Named By: William Conybeare & Henry De la Beche - 1821.
Classification: Chordata, Reptilia, Sauropterygia, Plesiosauria, Plesiosauroidea, Plesiosauridae.
Species: P. dolichodeirus (type).
Size: Approximately 3.5 meters long.
Known locations: England - Lias Group.
Time period: Possibly late Rhaetian of the Triassic through to Sinemurian of the Jurassic.
Fossil representation: Many specimens.
and most famous member of the plesiosaur group, Plesiosaurus
a stir upon its discovery as nothing like it was previously known.
Unfortunately Plesiosaurus suffered from the
wastebasket taxon effect
as any set of remains remotely similar to it ended up being
assigned to the genus without much further thought (a method of
classification that also affected many other prehistoric animals such
as the dinosaur Megalosaurus
and the pterosaur Pterodactylus).
Later study of Plesiosaurus fossils would reveal
that many of these
remains actually represented completely different plesiosaurs. New
plesiosaur genera created from the re-classification of plesiosaurus
species include Hydrorion, Occitanosaurus
How long Plesiosaurus spent in the water has long been a matter of debate. Classical art and reconstructions from the early years of marine reptile palaeontology depicted it as being just as capable of walking about on land as it could swim in the ocean. Also the long neck was almost always depicted as shooting out from the water and arcing around in strong curves, but today both of these depictions are thought to be highly unlikely.
The limbs of Plesiosaurus which were once legs in its ancestors have evolved into flippers which are actually quite stiff. This makes them better for paddling through the water, but cumbersome on land, and certainly not likely to be capable of lifting Plesiosaurus’s body off the ground. At best Plesiosaurus would be capable of pulling its body with the front flippers while pushing with the rear. This may make it capable of leaving the water but not for any great distance inland. Possibly more likely is Plesiosaurus pushing itself through the shallows where the water was not deep enough to float its entire body but still capable of supporting some of the weight and the bulk so that the flippers did not have to ‘lift’ as much.
The construction and makeup of the neck itself is actually taken as the strongest evidence for an entirely if not almost entirely aquatic lifestyle. Long presumed by many people to be capable of bending in strong curves, reconstruction of the vertebrae has revealed that the neck was surprisingly inflexible with only gentle arcs along the entire length of the neck being possible. This means that the neck was most stable when projecting horizontally level forwards. This also means that Plesiosaurus probably could not carry its head and neck high off the ground should it ever leave the water, and if it ever did the head and neck may have had to be rested on the ground to support the weight and bulk which the neck muscles were incapable of doing without the buoyant support of surrounding water.
It’s most likely that the neck was long in order to afford Plesiosaurus additional reach to strike out at prey. Given the necks inherent weakness and inflexibility the older theories depicting Plesiosaurus shooting its head and neck into the air and arcing down onto a shoal of fish are no longer considered accurate. Instead Plesiosaurus may have approached prey from the side or even below, hiding its large body in the murk of the lower depths so that fish did not realise the danger. The latter theory is often proposed for the elasmosaurid group of plesiosaurs that had the proportionately longest necks of all the plesiosaurs.
If Plesiosaurus was capable and if so how long it spent on land has also been part of the argument of whether it laid eggs or gave birth to live young. Plesiosaurus may have struggled its way up a beach like turtles do today, possibly even using a high tide to carry it as far up the shore as it was able to reach and then laying eggs in the sand just beyond the tidal reach. Such behaviour would have been risky as Plesiosaurus would likely need the tides to carry it back out to sea, and may have been vulnerable to terrestrial predators in the process.
While the above is a viable theory, it needs to be remembered that the precedent for live birth exists in other marine reptiles, and may go back to the plesiosaur ancestors the nothosaurs, as indicated by potential fossil material of the small nothosaur Lariosaurus. If live birth is the case for Plesiosaurus, then it’s a reasonable proposition that it may never have ventured onto land and spent its entire life in the water.