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There have been some great reports in the last week of folks hunting the Silurian and I wanted to add a report for my own serendipitous mini-trip from the last weekend. A few months ago, I had noticed a large pile of buff-colored stone dumped next to a retention pond in front of a local retail district. I thought they looked very similar to the Silurian dolomite I have seen and collected from elsewhere in Illinois, so I have been meaning to take a closer look. Last weekend I finally had some errands to run at Target with some free time on my hands, so I wandered over to the pile to check it out. In less than a minute I spotted a friendly face poking out of the corner of one piece of stone- Gravicalymene celebra! An iconic trilobite, and the biggest one I have found, with a cephalon just over 1 inch wide. It looks like it may be complete, although prep can be very difficult as @aek mentioned recently- at a minimum the cephalon appears to all be there. I looked around a little more and found a very poorly preserved cephalopod impression as well as one other rock with some intriguing shapes in it- it will need more prep though to say if it is anything. Since these were dumped next to a parking lot and there are no Silurian dolomite quarries within 60 miles, I can't say for sure what the source is. It seems likely to be the Racine or Joliet Dolomite of northeastern Illinois, though. I will definitely be returning when I have some free time and looking around some more- who knows, they may have used the same stone in other spots around the development!
andyrice11 posted a topic in Fossil IDThis post is about a well preserved Gravicalymene celebra molt I recently found in the Laurel member of the Salamonie formation of Southeastern Indiana. It is quite a peculiar specimen since it appears to have two very distinct mineral compositions. Most of the trilobite is composed of dolomite as is typical for fossils found in the Laurel. However, I initially noticed what appeared to be white calcitic pieces of the cephalon partially exposed at the anterior end of the specimen. The matrix surrounding these pieces was very easy to remove, having a fine sand like consistency. After some prep work, I was able to uncover a good portion of the glabella and concluded that these white pieces did indeed belong to the same specimen. My initial thought was that they are composed entirely of calcite, but I haven't been able to make that conclusion so I decided to post some detailed pictures in order to see what you all think. Figures 1&2. Specimen in ventral and lateral views. (Before prep work, the white rostral plate and lateral border (cephalic doublure?) which are quite obvious in the above picture, were only partially exposed. Initially my professor suggested that they likely belonged to a separate fossil specimen, perhaps a bryozoan.) Anterior end Figure 3. Anterior view showing the left and right lateral borders (cephalic doublures?), rostral plate and patrially exposed glabella. Figures 4&5. Magnified images of the rostral plate displaying uniform bumpy texture on the surface. \ Figure 6. Magnified image of dolomitized lateral border (cephalic doublure?). Note the absence of the bumpy texture seen in the previous images. So essentially my main questions are: 1. Could this white colored mineral be calcite, or something else? 2. Are the long narrow pieces considered cephalic doublures or just lateral borders? (In my research, I haven't been able to find a detailed description of Calymenid cephalic anatomy) 3. What exactly are the uniform bumps found on the white pieces? 4. Is double mineralization of a single specimen a rare occurrence, or has anyone seen something like this before? 5. What could this mineralization mean in terms of the taphonomic interpretation of this specimen. An interesting side note: A few weeks later I was once again fossil hunting in the spot at which I found the specimen described above. Along with some nice brachs and another full trilobite, I found a partial mold of a G.celebra thorax. I looked and looked for the specimen it may have once been attached to, but was unable to find anything. After returning to my lab, I noticed something quite interesting. It turns out that the mold belonged to the specimen I had collected just a few weeks before! I was glad to have found this mold, since it shows the morphology of part of the specimens posterior half which has been weathered away. Figure 7. The partial mold Figure 8. Specimen and mold side by side. Figure 9. Reunited and it feels so good!