Favosites turbinatus from Arkona

Arkona is one of the few location where I can reliably find Favosites turbinatus. This is a variety of the tabulate coral Favosites that has an odd growth pattern. As you can see below, the base of the coral is smooth and curved into a near coil while the top is a rounded mound.

It's thought that this growth patten is the result of the colony colonizing an upturned pelecypod or Gastropod shell. The smooth base walls and curved tip would have been in contact with the original shell while the rest of the coral was free and open. It makes some sense once you start to examine the coral and see that the smooth areas have the hexagonal pattern of the calicles, the structure that housed the individual coral polyps, (especially noticeable in the third picture) but do not show an opening. One of the questions I have is this: If the colony used a shell as it's base, what happened to the shell? Why is it not still attached? A possible explanation would be that the original base shell was slowly dissolved away over time since Pelecypod shells don't fossilize as well as Brachiopod shells. The above specimen was found in the Hungry Hollow member of the Widder Formation.  I've found this species before in the Centerfield Formation (equivalent to the Hungry Hollow) and the Jeffersonville formation.