Plesiosauria is an extinct group of marine reptiles that lived during the Mesozoic Era, primarily in the late Triassic to the end of the Cretaceous period. They are characterized by their long necks, small heads, and large, paddle-like limbs, which made them well-adapted for swimming in ancient oceans. Plesiosaurs are often divided into two main groups: long-necked plesiosaurs, which had elongated necks, and short-necked plesiosaurs, which had more robust bodies and shorter necks. Fossils of these creatures have been found worldwide, providing valuable insights into their diverse lifestyles and habitats in prehistoric seas.