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I have found scattered limestone clasts with submillimeter holes in them. I pick them up wondering if they are stromatoporoids, bryozoans, sponges or the like. The holes do not extend into the interior of the rock. Some of the rocks have lichen and algae growing on them. I finally found a soft dark lichen or algae growing in the holes in the rocks. Let me know if anyone can tell whether the dark spots are lichen or algae. If they are the cause then the rock exhibits bioerosian. Bioerosian was first described by Conrad Neumann in 1966 as “the removal of consolidated mineral or lithic substrate by the direct action of organisms.” The organisms probably secrete acid that dissolves limestone. Algae and lichens were early colonizers of the land. Bioerosian created soil that allowed new forms of life including plants and trees to colonize the land. Check your limestone rocks for these traces of bioerosian. Theoretical these could become trace fossils if buried for more that 10k years. Photos show holes in Tertiary limestone with plant fossils that are from 0.2 to 0.4 mm in diameter. Again, if you know what the black organism is let me know. https://aslopubs.onlinelibrary.wiley.com/doi/10.4319/lo.1966.11.1.0092 OBSERVATIONS ON COASTAL EROSION IN BERMUDA AND MEASUREMENTS OF THE BORING RATE OF THE SPONGE, CLIONA LAMPA by Conrad Neumann.