> First thing I found is that
>modern thraciids have "an aragonitic, granular homogeneous shell
>microstructure, whereas Runnegar found that thraciids from the Mesozoic have
>'nacreous'shells"...

They and some poromyoideans are the only anomalodesmatans I know of with
non-nacreous interiors.

>One thing about those nacreous interiors I'd like to ask...I was told by our
>same Walter Sage a long time ago that a nacreous interior reveals  close
>relationship to some very early molluscan forms.  That seems to be true with
>the gastropods, with mostly haliotids and slit shells and trochids and
>turbos and some patellids etc. having nacre. And the nautiloids are quite
>ancient, aren't they? But the bivalves don't seem to bear that out. Nut
>clams occur early in the taxonomic scheme, and oysters don't occur so very
>long after them maybe,  and the Trigonia and unionids after that. But the
>anomalodesmatids are "at the back of the book," so to speak.  Are there, in
>fact, any rules of thumb? Is nacre just a convergently evolved character? Or
>is there a bunch I am missing? (likely.)  What else is noteworthy about
>nacre?  It doesn't seem to hold up as long.  And it does seem to turn dark
>with extreme age...I found a fossil pearl at the Sarasota Fossil Pits, and
>it is blackened.

Nacre seems to be the ancestral condition for all bivalves; the earliest
non-nacreous bivalve is a Silurian nuculanoid, with all other Silurian taxa
with known microstructure being nacreous.  Anomalodesmatans appear as early
as the other major groups (at least early Ordovician) and generally seem to
have stayed content with it.

>And are American jingle shells (Anomia simplex) considered nacreous?  And,
>while I am thinking, aren't the Mytilidae nacreous?

Some mytilids retain nacre, but anomioideans do not.

Carter, 1990 and Taylor, Kennedy, and Hall 1973 probably provide more
information than you want about microstructure.

David Campbell

"Old Seashells"

Department of Geological Sciences
CB 3315 Mitchell Hall
University of North Carolina at Chapel Hill
Chapel Hill NC 27599-3315
USA

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"He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The
Gold Bug