the cryptoclidids Pantosaurus and Tatenectes. Based on stomach contents, the animal fed on cephalopods and fish, although it could also have fed on contemporary plesiosaurs. The Alaskan specimen also indicates that it would have occupied colder waters, where the fauna was less diverse.