What is "sauropterygia"

Sauropterygia \Sau*rop`te*ryg"i*a\, n. pl. [NL., fr. Gr. ? a lizard + ?, ?, a wing.] (Paleon.) Same as Plesiosauria.

Plesiosauria \Ple`si*o*sau"ri*a\, n. pl. [NL. See Plesiosaurus.] (Paleon.) An extinct order of Mesozoic marine reptiles including the genera Plesiosaurus, and allied forms; -- called also {Sauropterygia}.

Sauropterygia (" lizard flippers") is an extinct, diverse taxon of aquatic reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Mesozoic before they became extinct at the end of that era. When the End-Cretaceous extinction event occurred, only a single lineage, the Xenopsaria, represented by the two groups Elasmosauridae and Polycotylidae, was still present. Sauropterygians are united by a radical adaptation of their pectoral girdle, designed to support powerful flipper strokes. Some later sauropterygians, such as the pliosaurs, developed a similar mechanism in their pelvis.