How to Stump a Set of Scientists

To say we were stumped when a visitor stopped by with this object on Friday would be an understatement.  As you can see from the picture, it’s mostly spherical, about 5½” in diameter, and shiny, as if its been polished.  You can’t tell this from the picture, but it’s pretty lightweight, and the impression you get if you pick it up is that it’s hollow.  It was found floating in the Des Moines River about 60 years ago, and the owner stumped museum staff with it then as well.  This time, he asked staff at the UI Paleontology Repository and the Museum of Natural History before someone at the Office of the State Archaeologist finally solved the puzzle. 

We think this is an enterolith, or intestinal stone, presumably from a horse (our research suggests that they are common in horses, but found in some other animals too).  Enteroliths are a lot like gallstones or kidney stones in people, and also something like pearls in oysters.  They form in a horse’s intestines when the chemical conditions are right.  Most enteroliths seem to be formed of a mineral called struvite (ammonium magnesium phosphate), which forms crystals in concentric rings around some starting “seed” (as a pearl does around something like a grain of sand).  Horses seem to get them when they’re eating relatively high concentrations of protein (for example from alfalfa), which generates ammonium ions, and magnesium.  So enteroliths are more common in some places than others because the minerals in soil and water are different and because common food sources are different.  Small enteroliths can be passed naturally, but large ones need to be removed surgically.  According to an equine vet we asked about this, they are often much larger than this one, and they’re usually quite solid and heavy—they are, after all, stones!  That means we haven’t quite solved the mystery of this enterolith (if that’s what it is), because it feels light and hollow.  We wonder whether the mineral crystals inside the enterolith could have been dissolved by immersion in the river, leaving the hard shell… and if any chemists, veterinarians, or taphonomists out there want to do this experiment, we look forward to hearing what you find.

-Written by MNH Associate Director Trina Roberts
 

Examples of other enteroliths