Plesiosaurus

Genus:

Plesiosaurus

Genus author:

De la Beche and Conybeare, 1821

Classification:

Sauropterygia
Eosauropterygia
Eusauropterygia
Pistosauroidea
Pistosauria
Plesiosauria
Plesiosauroidea
Plesiosauridae

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 (De la Beche and Conybeare 1821). The first 1821 description was based on partial remains so the anatomy of Plesiosaurus – particularly the identity of the isolated bones and how they articulated together – involved some guesswork.

An attempted reconstruction of the forelimb of *Plesiosaurus* by De la Beche and Conybeare (1821). The humerus was correctly identified but with the proximal end pointing distally. It was articulated with two ilia interpreted as the radius and ulna, which in turn were articulated with a trio of bones (the actual radius and ulna, plus one extra ulna for good measure) wrongly interpreted as carpals.

The type species (P. dolichodeirus) was named a few years later (Conybeare 1824) following the discovery of a complete articulated specimen (NHMUK 22656) by Mary Anning in Lyme Regis. This skeleton provided clarity on the anatomy of Plesiosaurus, and showed that the animal had a long neck.

Holotype specimen of *Plesiosaurus dolichodeirus* in the Natural History Museum, London. Photo by Adam S. Smith.

For a long time Plesiosaurus was treated as waste-basket taxon and many different specimens were allocated to the genus Plesiosaurus under different species names, even when they were very different. By modern standards, most of these specimens deserve a generic name of their own or may be of dubious validity. 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, and ‘Plesiosaurus’ conybeari was renamed Attenborosaurus.

Reconstruction of *Plesiosaurus dolichodeirus* in lateral view. From Conybeare (1824).

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 of *Plesiosaurus dolichodeirus* in the Natural History Museum, London. Photo by Tom Challands.

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 a referred specimen of *Plesiosaurus dolichodeirus* in the British Geological Survey (BGS GSM 118412). Photo by Adam S. Smith.

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 (Owen 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. ‘Plesiosaurus’macrocephalus 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.