Their oldest relatives were small, semi-aquatic reptiles with four limbs that were adapted for paddling around in shallow water. By the end of the Triassic, they had grown to much larger animals fully adapted to a life at sea and were incapable of coming to shore.
Throughout the Jurassic and the Cretaceous developed a diverse range of body plans adapted for a life in the water. They went extinct at the end of the Cretaceous along with the dinosaurs.
The best known of the sauropterygians are the long neck Plesiosaurs but this taxon includes a whole host of other interesting Mesozoic marine reptiles. then flourished during the Mesozoic.
Sauropterygians are united by a radical adaptation of their pectoral girdle, designed to support powerful flipper strokes. While Tyrannosaurs ruled the land, and flying reptiles ruled the skies, the mighty Mosasaurs dominated the seas. They were late to the aquatic party, being the last clade to evolve. Photo: By Ryan Somma - PlesiosaurusUploaded by FunkMonk, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=6735975