Pantosaurus by Moondawg

Pantosaurus is an extinct genus of plesiosaur from the Late Jurassic (Oxfordian) of what is now Wyoming. It lived in what used to be the Sundance Sea.

The species Muraenosaurus reedii is in fact a junior synonym of Pantosaurus

Pantosaurus possesses between 35 and 40 cervical vertebrae, which are very similar in proportion and morphology to those of Muraenosaurus leedsii from the Oxford Clay Formation (Callovian, Middle Jurassic) of England. The forelimb of Pantosaurus however can be differentiated from that of Muraenosaurus, such as the relatively large size of the radius and the corresponding humerus-radius articulation. No Pantosaurus cranial material has yet been discovered.