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Markings on Gastroliths

I've put some pictures of two plesiosaur gastroliths up on a webpage at:

< http://www.oceansofkansas.com/gastro3.html >

They are from different specimens... one Kansas, one South Dakota, about
15-20 million years apart in age... but both have the same arc-shaped
scratches on them.

It's pretty obvious (to me at least) that these things were grinding
against each other and the food contents of the crop or stomach that
they were in...... but what process would create these very similar
marks?  Do the same markings appear on dinosaur gastros??

Relatively few stones from the same specimen have these markings...(most
of the gastroliths in the KUVP 1318 specimen were composed of the same
material as the pictured stone and were similarly marked).  I think
these two stones are fairly low on the hardness scale.. but I'm not a
geologist.   All gastroliths, including the quartz pebbles, were
similarly rounded.

Any ideas appreciated.....

Mike Everhart  (meverhar@fhsu.edu )
Adjunct Curator of Paleontology
Sternberg Museum of Natural History
Fort Hays State University, Hays, KS