The two types are related to the traditional strict division of the Plesiosauria into two suborders, the long-necked Plesiosauroidea and the short-neck Pliosauroidea.

Other naturalists during the seventeenth century added plesiosaur remains to their collections, such as John Woodward; these were only much later understood to be of a plesiosaurian nature and are today partly preserved in the Sedgwick Museum.

In 1719, William Stukeley described a partial skeleton of a plesiosaur, which had been brought to his attention by the great-grandfather of Charles Darwin, Robert Darwin of Elston.

The two finds revealed the unique and bizarre build of the animals, in 1832 by Professor Buckland likened to "a sea serpent run through a turtle".

In 1824, Conybeare also provided a specific name to Plesiosaurus: dolichodeirus, meaning "longneck".

In 1823, Thomas Clark reported an almost complete skull, probably belonging to Thalassiodracon, which is now preserved by the British Geological Survey as specimen BGS GSM 26035.

The same year, commercial fossil collector Mary Anning and her family uncovered an almost complete skeleton at Lyme Regis in Dorset, England, on what is today called the Jurassic Coast.

The term "plesiosaur" is properly used to refer to the Plesiosauria as a whole, but informally it is sometimes meant to indicate only the long-necked forms, the old Plesiosauroidea.

In 1605, Richard Verstegen of Antwerp illustrated in his A Restitution of Decayed Intelligence plesiosaur vertebrae that he referred to fishes and saw as proof that Great Britain was once connected to the European continent.

It is the earliest discovered more or less complete fossil reptile skeleton in a museum collection.

It can perhaps be referred to Plesiosaurus dolichodeirus.

Therefore, the purely descriptive terms "plesiosauromorph" and "pliosauromorph" have been introduced, which do not imply a direct relationship.