During the Early Jurassic, these diversified quickly into both long-necked small-headed plesiosaurs proper, and short-necked large-headed pliosaurs.

The demands of an aquatic environment caused the same features to evolve multiple times among reptiles, an example of convergent evolution.

Several analyses of sauropterygian relationships since the beginning of the 2010s have suggested that they are more closely related to archosaurs (birds and crocodilians) than to lepidosaurs (lizards and snakes).

[3][4] The cladogram shown hereafter is the result of an analysis of sauropterygian relationships (using just fossil evidence) conducted by Neenan and colleagues, in 2013.

[3] Pantestudines †Kuehneosauridae Lepidosauria †Prolacertiformes †Choristodera †Rhynchosauria †Trilophosaurus Archosauriformes †Ichthyopterygia †Thalattosauria †Eusaurosphargis †Hanosaurus †Helveticosaurus †Sinosaurosphargis †Placodontiformes †Yunguisaurus †Plesiosauria †Pistosaurus †Augustasaurus †Corosaurus †Cymatosaurus †Simosaurus †Germanosaurus †Nothosaurus †Lariosaurus †Diandongosaurus †Dianopachysaurus †Keichousaurus †Wumengosaurus †Anarosaurus-Dactylosaurus †Neusticosaurus-Serpianosaurus The cladogram shown below follows the most likely result found by an analysis of turtle relationships using both fossil and genetic evidence by M.S.