DIAGENESIS OF FOSSIL BRACHIOPODS: IS THAT REALLY ORIGINAL, 350-MILLION-YEAR-OLD SHELL MATERIAL?

I investigated the diagenetic fates of local, Devonian aged brachiopods to address a long standing question of area geologists and paleontologists: “are the 350 million year old ‘shells’ in local bedrock made of original shell material?”. I sampled brachiopod fossils from Late Devonian sedimentary rocks (siltstones and sandstones of the Chadakoin Formation) of southern Erie Country, PA. Four possible diagenetic outcomes were tested via thin section microscopy and X-ray diffraction. Results indicate that the original biogenic mineral, calcite (a common crystal form of CaCO₃), remains, albeit recrystallized in parts. The degree of recrystallization varies among specimens, from none to nearly complete. I speculate that this variation can be attributed to taxonomic differences and, also, variations in the physical characteristics (permeability, etc.) of the surrounding matrix.