To Henk, Andy R., and David,

The response was great; many thanks.  We touched on the stratigraphy,
taphonomy, conchology, lifestyle, taxonomy, nomenclature, and iconography
of a spectacular mollusk contemporary with the last of the dinosaurs - and
a website no less.

It doesn't get any better.

Thanks,
Harry

P. S. Can one of you give me a link to USGS publications?  It seems I never
got the third bivalve (N) volume issue of the Treatise...


At 09:02 AM 10/6/00 -0500, you wrote:
>Harry,
>
>Henk's comments are to the point.
>
>As the southernmost Cretaceous strata along U.S. Highway 231 are at Troy,
>more than 50 miles north of the Alabama-Florida state line, the locality
>cannot be "just above the Florida line in S. E. Alabama" if this is Exogyra.
>The genus became extinct at the end of the Cretaceous.
>
>Exogyra belongs to the "other" oyster family, the Gryphaeidae, and is
>related to Jurassic Gryphaea, Cretaceous Pycnodonte, and modern Hyotissa
>(which seems to have descended from Pycnodonte). They were very common in
>the Mesozoic, and large Hyotissa is common in the Pliocene beds at Sarasota,
>Florida. Like modern oysters, the gryphaeids are incredibly variable in form
>from individual to individual, and one has to look for a few distinctive
>characters. They have one large adductor scar and hinges much like ostreids.
>The adults usually have both laminated and finely vesicular ("bubbly") shell
>structure (invisible unless the shell is broken; and may be filled in with
>later calcite deposits) and a shelf-like structure along the pallial line.
>There may be an odd pattern of little ridges (chomata) on either side of the
>hinge, though it is not always present.
>
>Exogyra is very inequivalve. The larger valve is boldly curved in a low
>spiral, and tends to be thick-shelled. This is the valve that starts out
>being attached, commonly to another mollusk shell (frequently to a pteriid
>or another Exogyra), but most individuals attach to such small bits of shell
>that they evidently don't need this support as adults. The smaller valve is
>almost flat in most species and may even be concave, as in some modern
>scallops. The ornament is superficial and weathering commonly removes it,
>leaving a smooth valve and complicating identification of species.
>Collectors who don't know Exogyra well tend to pick up the larger valves and
>leave the less attractive smaller valves behind, but the smaller valves are
>actually easier to identify to species.
>
>In life, Exogyra probably rested on soft substrates (mud and sand) with the
>larger valve embedded in the substrate, as in some modern scallops. The
>valves commonly are extensively bored by sponges or other organisms, and may
>be encrusted with bryozoans and so on.
>
>With their thick, calcitic shells strewing some outcrops in abundance, the
>gryphaeids are well known to geologists, who use them to date the ages of
>Cretaceous strata. There is an extensive literature on them. For the
>southeastern U.S., I recommend:
>
>Stenzel, H. B., 1971, Oysters. In Moore, R. C., ed., Treatise on
>invertebrate paleontology, part N (Bivalvia), v. 3, p. N953-N1224.
>Geological Society of America and University of Kansas Press, Boulder,
>Colorado, and Lawrence, Kansas. [Including brief descriptions and photos of
>all oyster genera whether living or fossil, plus an extensive introduction
>on their ecology, growth, etc. In print; available from the Geological
>Society of America.]
>
>Stephenson, L. W., 1914, Cretaceous deposits in the eastern Gulf region and
>species of Exogyra from the eastern Gulf region and the Carolinas: U.S.
>Geological Survey, Professional Paper 81, 77 p., 21 pl. [The stratigraphy
>may be out of date, but the descriptions and full-size photos of species of
>Exogyra have never been surpassed, and their taxonomy has been only slightly
>modified. Out of print, but not a rare work; many university libraries have
>it, often in the documents section.]
>
>Many of the Cretaceous oysters, both ostreids and gryphaeids, had more
>regular forms than their modern counterparts, and some are quite attractive.
>Take a look at Charles Newsom's website, as Henk recommended.
>
>Andrew K. Rindsberg
>Geological Survey of Alabama

Harry G. Lee
Suite 500
1801 Barrs St.
Jacksonville, FL 32204
USA   904-384-6419
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Visit the Jacksonville Shell Club Home Page at:
http://home.sprynet.com/~wfrank/jacksonv.htm

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