Western Interior Seaway
Edgarosaurus is a genus of polycotylid plesiosaur from the Thermopolis Shale, containing one species, "E. muddi". The type specimen was found in Early Cretaceous (late Albian) rocks in the state of Montana in the United States. At the time, this location was covered by part of the Western Interior Seaway. "Edgarosaurus" was one of the first polycotylids that evolved to become native to the Western Interior Seaway.