Plesiosaurus

*Plesiosaurus dolichodeirus*

***Plesiosaurus*** (Greek: *πλησίος*/*plesios*, near to + *σαῦρος*/*sauros*, lizard) was a large 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, Russia 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. *P. brachypterygius*, *P. guilielmiiperatoris* and *P. tournemirensis* were assigned to new genera, *Hydrorion*, *Seeleyosaurus*, and *Occitanosaurus*. It lived from 200-65 mya (Jurassic-Cretaceous).

## Discovery

The first complete skeleton of *Plesiosaurus* was discovered by early paleontologist and fossil hunter Mary Anning in Sinemurian (Early Jurassic)-age rocks of the lower Lias Group in December 1823. Additional fossils of *Plesiosaurus* were found in rocks of the Lias Group of Dorset for many years, "until the cessation of quarrying activities in the Lias Group, early in this [20th] century."

*Plesiosaurus* was one of the first of the "antediluvian reptiles" to be discovered and excited great interest in Victorian England. It was so-named ("near lizard") by William Conybeare and Henry De la Beche, 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 earlier. *Plesiosaurus* is the archetypical genus of Plesiosauria and the first to be described, hence lending its name to the order. Conybeare and De la Beche coined the name for scattered finds from the Bristol region, Dorset, and Lyme Regis in 1821. The type species of *Plesiosaurus*, *P. dolichodeirus*, was named and described by Conybeare in 1824 on the basis of Anning's original finds.

## Description

*Plesiosaurus* was a fairly typical member of its order, and measured around 3 to 5 metres (10 to 16 feet) in total length. The snout was short, but the mouth was able to open very wide, and the jaws were provided with a series of conical teeth in sockets, much like those of the living gavial. The neck was long and slender, but this 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 was straight and tapered rapidly at the end. The pectoral and pelvic girdles which supported the paired limbs are greatly expanded, the pectoral arch being similar to the corresponding bones of turtles.
The limbs were 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 *Plesiosaurus* laid eggs on land like sea turtles or gave live birth in the water like sea snakes. The young might have lived in estuaries before going in the open ocean.

## Classification

*Plesiosaurus* has historically been a wastebasket taxon. This is due in part to few anatomical or taxonomic studies of the relevant fossils. Uncritical taxonomic work resulted in hundreds of species representing most of the world and most of the Mesozoic being assigned to *Plesiosaurus.* None of the younger Jurassic or Cretaceous species belong to *Plesiosaurus*. Review of the Early Jurassic species indicates that the only English species properly assigned to *Plesiosaurus* is *P. dolichodeirus*. Several other European Early Jurassic species have been assigned to new genera. *P. brachypterygius*, *P. guilielmiiperatoris* and *P. tournemirensis*, for example, were assigned to the new genera *Hydrorion*, *Seeleyosaurus* and *Occitanosaurus*.

## Stratigraphy

Unequivocal specimens of *P. dolichodeirus* are limited to the Lyme Regis area of Dorset. It appears to be the most common species of plesiosaur in the Lias Group of England. *Plesiosaurus* is best represented from the "upper part of the Blue Lias, the 'Shales with Beef,' and the lower Black Ven Marls"; using the Lias Group ammonite fossil zones, these rocks date to the early Sinemurian stage. Some other *Plesiosaurus* fossils are from later Sinemurian rocks. The oldest specimen may be a skull thought to come from late Rhaetian or early Hettangian rocks.

## Paleoecology

*Plesiosaurus* fed mainly on clams and snails, and are thought to have eaten belemnites, fish and other prey as well. 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. *Plesiosaurus* gave live birth to live young in the water like sea snakes. The young might have lived in estuaries before moving out into the open ocean. It has been postulated that the long neck of *Plesiosaurus* would have been a hindrance when trying to speed up, any bend in the neck creating turbulences. If that is the case then *Plesiosaurus* would have had to keep its neck straight to achieve good acceleration, something that would make hunting difficult. For this reason it may be possible that these animals would actually lie in wait for prey to come close instead of trying to pursue them.

## Appearance in Jurassic Park and other media

### *Jurassic Park*

* It can be created from DNA in Jurassic Park III: Park Builder.﻿
* It can be created from DNA in Jurassic World: The Game.﻿
* The Plesiosaur Pond was an attraction originally meant to appear in *Jurassic World* but was cut from the final film. It was to be a large pool like the Jurassic World Lagoon inhabited by *Plesiosaurus* and several unidentified species of ichthyosaurs. Visitors to Jurassic World would be able to visit the attraction by boarding a submarine attached to a monorail. Despite the attraction being cut from the film, *Plesiosaurus* is one of the animals that could be viewed on the Holoscape of the Innovation Center, possibly meaning it could have very well lived in Jurassic World.

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