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a subclass of reptiles, first appearing in the Permian period. Ancient lepidosaurians closely resembled the cotylosaurs, whose descendants they probably are. The skull of lepidosaurians, as in the archosaurians, with which they were previously classified, initially had two temporal arcades; but with the development of a movable articulation of the cranial bones (kinetism), the lower arcade was lost in lizards and both were lost in snakes. Mostly predators, either terrestrial or aquatic, the lepidosaurians comprise the Eosuchia, the Saura, the Serpentes, and a few other groups. Their significance for stratigraphy is small; an exception are the Mosasauridae, giant marine lizards of the late Cretaceous.