Genus: Styxosaurus WELLES, 1943
Etymology: Styx, mythological river in Hades, and Greek, sauros, “lizard”; Styx lizard.
= Alzadasaurus WELLES 1943 (partim)
Etymology: In reference to the town of Alzada, Montana, near where the specimen was found, Greek, sauros, “lizard”: Alzada lizard.
= Hydralmosaurus WELLES, 1943
Etymology: Greek, hydralme, “salt water” and Greek, sauros, “lizard”: Salt water lizard.
= Thalassonomosaurus WELLES, 1943
Etymology: Greek, thalassa, “sea”, Greek, nomo, “dwell” and Greek, sauros, “lizard”: Sea-living lizard.

Species: snowii (WILLISTON, 1890) WELLES, 1943
Etymology:
= Cimoliasaurus snowii WILLISTON, 1890
= Elasmosaurus snowii (WILLISTON, 1890) WILLISTON, 1906

Holotype: KUVP 1301

Locality: Hell Creek, Logan County (was Plum County), Kansas.

Horizon: Niobrara Formation, or Upper Smoky Hill Member.

Biostratigraphy: Spinaptychus sternbergi or Hesperornis zone.

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

Material: Skull and 28 cervical vertebrae.

WELLES, 1943, Holotype: DMNH 1588; A) right lateral and B) left lateral views.

Modifed from WELLES, 1943, Holotype: DMNH 1588; A) right lateral mouth closed and B) right lateral with mouth open.

Referred material:

CICIMURII & EVERHART, 2001

Locality: Logan County, Kansas.

Horizon: Sharon Springs Member, Pierre Shale.

Biostratigraphy:

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

Material:

NJSM 15435: Nearly complete skeleton lacking skull.
Note: Gastroliths (95) and fragments of Enchodus.

WILLISTON & MOODIE, 1917

= Ogmodirus ischiadicus referred material
WILLISTON 1906
= Styxosaurus snowii WELLES 1943 referred material.

Locality: Plum Creek, Logan County, Western Kansas.

Horizon: Niobrara Formation or Upper Smoky Hill Member.

Biostratigraphy: Spinaptychus sternbergi or Hesperornis zone.

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

Material:

YPM 1644 (YPM 636): Scapulae, coracoids, pubes and vertebrae.

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Species: marshii (WILLISTON, 1906) emend OTERO, 2016
= Elasmosaurus marshii WILLISTON, 1906 (nomen dubium)
Etymology: In honor of Othneil Charles Marsh.
= Thalassonomosaurus marshii (WILLISTON, 1906) WELLES, 1943 (nomen dubium)

Note: Otero (2016) placed Thalassonomosaurus marshi in Styxosaurus sp. However, because there is an assigned species name to the specimen, the name should be Styxosaurus marshii not Styxosaurus sp. Holotype: YPM 1645

Locality: Logan County, Kansas.

Horizon: Niobrara Formation or Upper Smoky Hill Member.

Biostratigraphy: Spinaptychus sternbergi or Hesperornis zone.

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

Material: 14 vertebral centra (cervical, pectoral and dorsal, listed as 32 or 33, but the difference now cannot be accounted for [STORRS, 1999]), numerous spine, centrum and rib fragments, left scapula, and a nearly complete left fore limb, numerous tiny girdle fragments.

Referred material:

YPM 2062: Scapulae.

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Species: pembertoni (WELLES & BUMP, 1949) emend OTERO, 2016
= Alzadasaurus pembertoni WELLES & BUMP, 1949
Etymology:

Note: Otero (2016) placed Alzadasaurus pembertoni in Styxosaurus sp. However, because there is an assigned species name to the specimen, the name should be Styxosaurus pembertoni not Styxosaurus sp.

Holotype: Museum of Geology, South Dakota School of Mines and Technology, No. 451

Locality: 25 miles south of Chamberlain and 11 miles northeast of Iona, Missouri River, Lyman 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

Material: Skull and nearly complete skeleton.
Note: Found with 253 quartzite gastroliths.
Note: Paleopathology, humerus shows avascular necrosis (ROTHSCHILDS & STORRS, 2003)

Referred material:

= Alzadasaurus cf. pembertoni NICHOLLS, 1988

Locality: Manitoba Province, Canada.

Horizon: Pembina Member, Pierre Shale.

Biostratigraphy:

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

Material:

P83.02.18: Disarticulated specimen.

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Species: kansasensis (WELLES, 1952) emend OTERO, 2016
= Alzadasaurus kansasensis WELLES, 1952
Etymology: In reference to the state of Kansas, where the specimen was found.

Note: Otero (2016) placed Alzadasaurus kansensis in Styxosaurus sp. However, because there is an assigned species name to the specimen, the name should be Styxosaurus kansasensis not Styxosaurus sp.

Holotype: YPM 1130

Locality: Probably along the Smoky Hill River, Logan County, Kansas.
Note: Was thought to be Wallace County, Kansas.

Horizon: Smoky Hill Member, Niobrara Formation.

Biostratigraphy: Hesperornis zone.

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

Material: 28 cervicals, 5 pectorals, 3 dorsals, 5 sacrals, 22 caudal vertebrae, associated spine and other vertebral fragments, rib fragments, the nearly complete right rear limb, 6 isolated phalanges, and most of the pelvis.

Referred material:

= Elasmosaurus ischiadicus WILLISTON, 1906 (Referred specimen.)
= Ogmodirus ischiadicus WILLISTON & MOODIE, 1917 (Referred specimen.)

Pelvis.

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Species: serpentinus (COPE, 1871) emend OTERO, 2016
Etymology:
= Elasmosaurus serpentinus COPE, 1871
= Hydralmosaurus serpentinus (COPE, 1871) WELLES, 1943

Note: Otero (2016) placed Hydralmosaurus serpentinus in Styxosaurus sp. However, because there is an assigned species name to the specimen, the name should be Styxosaurus serpentinus not Styxosaurus sp.

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.

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Species: 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.

Skull modified from WELLES, 1943: A) jaws closed and B) jaws opened.

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).

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Species: rezaci SMITH & O’KEEFE, 2024
Etymology: In honor of the Rezac family of Valparaiso, Nebraska, USA, who graciously donated the specimen to the Univeristy of Nebraska.
= Thalassomedon haningtoni CARPENTER, 1999

Holotype: USNM 50132

Locality: From the property of the Adolph Rezac Family farm, Valparaiso, Saunders County, Nebraska.

Horizon: Graneros shale.

Biostratigraphy:

Age: Lower Cenomanian Stage, Upper Middle Gallic Subepoch, Lowermost Gulf Epoch, Middle Cretaceous.

Material: Complete skull with brittle deformaiton, a complete series of 63 cervical vertebfae, 3 dorsal vertebrae, and a forelimb.
Note: Cephalopods and gastroliths found in the stomach region (CICIMURRI & EVERHART, 2001).

Styxosaurus rezaci after SMITH & O’KEEFE, 2024

Styxosaurus rezaci modified from SMITH & O’KEEFE, 2024: A) skull with mouth closed and B) skull with mouth open.

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