Any of the pebbles swallowed by animals and retained for a time in the gizzard or stomach, where they serve to grind up the food and in so doing become rounded and highly polished. Birds generally use such pebbles, as do some living reptiles, notably the crocodile and certain lizards. Some of the Mesozoic reptiles also used gastroliths. Articulated skeletons of the Plesiosaurs are generally associated with such highly polished cobbles, even when embedded in formations such as the Niobara chalk, which is otherwise free of gravel. In some instances as many as a half bushel of such stones have been found within the rib cage. Among the dinosaurs the great sauropods likewise used gizzard stones.