GIZZARD STONES
For many years rounded stones with a very high polish have been found in the sedimentary rocks which contain bones of extinct reptiles. The polish on these stones is very much higher than could have been applied by the action of water or wind. Some look as though they had been polished by a jeweler. Since we cannot attribute this very high polish to wind or water action, we must seek another agent.
Protiguanodon. NOTE GIZZARD STONES INSIDE RIB BASKET. (COURTESY, AMERICAN MUSEUM OF NATURAL HISTORY.)
Just as chickens swallow fine gravel for their gizzards to aid digestion, so it is thought that some large dinosaurs swallowed stones for the same purpose. There is some evidence to support this idea. Several specimens of a group of swimming reptiles, called plesiosaurs, which swarmed the Jurassic and Cretaceous seas, have been found with highly polished stones inside the rib basket. Also a mass of highly polished stones was found similarly associated with one dinosaur, Protiguanodon, in the Lower Cretaceous rocks of Mongolia.
On the other hand, no highly polished stones have been found associated with the specimens in the Dinosaur Quarry or anywhere in the quarry. A search of the many publications on dinosaurs has not turned up any mention of highly polished stones being associated with any of the many specimens found in North America. Thus the evidence which we have does not permit us to say that the dinosaurs found in the quarry did or did not possess gizzard stones.