Dear Palaeoconchlers,
 
I am no longer the only person on the Web who loves fossil oysters! Charles
Newsom (University of Iowa) has set up a website showing large Cretaceous
oysters from the US Gulf Coastal Plain (Alabama and Texas), with images of
several species of gryphaeids Exogyra and Pycnodonte (formerly placed in
Gryphaea, which is now restricted to Jurassic forms). The images are large
and take awhile to load, but should be helpful in identification. I
particularly like the image of Exogyra tigrina. Charles, a dedicated fossil
collector, is still working on this site and would appreciate constructive
criticism. You can reach him at [log in to unmask]
 
Oysters are useful because they are conspicuous, common fossils that
evolved fairly rapidly in the Cretaceous and Tertiary Periods. They lived
mainly in brackish to fully marine shallow-water environments. The shells
are made of calcite, a relatively stable form of calcium carbonate, except
for the muscle scar, which is aragonite (the other common form). In
near-surface conditions, aragonite tends to dissolve while calcite often
remains untouched for millions of years, which is why fossil oysters (and
scallops) can be found scattered on the ground at some outcrops when most
other mollusks are reduced to molds. Oysters and scallops were used as
index fossils to map the geology of the US Coastal Plains.
 
Andrew K. Rindsberg
Geological Survey of Alabama
Tuscaloosa, Alabama, USA