Megalneusaurus

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, without a specific assignment. The binominal name literally means "king of large swimming lizards", due to the measurement of the fossils of the holotype specimen.

Estimated to be around 7–9 metres (23–30 ft) long, Megalneusaurus is one of the largest known North American pliosaur. 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 has four paddle-like limbs, a short tail, and most likely an elongated head and short neck, suggesting that it is a thalassophonean-like pliosaurid. Some researchers consider the fossil material to be non-diagnostic, making Megalneusaurus a possible nomen dubium, although opposing views are nevertheless maintained.

The animal would have lived in the shallow waters of the Sundance Sea, an epicontinental sea covering much of North America during part of the Jurassic. Megalneusaurus 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 is not excluded that it would have attacked and fed on contemporary plesiosaurs. The referred specimen known from Alaska also indicates that it would have also occupied colder waters located further north of the continent, although the fauna is less diverse.