there were three families of long-necked plesiosaurs, who evidently had a different life-style from the pliosaurs. it was suggested by d.m.s. watson that their method was as surface swimmers, darting down to snatch smaller fish which were feeding on plankton. it is hard to see the benefit of a long neck under water; aquatic mammals operating under water all have a streamlined torpedo-shape, as did pliosaurs and ichthyosaurs. all the longer-necked familiers were, from the setting of the teeth and jaws, eaters of small fish. however, some at least were bottom-feeders, consuming various prey. digestion of shellfish was aided by gastroliths.