The grey/green stone on the top of your picture looks like marble. Marble is a metamorphic limestone, which means that recrystallization occurred, which means that you should not have fossils in it as much as in sedimentary limestones.

So maybe it's not a fossil but a trace of the excavation process? Is the stone as smooth as elsewhere in this area?

That being said, some very old limestone can be denser than marble without being recrystallized. I just don't know of any that aren't grey or beige.