Rhomaleosaurus (meaning "strong lizard") is an extinct genus of Early Jurassic (Toarcian age, about 183 to 175.6 million years ago) rhomaleosaurid pliosauroid known from Northamptonshire and from Yorkshire of the United Kingdom.

It was one of the earliest large marine reptile predators which hunted in the seas of Mesozoic era, measuring about 7 metres (23 ft) long.

It was collected from the A. bifrons ammonite zone of the Whitby Mudstone Formation, dating to the early Toarcian age, about 183 to 180 million years ago.

After a decade, still remaining undescribed, the specimen moved in the Royal Dublin Society museum and was officially described by Alexander Carte and W. H. Bailey as a new species of Plesiosaurus.

It was collected from the A. serpentines ammonoid zone, of the Whitby Mudstone Formation, Yorkshire, England, dating to the middle Toarcian stage, about 180-177 million years ago.

Adam S. Smith (2007), in his thesis on the anatomy and classification of the family Rhomaleosauridae, suggested that R. propinquus, is a junior synonym of Rhomaleosaurus zetlandicus.

[1][2] R. zetlandicus is known from the holotype YORYM G503 (pictured), a nearly complete skull and vertebral column in association with parts of the limbs.

However, according to Smith (2007), in his thesis on the anatomy and classification of the family Rhomaleosauridae, the genus Rhomaleosaurus has only three valid species: R. cramptoni, R. thorntoni and R.

[7] The name Thaumatosaurus, which means 'wonder reptile', belonged to a genus of plesiosaur that was described by palaeontologist Christian Erich Hermann von Meyer, back in 1841.

Meyer described the species Thaumatosaurus oolithicus based on partial skull, vertebral and limb remains, that were found in the Posidonia Shale of Holzmaden, Baden-Württemberg, Germany.