Ichthyosauria refers to a group of marine reptiles that lived during the Mesozoic Era, primarily from the Triassic to the Cretaceous periods. These creatures resembled modern-day dolphins and were well-adapted to life in the ocean, featuring streamlined bodies, large eyes, and long snouts filled with sharp teeth. Ichthyosaurs were among the first reptiles to return to the sea after the extinction of the dinosaurs. They varied in size, with some species reaching lengths of over 20 feet. Fossils of ichthyosaurs have been found worldwide, providing valuable insights into their evolution and ecology.