Order Sauropterygia is a term used in the field of paleontology to describe a group of extinct marine reptiles that lived during the Mesozoic era. These reptiles were thought to have evolved during the early Triassic period and went extinct during the late Cretaceous period. The Sauropterygia order includes several different families, including the Plesiosauridae, Nothosauridae, and Placodontidae. Synonyms for Sauropterygia include "sauropterygian reptiles," "sauropterygians," and "marine reptiles." These creatures played an important role in the evolution of marine ecosystems, and their fossils continue to be a source of fascination and study for scientists around the world.