Sauropterygia
Sauropterygia ("lizard flippers") is an extinct clade of diverse, generally marine reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosauria became extinct at the end of that period. The plesiosaurs would continue to diversify until the end of the Mesozoic, when they became extinct as part of the end-Cretaceous mass extinction.