Plesiosaurus De la Beche and Conybeare, 1821
Plesiosaurus was the first plesiosaur discovered and named. For a long time Plesiosaurus was treated as waste-basket taxon. This means that many different specimens were allocated a new species name under the genus Plesiosaurus, even when they were very different. By modern standards, these specimens would often deserve a generic name of their own, or on the other hand may be of dubious validity entirely. The lumping of species into the genus Plesiosaurus was initiated during the 1800s when plesiosaurs were first being studied and described. Many species formerly included in Plesiosaurus are now renamed and most of them do not even belong in the family Plesiosauridae. For example, 'Plesiosaurus' rostratus and 'Plesiosaurus' conybeari have been renamed Archaeonectrus and Attenborosaurus respectively, and they are both pliosauroids.
Storrs (1997) whittled down the number of valid species of Plesiosaurus to three. However, two of those have unique features that warrant generic separation: 'Plesiosaurus' guilielmiiperatoris is today regarded as Seeleyosaurus, a name proposed years ago and reinstated by Grossman (2007), and 'Plesiosaurus' brachypterygius is now known by the name of Hydrorion (Grossman, 2007). All this means that at present, Plesiosaurus contains only the single valid species, P. dolichodeirus. Some species arbitrarily referred to Plesiosaurus still remain today, pending revision. 'Plesiosaurus' macrocephalus, for example, is possibly a juvenile rhomaleosaurid.
Plesiosaurus belongs to the family Plesiosauridae and was regarded as the sole member of the group for several years. However, recent research into early Jurassic plesiosaurs has revealed a greater generic diversity of plesiosaurids.
Classification
Sauropterygia --> Eosauropterygia --> Eusauropterygia --> Pistosauroidea --> Plesiosauria --> Plesiosauroidea --> Plesiosauridae
P. dolichodeirus Conybeare, 1824
Material
Type specimen: BMNH 22656, complete skeleton.
Referred material: lots
Age and Location
Uppermost Sinemurian, Lower Jurassic. Lyme Regis, Dorset, England.
Plesiosaurus images
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