>The first known probable mollusc is Wixwaxia (almost certainly a mollusc) from the Burgess Shale.<
>Hmmm... also commonly spelled Wiwaxia.<

Wiwaxia (one x), from the Middle Cambrian Burgess Shale, has mollusk-like and polychaete-like features.  It probably represents an intermediate form between the two phyla.

Older mollusks include numerous monoplacophorans and the earliest bivalves, known from the Early Cambrian.  In addition, there are numerous Early Cambrian shells for which we do not know what kind of animal lived in them.  In many cases, these may represent a small part of the total skeleton (kind of like trying to guess what a chiton looks like if you have only one plate).  A few examples, such as Halkiera, have been found with all the spines and plates for one animal still together, confirming that several types of small shells all came from the same type of animal.  Many of these are mollusk-like, but again it is difficult to say whether they are true mollusks or transitions between mollusks and related phyla.  The oldest of the monoplacophorans and other small shelly fossils may be latest Precambrian.  Kimberella and the radula scrapings are also Precambrian.  Again, these are mollusk-like but not clearly true mollusks.

It does look as though the first true mollusks did not have a solid, one-piece shell, but they might have had lots of little spines.  (Note that defining a true mollusk is a problem, too.)


The Florida mystery shell looks like a cardiid to me.  I would expect Noetia to have a squarer shell.  A side view or a close-up of the hinge are would help.  Southwest Florida has moldic units ranging from Miocene to Pleistocene, and if they got their gravel from central Florida, Eocene or Oligocene would also be possible.

    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