Edgarosaurus is a genus of polycotylid plesiosaur, containing one species, E. muddi. The type specimen was found in Early Cretaceous (late Albian) rocks in the state of Montana in the USA. At the time, this location was covered by part of the Western Interior Seaway.
Druckenmiller, PS, 2002. Osteology of a new plesiosaur from the Early Cretaceous (Albian) Thermopolis Shale of Montana. Journal of Vertebrate Paleontology 22: 29-42. (Abstract)