Thursday, March 26, 2009

Ordovician Pelecypod - Ambonychia

I am continuing to read the book A Sea without Fish by David L. Meyer and Richard Arnold Davis about Ordovician fossils published in January 2009. Here are some pictures of a pelecypod (clam) from Trimble County, Kentucky. It is interesting because in their book they deal mostly with Ohio locations but they mention the site this fossil came from on pages 125-129. Their focus is on the gastropod beds in the Marble Hill Bed Rowland Member of the Drakes Formation (Katian - Richmondian) but I found this a layer below that.

What is nice about this fossil is the aragonite shell is pretty much intact. It appears this is somewhat of a rare occurrence. Usually, just a mold is left as they say in the book (pages 129-130) the shell must have had a high content of organic material. Using the Fossils of Ohio book and A Sea without Fish, this clam must be Ambonychia sp. Meyer and Davis refer to this pelecypod as an Epibyssate form which is one that "is attached to objects that are beneath the actual sediment-water interface of the sea floor."(page 284). I guess this would be opposed to ones that burrowed into the sediment.

This specimen is about 1 cm wide and 1.8 cm long. I found another that was 2.9 cm wide and 3.9 cm long but it was just a mold with on partial shell line impressions left at the back end of the clam .

You can see some other fossils found in Trimble County, Kentucky at the KYANA webpage on Ordovician fossils.

Note the pictures can be clicked on it make them show up larger.


Anonymous said...

G'day, Bill from S.E. Australia.
just found an identical shell in
early ordo. limestone outcrop.Is detached as is your one. Looks for all the world like its twin !Even down to the similar bits that are missing.

Kentuckiana Mike said...

Hi Bill,

Looking at an estimated global map of the continents during the Late Ordovician period (p.44 Fossils of Ohio book, Bulletin 70, Ohio Division of Geological Survey, 1996) both Kentucky in North America and southeast Australia were close to the equator in the southern hemisphere. I would guess they were about 10,000 km apart compared to approximately 15,000 km today.

It would seem to me that the clams would exist in both places in the tropical sea at that time.

Thanks for sharing that info.