Re: paleonet Fossil terminology

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>In the Texas Cretaceous we find lots & lots of ammonites preserved as marl steinkerns.  Almost all of them preserve the external topography of bumps, ridges, etc. (technical terms!) to some extent.  Many of these specimens also preserve some of the suture pattern on the same surfaces.  These would then be composite molds according to your definition, right?  (In fact, these may be some of the fossil molluscs that Michel was referring to.)<
>So the presence of external and internal topographies or textures on the same specimen is generally or always due to compaction?  <

In general, yes, but especially in thin-shelled species, sculpture may be evident on the inner surface of the shell and thus visible on a true internal mold.  Thus, the presence of bumps and wrinkles on an ammonite internal mold does not immediately demonstrate that it is composite.  If the "sculpture" is a fold of the entire shell, so that the shell is roughly constant in thickness, then the internal mold will show this sculpture.  On the other hand, if the feature is truly external sculpture, produced by a thickening or thinning of the shell, then it should not show up on the internal mold.  

By definition, to get external sculpture superimposed on the internal sculpture, there must be enough compaction to make up for the thickness of the shell.  However, this may not be very much.  

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

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