An integrated stratinomic model for the genesis and concentration of “small shelly fossil”-style phosphatic microsteinkerns in not-so-exceptional conditions

Abstract

Carbonate fluorapatite-filled millimeter-scale microsteinkerns are a key element of the “Small Shelly Fossil” (SSF) fauna generally associated with the late Proterozoic–Cambrian. The purported disappearance of these fossils and this style of phosphatic preservation during the Cambrian Stage 2 “Botomian extinction” has been linked to changes in ocean chemistry and/or changes to the physical environment due to the evolution of metazoans, as well to the changing role of metazoans in the marine environment. Through a qualitative meta-analysis of existing literature, we document a broader temporal distribution for SSF-style preservation than previously recognized. We examine these occurrences for common patterns of depositional conditions, and we offer a conceptual model for both the origin of the phosphatic microsteinkerns and their eventual concentration in certain sediments, based on known biogeochemical diagenetic processes of authigenic calcium fluorapatite precipitation (phosphogenesis) and subsequent concentration of phosphatic sedimentary particles. This polycyclic phosphogenic condensation (PPC) model makes predictions as to how and where this form of preservation might occur in Cambrian and non-Cambrian marine strata alike.