Geochemical analysis and paleoecological implications of phosphatic microspherules (otoliths?) from Frasnian–Famennian boundary strata in the Great Basin, USA
Phosphatic microspherules (<1 mm diameter) recovered from the Upper Devonian (uppermost Frasnian) Guilmette Limestone in eastern Nevada are interpreted here to be fish otoliths and thus provide insight into ocean water chemistry just prior to the Frasnian–Famennian mass extinction boundary. Analysis by scanning electron microscope (SEM) and electron microprobe indicates that they are apatite (francolite) and consist of radially aligned, concentrically banded crystals around a central nucleus. This type of microspherule, previously interpreted to be conodont pearls, has compositions more similar to fish teeth derived from the same unit than to conodonts. Microspherules and fish teeth have consistently lower concentrations in wt% of P2O5 (31.88–36.32), F (3.05–5.12), SrO (0.15–0.34), and analysis totals (indicating higher concentrations of OH and/or CO3; 90.66–96.09) and higher CaO (51.67–55.15), SO2 (0.50–0.90), MgO (0.09–0.15), and Fe2O3 (0.11–0.21) than the associated conodonts (P2O5: 37.32–40.01; F: 4.99–6.89; SrO: 0.32–1.79; totals: 96.07–100.55; CaO: 52.32–53.06; SO2: 0.05–0.21; MgO: 0.01–0.05; Fe2O3: 0.02–0.11). These differences in composition are consistent from core to rim in all microfossils analyzed and reflect primary biogenic compositions rather than diagenetic or metamorphic signatures. We interpret the apatite microspherules to be genetically related to the fish teeth rather than to the conodonts based on the geochemical analysis. The microspherules are morphologically similar to modern teleost fish otoliths. The fish teeth found associated with the microspherules are from Acanthodian or Actinopterygian fish, which possessed otoliths in the Devonian and are the distant ancestors to modern teleost fish. Modern fish otoliths normally have a calcium carbonate composition, but their trace element composition is highly sensitive to ambient water temperature and chemistry. We speculate that the stratigraphically restricted range and phosphatic composition of the Devonian otoliths reflects secretion by fish under conditions of excess dissolved reactive phosphorus in the water column that was most likely associated with upwelling and cooler-water conditions on the shelf during maximum transgression.