Regarding gastroliths,
according to Bakker (1986, Dinosaur Heresies), gastroliths are also
used to process food in crocodiles, as can be seen (according to him)
on x-rays.
More recent X-ray movies, presented at last year's SVP meeting (I think),
show that that's not the case: the gastroliths of alligators at least don't
move, they just lie around in the stomach. Apparently they're ballast.