Genus: Rhomaleosaurus SEELEY, 1874
Etymology: Greek, rhomaleos, “strong, robust” and Greek, sauros, “lizard”: Strong lizard.
= Thaumatosaurus FRAAS, 1910
Etymology: Greek, thaumato- (thauma), “wonder, monster” and Greek, sauros, “lizard”: Wonder lizard.

Species: zetlandicus (PHILLIPS, in ANON, 1854) TAYLOR, 1992
Etymology:
= Plesiosaurus zetlandicus PHILLIPS, 1854
= Thaumatosaurus zetlandicus (PHILLIPS, in ANON, 1854) LYDEKKER, 1889

Holotype: YORYM G503

Locality: Alum Mine, a large coastal quarry about 2 km northeast of Loftus, and about 18 km northwest of Whitby, Yorkshire County, England, Southern United Kingdom.

Horizon: Alum Shale Member (formerly termed the 'Cement Shales' and 'Main Alum Shales'), Whitby Mudstone Formation, Lias Group.

Biostratigraphy: Hildoceras bifrons zone.

Age: Lower Lias Epoch, Early Jurassic.

Material: Fragmentary skull and fragmentary skeleton.
Note: A single gastrolith.

Referred material:


= Plesiosaurus propinquus BLAKE, 1876 (nomen dubium)
Etymology:
= Rhomaleosaurus propinquus (BLAKE, 1876) WATSON, 1910 (nomen dubium)

Holotype: WM 851S

Locality: Kettleness, Whitby, Yorkshire County, England, Southern United Kingdom.

Horizon:

Biostratigraphy: Harpoceras falcifer zone.

Age: Upper Lias Epoch, Upper Early Jurassic.

Material: Skull and skeleton.

Referred material:

BENTON & TAYLOR, 1983

Locality: Saltwick, Whitby, Yorkshire County, England, Southern United Kingdom.

Horizon:

Biostratigraphy:

Age: Lias Epoch, Early Jurassic.

Material:

BMNH 33098-9, 33115:

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Species: cramptoni (CARTE & BAILEY, 1863) SEELEY, 1874
Etymology:
= Plesiosaurus cromptoni CARTE & BAILEY, 1863

Holotype: NMING F8785

Locality: Kettleness, near Whitby, Yorkshire County, England, Southern United Kingdom.

Horizon:

Biostratigraphy: Upper part of the Hildoceras bifrons zone.

Age: Upper Lias Epoch, Late Early Jurassic.

Material: Skull and skeleton.

Referred material:

BENTON & TAYLOR, 1983

Locality: Saltwick, Whitby, Yorkshire County, England, Southern United Kingdom.

Horizon:

Biostratigraphy:

Age: Lias Epoch, Early Jurassic.

Material:

BMNH 40123, R1008, R1365:

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Species: thorntoni ANDREWS, 1922
Etymology:

Holotype: BMNH R4853

Locality: Kingsthorpe, Northamptonshire County, England, Southern United Kingdom.

Horizon: Upper Liassic.

Biostratigraphy:

Age: Upper Lias Epoch, Late Early Jurassic.

Material: Anterior portion of the skull, and fragmentary skeleton.