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Plesiosaurs had a broad body, short tail, and four limb flippers. The term "Plesiosaur" includes the so-called "true Plesiosaurs", which had a long necks, and the Pliosaurs which had shorter necks and a larger head. Plesiosaurs varied greatly in size, the adults varying between 10 feet (3 meters) and 66 feet (20 meters) in length, depending on the particular species. Perhaps the most remarkable are the family of Plesiosaurs known as "Elasmosauridae", which includes species such as Elasmosaurus and Thalassomedon, who had necks that made up half or more of the length of the creature.
Plesiosaurs are often drawn with their necks positioned upwards in a swan-like posture. However, this is now thought to be unlikely to be an accurate depiction - it is thought they necks were probably not flexible enough to do this, and, in any case, such a posture would probably have tipped them over.
In life, Plesiosaurs are believed to have been the top ("apex") predators of the oceans. It is thought perhaps they swam slowly below the surface of the water, and then used their long necks to position to snap up fish or cephalopods. Stones have been found in the stomach area of some Plesiosaurs (including Thalassomedon), and it is thought perhaps that these were used as ballast and/or as an aid to digestion (similar to gizzard stones in modern birds being used to grind up food).
The first recorded Plesiosaur fossils were found in England - these include a partial skeleton found by Stukely in Nottinghamshire in 1719, and others found by Mary Anning. As a result of this, an impressive collection of Plesiosaur fossils are now found in the Natural History Museum and London.
A number of alleged lake monsters (animals supposedly living in freshwater lakes) are sometimes claimed to be living Plesiosaurs. These include Champ which supposedly lives in Lake Champlain in the United States, Ogopogo which supposedly lives in Lake Okanagan in Canada, and most famously of all, Nessie which supposedly lives in Loch Ness in Scotland. Mainstream scientists are generally skeptical of these claims for a variety of reasons including lack of physical evidence, the lakes are generally too small to support a viable breeding population, the lakes are often too cold, and bodies of dead animals have not been found. Cryptozoologists (people who search for rumored or mythical creatures whose existence is uncertain), and of course tourists to these areas, however continue to look for these creatures.
Plesiosaurs were marine reptiles that lived between 228 and 65 million years ago
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