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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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