>(it is known that Cerithium in which the gonads are destroyed by cestode (tapeworm) parasitism grow much larger than normal, so either live longer or have excessive production of growth-hormones... or both).<

Another possibility is that the energy that would normally be put into reproduction goes into growth instead.  Instead of producing a bunch of gametes, it produces an extra cm or whatever of growth.  This wouls also explain the extra-large individuals sometimes found near the periphery of ranges, e.g., some Caribbean taxa in Bermuda that seem to be non-reproducing individuals carried in as larvae by the Gulf Stream.

On the Caribbean Eocene Campanile, I looked up Jung.  He estimates ca. 90 cm for his reconstructed specimen, and claims literature reports of ca. 1 m for the European one, though I do not know how precise the original source of those measurements might be (I just read Wallace on an orangutang specimen that shrunk 1 foot from the hunters' account to that of the boat captain and 2 more feet by the time it got measured in a museum.)  Incidentally, there's trouble on the type of Campanile-the French Eocene and the extant Australian one were both included in the original description, and there are competing subsequent type designations and interpretations.

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