Species: serpentinus (COPE, 1871) emend OTERO, 2016
Etymology: Latin, serpentis, "snake." In reference to its long, snake like neck.
= Elasmosaurus serpentinus COPE, 1871
= Hydralmosaurus serpentinus (COPE, 1871) WELLES, 1943

Holotype: AMNH 1495

Locality: A bluff of blue shale in Cedar, Dixon or Dakota County, Nebraska on the southwest side of the Missouri River between Yankton, South Dakota and Sioux City, Iowa.

Horizon: Smoky Hill Member, Niobrara Formation.

Biostratigraphy:

Age: Santonian or Campanian Stage, Senonian Subepoch, Gulf Epoch, Late Cretaceous.

Material: 58 cervicals, 3 pectorals, 19 dorsals, 4 sacral, 20 caudal vertebrae, coracoid, humerus, femur, and fragmentary pectoral girdle.

Referred material:

= Styxosaurus browni WELLES, 1952
Etymology:

Holotype: AMNH 5835

Locality: Red Bird, Niobrara County, Wyoming.
Note: Note Mule Creek, 15 miles west of Edgemont, South Dakota as reported by WELLES, 1952, as per CARPENTER, 1999.

Horizon: Sharon Springs Member, Pierre Shale.

Biostratigraphy:

Age: Upper Lower to Lower Middle Campanian Stage, Senonian Subepoch, Upper Gulf Epoch, Late Cretaceous.

Material: Skull, 75 vertebrae, pectrum and a pectoral limb.

Note: Not type of Styxosaurus WELLES 1952

Referred material:

BROWN, 1904

Locality: Head of Hat Creek Basin, 18 miles, southwest of Edgemont, Fall River County, South Dakota.

Horizon: Sharon Springs Member, Pierre Shale.

Biostratigraphy:

Age: Upper Lower to Lower Middle Campanian Stage, Senonian Subepoch, Upper Gulf Epoch, Late Cretaceous
Note: Now believed to be from the Younger Pierre Shale (MARTIN, SCHUMACHER PARRIS & GRANDSTAFF, 1998)

Material:

AMNH 5803: Fragmentary skeleton with gastroliths.
Note: Pteranodon sp found in the stomach region along with a teleost fish, Scaphites and gastroliths (CICIMURRI & EVERHART, 2001).