I think the gastroliths were the stones that the animals swallowed.

gastrolith
Stone that was once part of the digestive system of a dinosaur or other extinct animal. Rock fragments were swallowed to assist in the grinding process in the dinosaur digestive tract, much as some birds now swallow grit and pebbles to grind food in their crop. Once the animal has decayed, smooth round stones remain – often the only clue to their past use is the fact that they are geologically different from their surrounding strata.

http://www.tiscali.co.uk/reference/encyclopaedia/hutchinson/m0034009.html


TEd