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Gastroliths

A little rock appetizer with that salad?

On a “global warming”  60 degree day this January, I was out working on a fence line less that 200 yards from my house.  I habitually scan grass less areas of ground looking for indian artifacts or traces of dinosaur fossil material.  (Though I need glasses for every other distance, I see perfectly at 6 feet to the ground.) I spied a nicely rounded black cherty pebble about 3/4 of an inch long with a slight polish.  I immediately dropped what I was doing and got on my knees to closely examine the area.  Deftly avoiding all but the aroma of the freshly derived bovine land mine nearby, I started picking up more of these pebbles right off the surface of an area about 2 yards on a side.  When I was done, I had a pretty full handful of small slightly polished pebbles made of various material and all nicely rounded.  I was excited.

If you remember nothing from the Geologic Column, remember this.  “Always look for things that are out of place when looking for collectable things in nature”. Consistantly, up in this sandstone country, cherty/quartitic pebbles are not common at all and are infact rare.  In stream valleys such as the Powder River Valley, they are very common but not on my highland ranch.  Since they do not outcrop naturally here then any occurrence of them must be treated as a discovery so some kind.  Additionally, they were out on cattle land away from driveway or path, they were all of similar size, variously rounded, smooth and semi polished. I concluded they were not county crushed gravel falling off a muddy ranch truck.  Indians did not bring them in because they were too small to make into any effective tool. Where did they come from?

I have given you several clues.

Answer:  Vegetarian dinosaurs ate lots of roughage and needed help to break down the woody pulp in the bellies.  To facilitate this, they often ate gravel which tumbled around in their gut along with the roughage and aided their digestion.  (Chickens do the same thing today to fill their gizzard with pebbles.) These dinosaur gizzard stones are called gastroliths.  They are very collectable by themselves and are fairly common in areas where the Cretaceous Hell Creek formation outcrops. Their source however may have been from hundreds of miles away where such stones naturally occur and they were all carried here by the critter that swallowed them  They are not actually fossils but are considered trace fossils (ichnofossils) by paleontologists. An ichnofossil is just an indication that an animal was there, not an actual fossilized part of biological material.  This type of structure might also be tracks, trails, tail drag marks, and fossil poop (to be covered in a later Geologic Column).

Gastroliths come in all sizes (as do dinosaurs).  I have even found 10 pound stones half the size of a volley ball in direct association with dinosaur bones on the surface.  Just imagine the size of a creature that would swallow a 10 pound rock to help digest food.  I also have found pea sized stones obviously used by smaller creatures for a digestive aid.  They come in a variety of material but are almost all quartzitic, silicic, or chalcedonic in composition.  In other words they are made of hard stuff.  The softer ones got ground down very quickly or dissolved by the stomach acid.  Gastroliths may have a calcium carbonate coating from ground water flowing over them or they may be iron stained brown by the same process.  They are almost all semi-polished as if from the rock tumbler sitting on a dusty shelf in your garage.  And they usually are not single isolated stones often occurring in numbers that would have partially filled a stomach some 65 million years ago.  If you find one, keep on looking.

Before there was Tums for your tummy, there were stones for your stomach.

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