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Daniel Quarto

Historical Geology

Spring 2008         

Stomach Rocks through the Ages


The word gastrolith is comprised of two words; Gastro, being related to the digestive system, and Lith, which means rock. Gastroliths are just that, a stomach rock; any dirt, gravel, rock, or even cobble that was once ingested by an animal to aid in digestion. Just as our modern chickens use gravel to grind their food, so did the dinosaurs of the Mesozoic Age, and more specifically, the Plesiosaur, recognized widely as “that dinosaur that looks like the Loch Ness Monster.”

This photo actually shows an inconsistency, since

it has since been discovered that Plesiosaurs probably

couldn’t lift their necks up at a “swan-like” angle as usually



While plesiosaurs certainly aren’t the only animals who ingest rocks to grind their food (crocodiles, seals, sea lions, all manner of fowl) they are perhaps the best example of the gastrolith as a historical tool. It is thought that these unique animals used gastroliths not only for digestion, but they also served as a “ballast” to keep the creatures’ balance and center of gravity in the water.

In 1998, the Cinncinati Museum of Natural History completed a dig seven years in the making. Discovered by a rancher in 1991, Plesiosaur bones along with eroded round stones were strewn along an exposure of Pierre Shale on a hillside in Kansas. A varied collection of bones and gastroliths were unearthed and displayed.


Gastroliths can be tricky to identify. The main distinctive features of a gastrolith are similar to ordinary sedimentary rocks. Often, a sample can only be determined to be a gastrolith if it is found in an area geologically inconsistent to its composition, or, if the sample is discovered with the remains of an animal. These limited means of identification have been addressed by paleontologists who research new methods of identifying gastroliths without remains present, so they can learn such crucial information, such as migration patterns.


            All gastroliths, like sedimentary rocks, are rounded and polished due to saltation, or the process by which a material is sanded and abrated by other materials and thus becoming smooth and round. Unlike sedimentary rocks, which receive this treatment in river beds, gastroliths are given their shape and sheen through tumbling with other materials in the body, such as tough foods, plants, and other gastroliths. These rocks could either pass through the animals digestive system or be released through decomposition following the animals’ death. In the former possibility, gastroliths would then be found in samples of fossilized dung, while with the latter option usually be discovered alongside the animals’ bones as described earlier.


Gastroliths aren’t fossils in a conventional sense, citing the fact that they were not generated naturally from a living being. But, they are nevertheless fossils because they have metamorphosed, or undergone a significant change due to their use by the animals. The discovery of gastroliths in either animal feces or remains gives unique insight into the dietary habits that we’d otherwise know little or nothing about in creatures long since extinct. In another, and admittedly narrow viewpoint, the comparison drawn from the use of rocks to grind food in both modern birds and ancient dinosaurs gives ammo for those who wish to cite additional evidence toward the debated theory that dinosaurs evolved into birds.


Is this a Gastrolith?


            Pictured above is a rock I recently had the pleasure of identifying for a coworker. Found in a wash here in Sierra Vista, this particular rock was about the size of a Klondike bar and was very smooth to the touch. I had immediately recognized it as a sedimentary rock due to the bedded layers that were along its’ sides.

Larger photo, for reference.

            It was square in shape, but it was incomplete. Pictured above, on the left side of the rock, there was an indentation that had the same polished veneer as the rest of the rock. This is an indication that the rock had been chipped at that place before it was subjected to saltation. The reason I bring this point up:


            The rock had been chipped after the saltation occurred as well. That bright, cream colored area confirmed several things. First, there are no individual crystals visible on the exposed space. It is a very fine grained material, later identified as bedded chert. Also, since this area is unpolished, this break occurred much more recently than the former one.


            I came to a lot of conclusions on my own, but I never imagined it to be a gastrolith until I got a professional opinion on the rock. So I consulted my geology teacher, Professor Roger Weller, who looked it over with his usual bemused interest. My timing, as it had been in my experience in the class, had been optimal, as what he then told me would come up very soon in class lecture.


            This rock is indeed a gastrolith. And we can tell because there are subtle differences between sedimentary polished rocks and stomach tumbled polished rocks. Look once more at the last picture, The polish covers the entire rock except the depressions on its surface. This is one of the ways to differentiate the two. Another way is that gastroliths contain microscopic hairline fractures in its surface, or tend to have the shape of actual teeth.


            And so, with new insight, I returned the stone to the procurer and relayed all I had learned about it, fully confident that they would appreciate my time, and know that they had come to the right place. They told me “I’d still like to think that its petrified soap”



Works Cited  
title photo and intro story on digsite   
plesiosaur picture


Henderson, D. M. 2006. Floating point: a computational study of buoyancy, equilibrium, and gastroliths in plesiosaurs; Lethaia; 39 pp.227-244
on the neck lifting & gastroliths as a ballast.