Fossil range: Early Jurassic
| Scientific classification
Plesiosaurus was a large (about 3 to 5 meters long), marine sauropterygian reptile that lived during the early part of the Jurassic Period, and is known by nearly complete skeletons from the Lias of England and Germany. It was distinguished by its small head, long and slender neck, broad turtle like body, a short tail, and two pairs of large, elongated paddles. It lends its name to the order Plesiosauria, of which it is an early but fairly typical member.
Plesiosaurus was one of the first of the "antediluvian reptiles" to be discovered (by Mary Anning), and excited great interest in Victorian England. It was so-named ("near lizard") by William Conybeare, to indicate that it was more like a normal reptile than Ichthyosaurus, which had been found in the same rock strata just a few years previously.
Plesiosaurus was the first plesiosaur to be discovered and was the first to be named. The genus Plesiosaurus is the sole member of the family Plesiosauridae. 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. There are now no more than three valid species of Plesiosaurus (Storrs, 1997) although some remain e.g. 'Plesiosaurus' macrocephalus pending revision. The valid species (dolichodeirus, guilielmiiperatoris and brachypterygius) comprise the family Plesiosauridae.
The snout is short, but the mouth was able to open very wide, and the jaws are provided with a series of conical teeth in sockets, much like those of the living gavial. The neck is long and slender, but seems to have been rather stiff, because the vertebrae are nearly flat-ended, which indicates that it could not have been bent in the swan-fashion represented in many old restorations. The other vertebrae are similarly almost flat-ended and firmly united, and there is no sacrum. The ribs are single-headed, and in the middle of the trunk, between the supports of the paired limbs, they meet a dense plastron of abdominal ribs. The short tail is straight and rapidly tapering,
The pectoral and pelvic girdles which support the paired limbs are greatly expanded, the pectoral arch being similar to the corresponding bones of turtles.
The limbs are elongated paddles, with five complete digits, although each consists of a very large number of phalanges. Some traces of skin discovered suggest that it was smooth, not scaly.
Plesiosaurus fed on belemnites, fish and other prey. Its U-shaped jaw and sharp teeth would have been like a fish trap. It propelled itself by the paddles, the tail being too short to be of much use. Its neck could have been used as a rudder when navigating during a chase.
It is unknown if Plesiosaurs laid eggs on land like sea turtles or give live birth in the water like sea snakes. The young might have lived in estuaries before going in the open ocean.
At one time, Plesiosaurus was a wastebasket taxon used to describe any Mesozoic plesiosaur of generally similar appearance. More recently there has been a number of revisions in sauropterygian taxonomy, and many species previously included here have been moved to other genera and families. Only two species are unambiguiously recognised.
- Plesiosaurus dolichodeirus is the type species, known from the Lower Lias (Sinemurian) of Lyme Regis, which was about three metres long. Other plesiosaurs from the same formation measure between five to six meters in length.
- Plesiosaurus guilelmiimperatoris is known from a large almost complete skeleton from the Upper Lias (Toarcian) of Württemberg. There seems to be the impression of a rhomboidal flap of skin in a vertical plane; if so, many plesiosaurs may have been equipped in this way.
- 'P'. macrocephalus is quite distinct from other Plesiosaurus and deserves generic status of its own. It is currently under revision. An outdated description is given by Andrews (1896).
- 'P'. brachypterygius
- Richard Owen, Fossil Reptili of the Liassic Formations, pt iii. (Monogr. Palaeont. Soc., 1865)