A few of the mollusks also had partially calcite shells back then, especially some of the pterioid bivalves.  Various taxa today also have calcite.  There are also some rock units (including some around Cinncinnati) that have the original aragonite shells replaced with calcite or silica in a way that preserves fine details, even including internal structure of the shell.  However, the majority of well-preserved shells that look sort of molluscan in those rocks are actually brachiopods.
The oldest known examples of preserved aragonite come from a bit southeast from Cinncinnati, in the Pennsylvanian of eastern Kentucky.  However, deposits as young as Pleistocene may have the aragonitic shells entirely lost.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa