Plesiosaurids were a group of marine reptiles that lived during the Mesozoic Era, particularly from the late Triassic to the end of the Cretaceous Period. They are known for their distinctive body shape, featuring a long neck, small head, and a broad, flat body. Plesiosaurids had four flippers, which they used to navigate through the water, making them efficient swimmers. These creatures were primarily carnivorous, feeding on fish and other marine animals. Fossils of plesiosaurids have been found on every continent, indicating their widespread presence in ancient oceans. Their unique adaptations allowed them to thrive in various marine environments.