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.

Foerstephyllum Colonial Coral

Here is a colonial coral from the Upper Ordovician time period. It was found in eastern Jefferson County, Kentucky. It is probably 14 cm in diameter. It is probably some sort of Foerstephyllum (Bassler, 1941) from the extinct Tabulata coral group. There is a relatively new book entitled A Sea Without Fish - Life in the Ordovician Sea of the Cincinnati Region by David L. Meyer and Richard Arnold Davis with a chapter by Steven M. Holland. This type of coral is shown pages 76-78. They have a picture of an octagonal tool house built sometime in the early 1900s that is composed of colonial corals from the Richmondian coral beds in Madison, Indiana. The structure can be found in John Paul Park in Madison, Indiana. They also cite the work of Ruth Browne on these types of coral. Ruth was a Louisville geologist who did some research back in the 1960s and 1970s in this area.

Trilobite Phacops rana

Most common trilobite I find in the area is the Phacops rana from the Devonian period. They are found in the Jeffersonville Limestone in Clark County, Indiana.

This first picture is of molts of the pygidium (tails) and cephalon (head).


The next picture shows the bottom of the trilobite with a prominate feature showing the cephalic border.

Here is an enrolled thorax of trilobite and this one is probably not a molt.

Looking at the Maryland Geological Survey Middle and Upper Devonian, Baltimore, The John Hopkins Press, 1913 the Phacops rana was named by Green in 1832? and maybe described by Hall in 1861. The description is "General form elongate suboval; greatest width (measured at the posterior margin of the cephalon) to axial length as 1 to 2; the cephalon, thorax, and pygidium are to one another in length as 1.5 to 2 to 1. Cephalon subsemicircular, the regularity of the outline interrupted by the slight protrusion of the glabella and the genal extremities, frontal margin obscure, concealed by the overhanging glabella; facial sutures rarely discernible; glabella large, gibbous, outline subpentagonal, greatest width anteriorly, posterior furrow extending clear across the glabella; cheeks abruptly sloping to the margin, narrowing anteriorly and reflected ventrally to form the doublure; eyes prominent, scarce reaching the height of the glabella in uncompressed specimens, visual area lunate, separated from the cheek by a strong, smooth sulcus, average number of lenses in normal adults is between forty and fifty for each eye.

Thorax subquadrate, lateral margins slowly tapering, surface strongly trilobate; axis flattened at the margins, evenly convex in the middle, widest at the third or fourth segment, tapering very slowly to the ninth, and thence much more rapidly to the pygidium; pleurae flat for about one-third their width from the axis and thence abruptly deflected to the margin, each segment bears a furrow which becomes obsolete at the fulcrum. Pygidium relatively small, regularly and evenly rounded margin the posterior part of which forms the arc of a circle; axis composed of nine annulations, rapidly and evenly tapering from the last segment of the thorax, reaching an acute termination just within the posterior margin; pleurae seven in number, broad, depressed-convex, and sloping evenly to the posterior margin. Surface of the test ornamented with tubercles, which are largest and most closely set upon the glabella."

It goes on to say that mostly heads or tails are found in Maryland which would be the molts. The holotype is stored with the State Paleontologist of New York.