Mystery Nodule

Click on images to enlarge, found in Benton county, Arkansas, Boone limestone formation

This one is a total mystery. It was found in some hard limestone that was mostly layer upon layer of three of four species of brachiopods. There was also a lovely trilobite in the cube-shaped rock that was about ten inches square. I was able to repeatedly cleave it with the grain so that I kept uncovering layer after layer of goodies.