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an order of fossil marine reptiles of the subclass Synaptosauria. Placodonts lived during the Triassic. They resembled turtles owing to their armored back and abdomen. Their elongate body, which reached 2.5 m long, resembled that of a dugong. The skull was massive and enlarged posteriorly. The teeth were powerful and adapted for grinding the shells of mol-lusks—the principal food of placodonts. The limbs were flipperlike and had shortened phalanges. Placodonts have been found in Western Europe.