Plesiosaur means 'near lizard'.
It was once thought that plesiosaurs were the descendents of the more primitive nothosaurs. However, some features of plesiosaurs are less derived than those of nothosaurs. It may be that plesiosaurs evolved from a more primitive diapsid, and not nothosaurs.
Plesiosaurs were larger than nothosaurs, and were more specialised for marine life. They appeared at the Triassic/Jurassic boundary (200 million years ago), and persisted until the end of the Cretaceous Period (65 million years ago).
Plesiosaurs are divided into 2 groups:
1. More primitive Plesiosauridae (short neck, large head).
2. More specialised elasmosaurs (long neck, small head).
Both groups swam with limbs that had evolved into paddles. Hyperphalangy (addition of joints to toes) increased the size of the paddles, and made them more efficient in swimming. Plesiosaurs used these paddles for propulsion instead of the tail.
In both groups, the nostrils were high on the head and just in front of the eyes.
The genus Elasmosaurus is found in the upper Cretaceous, and its' lineage can be traced back to Plesiosaurus in the lower Jurassic.
The Elasmosaurus line is characterised by a progressive increase in the length of the neck, and a reduction in the size of the head. The size of the paddles relative to body size decreases from the Jurassic to the Cretaceous.
Elasmosaurs were not very streamlined, and this streamlining became poorer as the neck became longer. Elasmosaurs were consequently not fast swimmers. They were also not able to make swift movements of the head to capture prey, as the neck vertebrae were very stiff and inflexible. They may instead have pivoted the entire body, swinging the head rapidly through a large arc and enabling them to catch prey efficiently.
Pliosaurs and early elasmosaurs both started with a body form that was very similar, but they followed different evolutionary pathways. The pliosaur pathway led to increasingly streamlined forms - the neck became shorter, and the paddles larger. Some pliosaurs reached very large sizes (e.g. Liopleurodon was 12-15m long), and ate fish, as well as smaller ichthyosaurs and plesiosaurs.
Like nothosaurs, plesiosaurs probably had to return to land to lay eggs, despite their increased adaptation to aquatic life.
Author: Rachel Jennings
Last updated: 22/11/2005
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