Sauropod dinosaurs appear to have used stones to grind tough plant matter. An example of this is the early Cretaceous theropod "Caudipteryx zoui" from northeastern China. It was discovered with a series of small stones, in the area of its skeleton that would have corresponded with its abdominal region. Aquatic animals, such as plesiosaurs, may have used them as ballast, to help balance themselves or to decrease their buoyancy, as crocodiles do. Some fossil gastroliths are rounded and polished, many stones in living birds are not polished at all. Gastroliths associated with dinosaur fossils can weigh several kilograms.