Ford and Martin (2010) proposed that "Psittacosaurus" was semi-aquatic, swimming with its tail like a crocodile, and paddling and kicking. They based their interpretation on evidence including: the lacustrine (lake) depositional setting of many specimens; the position of the nostrils and eyes; interpretations of the motions of the arms and legs; tails with long chevrons (and with the bristles on the tail interpreted as possibly skin-covered, forming a fin), providing a propulsive surface; and the presence of gastroliths, interpreted as ballast. They further suggested that some species of "Psittacosaurus" were more terrestrial than others.