Genus: Seeleyosaurus WHITE, 1940
Etymology: In honor of Harry Govier Seeley (1839-1909), British paleontologist, Greek, sauros, “lizard”: Seeley’s lizard.
= Plesiopterys O'KEEFE, 2004
Etymology: Greek, Plesio, 'near', and Greek, pterys, 'wing': The latter also refers to the pterygoid bone, which is an important feature of this taxon.

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. (MARX, SJOVALL, KEAR, JARENMARK, ERIKSSON, SACHS, NILKENS, De BEECK, & LINDGREN, 2025).