Tuesday, March 24, 2009

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.

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