Backyard Firepit Rock with Surprise

I collected these rocks from the fields here in Trigg County where I live to make a firepit. They worked great and when the firepit was moved, some of the rocks broke open and WaLaa - more brachiopods, clams, and bryozoans with their respective imprints. What a surprise! I think the rock they are embedded within is a mineral rock (maybe a type of Dolomite?). I'm learning as I go so correct me if needed :)