Genus: Seeleyosaurus WHITE, 1940
Etymology: In honor of Harry Govier Seeley (1839-1909), British paleontologist and Greek, sauros, “lizard”: Seeley’s lizard.

Species: guilelmiimperatoris (DAMES, 1895) WHITE, 1940
Etymology:
= Plesiosaurus guilelmi imperatoris DAMES, 1895
= Plesiosaurus imperatoris guilelmi MENNER, 1992
= Microcleidus guilelmi imperatoris (DAMES 1895) MENNER, 1992

Holotype: MB.R. 1992

Locality: Holzmaden, Baden-Wurttemberg State, South Germany.

Horizon:

Biostratigraphy:

Age: Upper Lias Epoch, Late Early Jurassic.

Material: Skull and skeleton.

Note: With skin impression on the tail and trailing edge of right flipper. And gut contents comprising a coarse sediment mass with gastropod shell and possible belemnite phragmocone/guard and onychite (arm hook) (MARX, SJOVALL, KEAR, JARENMARK, ERIKSSON, SACHS, NILKENS, De BEECK, & LINDGREN, 2025).

Referred material:

MENNER, 1992

Locality: Siberia, Asian Russia.

Horizon:

Biostratigraphy:

Age: Middle Jurassic.

Material:

Number: Not given:

= Seeleyosaurus holzmadensis WHITE, 1940
Etymology: In reference to Holzmaden, Baden-Wurttemberg State, South Germany, where the specimen was found.
= Plesiosaurus guilelmi imperatoris FRAAS, 1910 non DAMES, 1895
Etymology:

Holotype: SMNS 12039

Locality: Hauses near Holzmaden, Baden-Wurttemberg State, South Germany.

Horizon:

Biostratigraphy:

Age: Upper Lias Epoch, Late Early Jurassic.

Material: Skull and skeleton.