Charles Newsom reminds me that I forgot to include his URL. Apologies!
 
http://www.physics.uiowa.edu/~cnewsom/fossils/Oysters/
 
Emilio Jorge Power asked,
How are they [oysters] distributed across the Cretaceous/Tertiary
boundary (K/T)? I suppose they survived the mass extinction?
 
Well, some of them did and some of them didn't. There are three families of
oysters: ostreids (including most of the familiar modern oysters. Ostrea,
Crassostrea, etc.), gryphaeids (including modern Hyotissa), and
palaeolophids (an extinct family including Rastellum). One of the simplest
ways to distinguish the ostreids and gryphaeids is by examining a broken
piece of shell. Ostreid shells are built of solid lamellae, though some
lamellae may be chalky and dissolve readily. Gryphaeid shells include
vesicular lamellae; that is, flattish layers that appear bubbly under low
magnification. The vesicular structure is barely visible to the naked eye.
 
Cretaceous oysters were diverse right to the end, and included some
elegantly shaped species, like Agerostrea falcata and Exogyra costata and
Rastellum carinatum; and some very large species, like Pycnodonte
mutabilis. It is by no means unusual to see five or six species of oyster
at a single outcrop of marine strata. Some genera sailed through the
Cretaceous-Tertiary boundary, like Crassostrea, then, as now, a
brackish-water genus. But most genera died, including Exogyra and all the
palaeolophids. Paleocene (earliest Tertiary) survivors tend to be small and
lumpish. There is an abundant earliest Paleocene species of Pycnodonte, P.
pulaskensis, but it is no larger than your fingernail, a far cry from the
glory days of the Cretaceous when it was one of the largest elements of the
molluscan fauna. The gryphaeids never became as diverse as they were
before, though they can be locally common, like Hyotissa in the Pliocene
beds at Sarasota, Florida. The ostreids recovered in numbers, but, in the
main, they are not as diverse or beautifully shaped as the Cretaceous
forms.
 
Incidentally, there is no true Cretaceous Ostrea or Gryphaea. These genera
were named early, and hundreds of species have been named. One by one, they
are being reassigned to other genera as specialists figure out how to
subdivide these family-sized "genera". Ostrea is now considered to be
restricted to the Cenozoic (=Tertiary + Quaternary, including modern). And
Gryphaea is Jurassic.
 
Sources of information on fossil oysters include:
 
Stenzel, H. B., 1971, Oysters, in Teichert, Curt, ed., Treatise on
invertebrate paleontology, pt. N (mollusca 6), v. 3: Boulder, Colorado,
Geological Society of America, and Lawrence, Kansas, University of Kansas
Press, p. i-iv + N953-N1224. The late Stenzel included modern oysters in
this masterpiece, and the book has been reprinted and is still available.
See the Geological Society of America's website for ordering information.
Also, most university libraries have a set of the Treatise.
 
Malchus, Nikolaus, 1990, Revision der Kreide-Austern (Bivalvia:
Pteriomorpha) Aegyptens (Biostratigraphie, Systematik): Berliner
geowissenschaftliche Abhandlungen, Reihe A, Band 125, 231 p., 27 pl.
Malchus is the world's foremost expert on fossil oysters. In this hefty*
work, he named a new family, Palaeolophidae. Despite the title, the book is
not just about Egyptian oysters, as it includes a great deal of
reclassification. Good luck on finding a copy. Interlibrary Loan comes to
mind.
 
As Dr. Malchus is not very old (even younger than that young sprat Doug
Shelton, I would guess), the question naturally arises, Would an expert on
worldwide fossil oysters exist today if he did not study them? Apparently
not, because Malchus has little competition in this field despite its
importance to stratigraphy and paleontology. A great many living and fossil
groups have no specialist to study them. So I would encourage those who
wish to be considered The World Expert on Something to find a group of
interesting organisms, develop a collection and expertise in them, and
write their own book. (Sorry, the cypraeids seem to be taken... )
 
Andrew K. Rindsberg
Geological Survey of Alabama
 
*Yes, Emilio, that was a pun.