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.