Edgarosaurus, recognized as a genus of polycotylid plesiosaur, is represented by a single species, E. muddi. Fossils of this creature were unearthed in the Early Cretaceous period, specifically from late Albian rock formations in the state of Montana, located within the United States. During this period, the area that is now Montana was submerged beneath the waters of the Western Interior Seaway. Edgarosaurus holds significance as one of the initial polycotylids known to have adapted to this seaway environment.

The holotype, which currently stands as the sole known specimen, comprises the skull, neck, sections of the spine, and the left front flipper. Estimations suggest that Edgarosaurus had an approximate total length ranging between 3.2 to 3.7 meters (equivalent to 10 to 12 feet). These estimations are derived from comparisons with the proportions of similar plesiosaurs like Dolichorhynchops and Peloneustes.

Notably, Edgarosaurus possessed a relatively short neck compared to many other plesiosaurs, consisting of a total of 26 vertebrae. The neck and skull, when combined, were estimated to constitute up to 45% of the animal's total length. Additionally, the lower arm bones of Edgarosaurus were characterized by their short and wide structure. Its forelimb featured a complex arrangement of digits, with the first consisting of seven bones, the second with eleven, the third with ten, and the fourth and fifth having nine each.