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Dr. Benjamin Dattilo
Department of Geosciences
Indiana University – Purdue University Fort Wayne
Some of the earliest fossils of multi-cellular animals are the “small shelly faunas” from Lower Cambrian (~540 million years old) sediments, marking the “Cambrian Explosion”. Small shelly faunas are assemblages of extremely small (millimeter-sized) phosphatic molds of shells such as snails or clams. While small shelly faunas are found in sediments ranging up to recent times, these later occurrences are often poorly studied or ignored entirely if there are larger fossils in the same sediments.
The Elgin Member in the lower part of the Maquoketa Shale in Graf, Iowa is Upper Ordovician (~450 million years old) in age, and contains a small shelly fauna. This fauna has received attention because the small shellies make up the bulk of the rock, which is rich enough in phosphorus to be called a “phosphorite”. Partly because of their small size and partly because of the geochemistry of phosphorus, previous workers have concluded that the Elgin Member phosphorites were deposited under low oxygen conditions which resulted in the animals being “dwarfed” or “stunted”. Phosphorous can be the limiting nutrient in oceanic environments, so it is not commonly found dissolved in sea-water in large quantities. When oxygen is low, however, there are not many organisms to consume the phosphorous. This is why high quantities of dissolved phosphorous might occur in anoxic deep water bodies. For this reason, the Maquoketa Shale/Elgin Member phosphorite has been attributed to upwelling of such anoxic bottom waters into the continental sea environment, where phosphorus precipitated directly into the sediment.
However, living things are still “in” the water, so oxygenated water with biomass can contain as much phosphorus as anoxic waters with phosphorus. The burial of settled organic matter is a probable a delivery mechanism for sedimentary phosphorus. Therefore an alternative hypothesis is that phosphatic sediments can form in oxygenated water, and might be concentrated by accumulation over long periods of time rather than by rapid precipitation from phosphorus-saturated water. If phosphatic preservation is size-selective and if there were evidence of the presence of normal marine fauna in these deposits, that would argue against the oceanic anoxia hypothesis. The organic burial hypothesis can be tested by identifying sizeselective preservation, or “normal” fossils.
Rock samples were taken from two sedimentary beds at Graf. Thin sections are being made from these samples, so that fossils can be examined in context. Remaining samples were dissolved in acetic acid, which releases the small phosphatic fossils from the rock. Among the acid residues Full sized specimens are being found: bryozoan, crinoid columnals, and umbilical molds of larger snails, all of which are small parts of larger animals. Also certain clams and snails are only partially filled with phosphate. These findings suggest that phosphate filled small pores and could not fill larger ones. Therefore preservational bias favored small shells, or small parts of larger shells. This suggests that the original fauna may not have been dwarfed at all, which negates the idea that oxygen poor deep-water masses caused phosphate precipitation.
Earth Sciences | Physical Sciences and Mathematics
Reeder, Jessie and Argast, Anne S., "The Graf Phosphatic Fauna: Is It Dwarfed?" (2015). 2015 IPFW Student Research and Creative Endeavor Symposium. 57.