The family Plesiosauridae consists of primitive long-necked plesiosauroids all belonging to the genus Plesiosaurus. It is a rather poorly understood family, mostly because Plesiosaurus is a waste-basket taxon. This means that many different specimens have all been allocated to the genus Plesiosaurus, even when they were very different and probably deserve a name all of their own. This confusion was initiated during the 1800’s when plesiosaurs were first being studied and described. Many of the species 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; they are actually both pliosaurids. 

Holotype specimen (NHMUK PV OR 22656) of Plesiosaurus dolichodeirus on display in the Natural History Museum, London. Photo by Chris Crump.

There are no more than three valid species of Plesiosaurus within the Plesiosauridae (Storrs 1997)