I love this kind of fossil, which explains why you’ve seen so many examples on this blog. We are looking at an encrusted external mold of the bivalve Anomalodonta gigantea found in the Waynesville Formation exposed in Franklin County, Indiana. I collected it many years ago as part of an ongoing study of this kind of preservation and encrustation.
To tell this story, I’ve lettered the primary interest areas on image above. First, an external mold is an impression of the exterior of an organism. In this case we have a triangular clam with radiating ribs in its shell. The exterior of the shell with its ribs was buried in sediment and the shell dissolved, leaving the basic impression above. It is a negative relief. Please now refer to the letters for the close-up images below.
A. At the distal end of the bivalve mold is what looks at first to be the original shell. It is calcitic, though, and we know this bivalve had an aragonitic shell. A closer look shows that this is actually the attaching surface of an encrusting bryozoan that bioimmured the original bivalve shell, which has since dissolved away. This smooth surface is the bryozoan underside; we see the characteristic zooecia (tubes holding the individual zooids) only when this surface is weathered away.
B. These tubular objects are infillings of borings (maybe Trypanites)that were cut into the original aragonitic shell of the bivalve. The tunnels of the borings were filled with fine sediment, and then the shell dissolved away, leaving these casts of the borings.
C and D. In the middle of the external mold is this curious circular feature (C) mostly surrounded by a bryozoan (D). There was at one time a circular encruster, likely an inarticulate brachiopod like Petrocrania, that sat directly on the external mold surface. The bryozoan colony grew around but not over it because it was alive and still opening and closing its valves for feeding. The bryozoan built a vertical sheet of skeleton around it as a kind of sanitary wall. You may be able to see the other three or four structures in the top image showing brachiopod encrusters that left the building. This is an example of fossils showing us a living relationship, even if one is not longer preserved.
This fossil and its sclerobionts (hard substrate dwellers) show us that soon after the bivalve died its aragonitic shell dissolved away, leaving as evidence the external mold in the sediment, the bioimmuring bryozoan, and the boring casts. Very soon thereafter bryozoans and brachiopods encrusted the available hard substrate. This is a typical example of early aragonite dissolution on the sea floor during a Calcite Sea interval.
Palmer, T.J. and Wilson, M.A. 2004. Calcite precipitation and dissolution of biogenic aragonite in shallow Ordovician calcite seas. Lethaia 37: 417-427.
Taylor, P.D. 1990. Preservation of soft-bodied and other organisms by bioimmuration—a review. Palaeontology 33: 1-17.
Taylor, P.D. and Wilson, M.A. 2003. Palaeoecology and evolution of marine hard substrate communities. Earth-Science Reviews 62: 1-103.
Wilson, M.A., Palmer, T.J. and Taylor, P.D. 1994. Earliest preservation of soft-bodied fossils by epibiont bioimmuration: Upper Ordovician of Kentucky. Lethaia 27: 269-270.