Plesiosaurus

Genus:
Plesiosaurus
Genus author:
De la Beche and Conybeare, 1821
Classification:
Age:
Late Sinemurian, Early Jurassic
Location:
Lyme Regis, Dorset, England
Referred material (sp.):
To compile
Type species:

P. dolichodeirus

Species:
Plesiosaurus dolichodeirus
Species author:
Conybeare,1824
Type specimen:
NHMUK PV OR 22656
Age:
Early Jurassic, Sinemurian
Geological formation:
Charmouth Mudstone Formation
Type location:
Lyme Regis, Dorset, UK
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 (a complete jaw purchased from the Edgerton collection in 1882),R.1313(collected by Mary Anning, 1829; Buckland's Geology and Mineralogy, "The Bridgewater Treatise," 1837:Plate 16, Figure 2), R.1314 (a well-preservedright forelimb), ?R.1315,R. 1316(Owen, 1865:Plate3, Figures 4 and 6), R. 1316b(Owen, 1865:Plate 4, Figures 3 and 8), ?R.1330,R.1756 (a partial skeleton with good right limbs; Lydekker, 1890:277), ?BRSMG Ce17972, JMM FC M 032, NMHN A.-C. 8592, NMING F:8758, OXFUM J.10304, J.13809, J.28586 (typical dentaries from the Philpott collection, measured by Owen, 1840a:61), J.28587,TM 13286 (Winkler, 1873:Plate 7), YPM 1654 (an isolated pair of dentaries with well-preserved teeth), YPM-PU 3352 (a juvenile specimen), and number of isolated, but frequently non diagnostic, bones assigned to P. dolichodeirus are contained in virtually all collections of Lias material.

Species:
'Plesiosaurus' macrocephalus
Species author:
Owen 1838
Type specimen:
NHMUK PV R 1336
Age:
Early Jurassic, Sinemurian
Geological formation:
Charmouth Mudstone Formation
Type location:
Lyme Regis, Dorset, UK
Referred material:
N/A

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.

Plesiosaurus dolichodeirus

The holotype specimen of P. dolichodeirus (Conybeare 1824) is NHMUK 22656, a complete skeleton, discovered and collected by Mary Anning.

Holotype specimen (NHMUK PV OR 22656) of Plesiosaurus dolichodeirus on display in the Natural History Museum, London. Photo by Chris Crump.
Holotype specimen of Plesiosaurus dolichodeirus in the Natural History Museum, London. Photo by Tom Challands.
Holotype specimen of Plesiosaurus dolichodeirus in the Natural History Museum, London. Photo by Adam S. Smith.
Skull referred to Plesiosaurus dolichodeirus in the Natural History Museum, London.
Referred specimen of Plesiosaurus dolichodeirus in the James Mitchell Geology Museum, Galway, Ireland. Photo by Adam S. Smith.
Cast of referred specimen of Plesiosaurus dolichodeirus (BGS GSM 118412) in the British Geological Survey.
Skull of Plesiosaurus dolichodeirus in lateral view. From Storrs (1996).
Skull of Plesiosaurus dolichodeirus in dorsal and ventral views. From Storrs (1996).

‘Plesiosaurus’ macrocephalus

This iconic plesiosaur skeleton (NHMUK PV R 1336) was discovered by Mary Anning in Lyme Regis in December 1830. Regarded as her last major discovery, it was first figured under the name “Plesiosaurus macrocephalus” by William Buckland in 1836, and later described by Richard Owen in 1840. The fossil was originally purchased by Lord Cole in 1831. It was subsequently acquired from him by the Natural History Museum, London, where it has remained on display in the museum’s ‘Fossil Marine Reptiles’ hall.

The skeleton has a proportionally large head and small flippers, poorly ossified limb bones and vertebrae, and a small overall body size. These characters suggest the individual was immature. ‘Plesiosaurusmacrocephalus certainly does not belong to the long-necked, small-skulled genus Plesiosaurus. It is probably a juvenile rhomaleosaurid but has not yet been assigned to a new or existing genus. So, pending revision of the taxon and referral to a different or other existing genus, it is still named ‘Plesiosaurus‘, following Owen.

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.