What is a Plesiosaur?

Mary McMahon

A plesiosaur is a prehistoric creature from the order Plesiosauria. Although these creatures are often called “dinosaurs” by lay people, they were not, in fact, dinosaurs, and the two groups of animals were very different, with markedly different habitats and lifestyles. The plesiosaur is generally believed to be extinct, although apocryphal examples of living specimens occasionally crop up in the news.

Some people look at giraffes as an example of the way plesiosaurs could have evolved.
Some people look at giraffes as an example of the way plesiosaurs could have evolved.

The term “plesiosaur” means “almost lizard” in Greek, and these marine reptiles do indeed have several traits more closely associated with land-bound reptiles. It is assumed that they reproduced through eggs, possibly laying them on or near shore, and evidence suggests that these creatures died out during the Cretaceous-Tertiary extinction event, which claimed the lives of numerous and unusual plant and animal species who could not adapt to changing conditions on Earth.

Plesiosaurs lived in the ocean, with bodies specially adapted for swimming. The examples we have found in fossil form all have broad bodies, flipper-like limbs, and short tails, and biologists have suggested that they probably moved through the water much like penguins. Both short-necked and long-necked fossilized plesiosaur specimens have been discovered, although the long-necked variety is probably more famous. Some very fine examples of plesiosaur skeletons can be seen in many natural history museums.

The fossilized remains of plesiosaurs seem to indicate that the animals probably couldn't swim very rapidly, but they could swim well. Their four limbs would have acted like paddles, making it very easy for the creatures to rapidly swivel their bodies while seeking prey. Evidence suggests that plesiosaurs probably drifted below the surface, waiting for unwary victims to pass overhead, and then snapped them up with their extremely powerful jaws.

The long-neck plesiosaur is of immense interest to some fans of prehistoric creatures. In many drawings, the animals are depicted with raised necks, suggesting that they had very muscular necks and bodies. Biologists have suggested, however, that their necks may not have been as mobile as people believe; the sheer weight and size of the neck would have made it hard to move. Others point to giraffes, a land-based creature known for having an extraordinary neck, to demonstrate the possible ways in which plesiosaurs might have moved.

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