Re: What identifies and distinguishes a gastrolith from other rocks?

Greetings,
I found some interesting information on Gizzard Stones and Dinosaurs from 
the Geophysical Institute in Alaska and also T. Neil Davis a seismologist 
at the institute.
Hope you find it both informative and instructive.

Just about everywhere one cares to look, Nature has placed objects to 
fascinate the mind. Even in the dirt particles beneath our feet there are 
many intriguing stories about the present and the past; the trick is to be 
observant and to question what one sees.

Several years ago, archeologists sieving Fairbanks area dirt in search of 
small prehistoric bones and human artifacts found in their screens many 
highly-polished small pebbles of quartz and chert. Puzzling over the origin 
of these usually rounded but sometimes angular shiny stones, the scientists 
considered several possibilities.

They decided that wind-blown sand or ice crystals could not be the answer 
because these agents would carve facets on the polished stones. Nor did 
polishing from natural tumbling of the soil by frost action seem to explain 
the combination of rounded and angular shapes found, since tumbling causes 
all stones to become round.

Then Charles Hoskin and his coworkers realized that these objects might be 
gizzard stones, called "gastroliths", ejected from grouse and ptarmigan of 
long ago.
Comparison of the stones with those taken from the gizzards of modern 
grouse and ptarmigan led to the conclusion that the polished stones were, 
indeed, gastroliths.
The polishing given to bird gastroliths comes from chemical action and 
physical grinding as willow and spruce buds, seeds and other 
hard-to-assimilate foodstuffs
are ground up in the birds' gizzards. Sharp, angular stones are picked up 
in spring and summer; as the year progresses, the stones become more 
rounded.

One mystery remains. Some of the gizzard stones found in the soil are 
larger than present-day birds are known to use,a hint that some grouse of 
ptarmigan of the past were larger than those that live now. 
http://dogbert.gi.alska.edu

The following information from Emory University should shed further light 
on DINOSAUR GASTROLITHS

Some modern birds will swallow stones, which then reside in their gizzards 
and aid in digestion of food by helping to grind tough food material. 
Because birds do not have teeth, they need their
gizzards to grind their food, which helps to increase the surface area of 
the food for easier
digestibility. Stones used to help in the mechanical breakdown of food 
within a digestive tract are called gastroliths ("gastro" = "stomach" and 
"lith" = "stone") and a colloquial term for gastroliths is
"gizzard stones." Apparently dinosaurs also swallowed stones for the same 
reason as birds;numerous polished stones are associated with some dinosaur 
remains and are especially convincing
when found within the thoracic regions of a skeleton. These stones are 
typically smooth, polished,and oblate to semispherical. 

However, these trace fossils (more so than most other dinosaur trace 
fossils) are susceptible to
secondary reworking by sedimentary processes or could be produced by 
abiogenic means. Therefore you should be very skeptical of a dinosaurian 
affiliation for any polished stones from Mesozoic deposits unless they were 
picked out of the rib cage of a dinosaur skeleton. 
           
These could be gastroliths, or they could be just polished stones. One of 
the major criteria for suspecting gastroliths in this case is that these 
specimens were found in Mesozoic rocks that were
known to contain dinosaurs. Specimens are in the Museum of Western 
Colorado's Dinosaur Valley,Grand Junction, Colorado. 
 http://www.emory.
edu/GEOSCIENCE/HTML/TFW3.HTML

Hope this answers your question.

June Wingert
Baylor College of Medicine
Houston, Texas