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 to the genus Plesiosaurus under different species names, even when they were very different. By modern standards, many of these specimens deserve a generic name of their own, or may be of dubious validity entirely. The lumping of many 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 was renamed Archaeonectrus, ‘Plesiosaurus’ conybeari was renamed Attenborosaurus.
Storrs (1997) reduced 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 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.
The holotype specimen of P. dolichodeirus (Conybeare 1824) is NHMUK 22656, a complete skeleton.
Plesiosaurus dolichodeirus
Species
Plesiosaurus dolichodeirus
Author
Conybeare,1824
Classification
Plesiosaurus
Age
Early Jurassic, Sinemurian
Type location
Lyme Regis, Dorset, UK
Type specimen
NHMUK PV OR 22656
Referred material
BMNH 28332,33287 (Owen, 1865:Plate4,Figures1and 2), 36183(Owen, 1865:Plates1and2), 39490(Owen, 1865:Plate3, Figures1and 3), 39491 (Owen, 1865:Plate3, Figure 2), 41101 (Andrews, 1896:Figure1), R.255 (acompletejaw purchasedfrom theEdgertoncollectionin 1882),R.1313(collected by Mary Anning, 1829; Buckland'sGeology and Mineralogy, "The Bridgewater Treatise," 1837:Plate 16, Figure 2), R.1314 (a well-preservedright forelimb), ?R.1315,R. 1316(Owen, 1865:Plate3, Figures4 and6), R. 1316b(Owen, 1865:Plate 4, Figures 3 and 8), ?R.1330,R.1756 (a partial skeletonwith good right limbs; Lydekker, 1890:277), ?BRSMG Ce17972 (the smallest potential Plesiosaurus skeletonknown; Storrs, in press),JMM FC M 032, NMHN A.-C. 8592, NMING F:8758, OXFUM J.10304,J.13809,J.28586(typical dentariesfrom the Philpott collection,measuredby Owen, 1840a:61),J.28587,TM 13286(Winkler, 1873:Plate 7), YPM 1654 (an isolatedpair of dentarieswith well-preservedteeth), YPM-PU 3 3 5 2 ( a j u v e n i l e s p e c i m e n ) , a n d n u m e r o u si s o l a t e d , b u t f r e q u e n t l y n o n d i a g n o s t i c , bonesassignedto P. dolichodeirusarecontainedin virtually all collectionsof Lias material.
Under construction
Plesiosaurus macrocephalus
Species
'Plesiosaurus' macrocephalus
Author
Owen 1838
Classification
Age
Early Jurassic, Sinemurian
Type location
Lyme Regis, Dorset, UK
Type specimen
NHMUK PV R 1336
Referred material
N/A
Holotype specimen of ‘Plesiosaurus’ macrocephalus in the Natural History Museum, London. Photo by Tom Challands. 
Cast of the holotype of ‘Plesiosaurus’ macrocephalus in the Bath Royal Literary and Scientific Institute. Photo by Adam S. Smith.
The holotype specimen (NHMUK PV R 1336) was first and last described by Owen, 1840. ‘Plesiosaurus’ macrocephalus is probably a juvenile rhomaleosaurid, and certainly doesn’t belong to the genus Plesiosaurus. However, pending revision of the taxon and referral to a different or other existing genus, it is still named ‘Plesiosaurus‘, following Owen.