Genus: Megalneusaurus KNIGHT, 1898
Etymology: Greek, megal- (megas), “great”, Greek, neus- (neo), “swim”, Greek, sauros, “lizard”: Great swimming reptile.

Species: rex KNIGHT, 1898
Etymology:

Holotype: UW 4602

Locality: Wind River Basin, Natrona County, Wyoming.

Horizon: Upper Redwater Shale Member, Sundance Formation.

Biostratigraphy:

Age: Late Oxfordian Stage, Lower Malm Epoch, Late Jurassic.

Material: Cervical, dorsal and caudal vertebrae, humerus, ulna, radius, phalanges, ribs and greater portion of the pectoral girdle.

New material from type locality, WHAL, ROSS & MASSARE, 2007: Vertebral centrum, matrix filled neural arch, possible limb girdle element, adn epipodial element.
Note: Paleopathology, 2 humeri shows avascular necrosis. (ROTHSCHILDS & STORRS, 2003)

Note: WHAL, ROSS & MASSARE, 2007 have found possible gut contents consisting of coleoid (belemnite) tentacle hooklests and jaw elements.