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.
[Illustration: _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.