Planko Posted May 21, 2020 Share Posted May 21, 2020 Hey all. Couple questions about North Sulfur River Tx. Why can I pull one gastropod that will be black (one species) and then within arms reach (another species) will be red? Both semi covered in the red matrix. Has anyone found a large complete red ammonite? I find pieces of large ammonites but always just pieces. I have found a complete Pachydiscus that is around 6 inches in diameter. That is the largest complete one I have found. Thanks for any help. Still learning this stuff. Link to comment Share on other sites More sharing options...
Fossildude19 Posted May 21, 2020 Share Posted May 21, 2020 Color variations are usually due to different minerals leaching into/percolating through the matrix during fossilization, and sometimes much later. From what I gather, reading posts here, (and I could be totally wrong!) is that the red material comes from a bit higher up in the banks, and is broken up more by the time it is found. In my years here on the Forum, I have only seen smaller red ammonites from the NSR posted here. A 6 incher I think is pretty good. 1 Tim - VETERAN SHALE SPLITTER VFOTM --- APRIL - 2015 __________________________________________________ "In every walk with nature one receives far more than he seeks." John Muir ~ ~ ~ ~ ><))))( *> About Me Link to comment Share on other sites More sharing options...
FossilDAWG Posted May 21, 2020 Share Posted May 21, 2020 I think the red color comes from oxidized iron minerals in the rock. This might come from oxidation during weathering of the rock, in which case the layers closest to the surface (i.e. the uppermost layer, in the process of breaking down into soil) might be the most red. In other cases the amount of oxidation reflects the condition of the sediment as it was deposited. As Tim noted most of the complete ammonites from the NSR are smaller, maybe a few inches in diameter. Some of the species just tend to be smaller, but the pachydiscids could get to be quite large. However, everywhere such ammonites occur large complete specimens are found in fresh outcrops, or protected inside of concretions. In exposures such as at NSR, where fossils erode from the river banks, large fossils tend to break up as they erode, so complete specimens are very unusual. Often the "complete" smaller specimens started out as the inner whorls of a large specimen that has broken up. Don 2 Link to comment Share on other sites More sharing options...
DPS Ammonite Posted May 21, 2020 Share Posted May 21, 2020 The black fossils are phosphatic steinkerns of mostly mollusks that probably formed in oxygen poor environments. The red fossils and rocks are iron rich carbonates that probable originally had pyrite. Pyrite can also indicate oxygen poor conditions. My goal is to leave no stone or fossil unturned. See my Arizona Paleontology Guide link The best single resource for Arizona paleontology anywhere. Link to comment Share on other sites More sharing options...
Planko Posted May 23, 2020 Author Share Posted May 23, 2020 Thank you for your answers. Much appreciated. Link to comment Share on other sites More sharing options...
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