Taphonomy of Cambrian Phosphatic Small Shelly Fossils

Abstract

Small shelly fossils are preserved as apatite steinkerns in the Cambrian Series 2–3 Thorntonia Limestone, Australia. Petrological observations indicate that phosphorus delivered to Thorntonia sediment was remobilized before precipitating in microenvironments defined by the matrix-filled interiors of small, mostly conical skeletons. A previous geochemical study concluded that both organic matter and iron oxides sourced phosphorus to Thorntonia sediments, and that anoxia governed phosphorus remobilization within the sediment column. This contribution asks: What factors allowed for the selective preservation of skeleton interiors, and what biases result from this preservation? We find that small shells physically trapped phosphorus-laden pore waters, creating local conditions where kinetic barriers to apatite precipitation could be overcome. Only a subset of Thorntonia Limestone skeletons is preserved by apatite, showing evidence of selectivity with respect to shell size, shape, and orientation. Both the biological and physical factors that govern phosphorus remineralization and precipitation have changed through time, accounting for the opening and closing of the Ediacaran-Cambrian phosphatization taphonomic window. The opening of this window may have required a global increase in phosphate delivery to the oceans.