Closing the Phosphatization Window: Testing for the Influence of Taphonomic Megabias on the Pattern of Small Shelly Fossil Decline

Small shelly fossils (SSFs) are a group of mostly problematic, small skeletal elements preserved primarily through secondary phosphatization. They dominate lower Cambrian diversity, but appear to suffer a sharp decline in the Botomian Stage or equivalent levels outside Siberia. This observed decline coincides with a significant reduction in phosphogenesis, suggesting that it may be attributable to the closure of a phosphatization taphonomic window. The influence of taphonomic bias on observed patterns of SSF extinction at the end of the Botomian was tested using a dataset consisting of 558 Cambrian skeletal genus occurrences compiled from 109 references. Analyses indicate that SSF preservation is significantly enhanced by, and for most taxa, restricted to, a phosphatization window. Independent proxies indicate that prevalence of secondary phosphatization declined from 74% and 64% of all preservational modes during Nemakit-Dal’dynian 1 Tommotian and Atdabanian 1 Botomian times, respectively, to 40% of all modes during Toyonian 1 middle Cambrian times, coincident with a severe reduction in observed SSF diversity. Subsampling methods that control for variations in the phosphatization window were used to test whether observed SSF diversity trajectories are biased. The corrected curve suggests that although the decline of SSFs was real, it may have been significantly exaggerated by the closure of a phosphatization window.