The “fossilized turtle shell” pictured above is actually one of the more commonly misidentified pseudo-fossils in the rock record, a septarian concretion. Concretions aren’t the remains of animals or their shells at all: They are inorganic nodules that grow in rocks by precipitation from groundwater. This kind of concretion is called septarian – a Latin word for partition – because of the cracks that divide it into polygons.
Though these are not uncommon, how they form isn’t well understood. Explanations for the cracks range from dehydration and shrinkage (similar to mud cracks) to fracturing by either crushing or violent shaking (such as in an earthquake). Once fractured, circulating ground water deposits more minerals into the cracks.
This septarian concretion was collected from the Fayetteville Shale near Leslie, Arkansas in the Ozarks.
To see more views of septarian concretions, click here